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Thermodynamics of Dehydration and Hydration in Natrolite and Analcime


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THERMODYNAMICS OF DEHYDRATION AND HYDRATION IN NATROLITE AND ANALCIME By JIE WANG A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Jie Wang

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iii ACKNOWLEDGMENTS Funding for this project was provided by an NSF-EAR grant to Dr. Philip Neuhoff at the University of Florida. First, I gr atefully acknowledge Dr. Neuhoff for his support during the past two years in my study and livi ng in United States. His patience and advice helped me overcome a lot of difficulties and fi nally finish this project. I would like to thank my committee (Dr. Mike Perfit and Dr. Jon Martin) and Dr. Guerry McClellan for their support and guidance. I would also lik e to thank Laura Ruhl and Scott Keddy for their assistance with sample preparation, Jane Gustavson and Gokce Atalan for their help in my research. Special thanks also go to Ryan Francis, Shawn Malone, Susanna Blair, Derrick Newkirk and all my other friends fo r their help with my spoken and written English. Finally, I would like to thank my parents for all their love and support throughout my life.

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iv TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES.............................................................................................................vi LIST OF FIGURES..........................................................................................................vii ABSTRACT.......................................................................................................................ix CHAPTER 1 INTRODUCTION........................................................................................................1 Background Studies......................................................................................................3 Occurrence of Zeolites..................................................................................................5 Industry Application of Zeolites...................................................................................6 Thermodynamics of Dehydration in Zeolites...............................................................6 Heat Capacities of Hydrous Zeolites............................................................................8 Calorimetric Technique..............................................................................................10 2 ENTHALPy OF HYDRAT ION IN AnalCIME.........................................................15 Introduction.................................................................................................................15 Methods......................................................................................................................17 Sample and Characterization...............................................................................17 Absorption Calorimetry.......................................................................................17 Heat Capacity Measurements..............................................................................18 Results........................................................................................................................ .19 Enthalpy of Hydration in Analcime....................................................................19 Heat Capacities of Hydrated and Dehydrated Analcime.....................................21 Discussion...................................................................................................................21 Comparison of Present Results with Previous Studies........................................21 Temperature Dependence of Heat of Hydration.................................................22 Behavior of Heat Capacity of Hydration.............................................................24 3 EXCESS HEAT CAPACITY IN NATROLITE HYDRATION...............................39 Introduction.................................................................................................................39 Methods......................................................................................................................40

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v Sample and Characterization...............................................................................40 Heat Capacity Measurements..............................................................................41 Absorption Calorimetry.......................................................................................42 Results........................................................................................................................ .43 Heat Capacities of Hydrated and Dehydrated Natrolite......................................43 Enthalpy of Hydration in Natrolite......................................................................43 Discussion...................................................................................................................45 Comparison with Previous Results......................................................................45 Heat Capacity of Hydration in Natrolite.............................................................46 Nature of the Natrolite-Dehydrat ed Natrolite Solid Solution..............................48 4 CONCLUSION...........................................................................................................62 Comparison of the Results of Analcime and Natrolite...............................................62 Heuristic Outcomes of This Study..............................................................................63 Future Work................................................................................................................64 LIST OF REFERENCES...................................................................................................65 BIOGRAPHICAL SKETCH.............................................................................................74

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vi LIST OF TABLES Table page 2-1 Isothermal immersion calor imetric data for analcime..............................................27 2-2 Heat capacities of hydrated and dehy drated analcime, steam, and hydration of analcime at different temperatures...........................................................................28 2-3 Enthalpy of hydration in analcime...........................................................................29 3-1 Heat capacities of hydrated and dehy drated natrolite, steam, and hydration of natrolite at different temperatures............................................................................51 3-2 Isothermal immersion calor imetric data for natrolite...............................................52 3-3 Enthalpy of hydration in natrolite............................................................................53

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vii LIST OF FIGURES Figure page 1-1 View of the crystal structures of natr olite (a) (after Peacor, 1973) and laumontite (b) (after Fridriksson et al., 2003) projected along the c axis..................................12 1-2 Thermogravimetric analysis (TGA) curv es depicting the change in mass with increasing temperature of natr olite (a) and laumontite (b).......................................13 1-3 Schematic representation of the simultaneous DSC/TGA system...........................14 2-1 Crystal structure of analcime viewed down the b crystallographic axis (after Mazzi and Galli, 1978).............................................................................................30 2-2 Example isothermal immersion experiment on analcime at 403K...........................31 2-3 Plot of DSC versus dTGA for the e xperimental results shown in Fig. 2-2..............32 2-4 Heat evolved during absorption of wate r into analcime as a function of mass absorbed...................................................................................................................33 2-5 Enthalpy of hydration in analcime as a function of temperature.............................34 2-6 The mass change (a) and DSC response (b) of hydrated and dehydrated analcime collected at a scanning rate of 15 K/min under ultrapure N2...................................35 2-7 Experimental Cp data as a function of temper ature for hydrated and dehydrated analcime...................................................................................................................36 2-8 The heat capacity of hydration in an alcime as a function of temperature................37 2-9 Partial molar enthalpies of hydra tion in analcime vs. temperature..........................38 3-1 Crystal structures of natrolite down the c crystallographic ax is. (after Peacor, 1973).........................................................................................................................54 3-2 The mass change (a) and DSC response (b) of hydrated and dehydrated collected at a scanning rate of 15 K/min under ultrapure N2...................................................55 3-3 Heat capacities of hydrated and dehydrated natrolite as a function of temperature from 320 to 680 K....................................................................................................56

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viii 3-4 Example immersion calorimetric experi ment on natrolite conducted at 412 K.......57 3-5 The enthalpy of dehydration in natr olite as a function of temperature....................58 3-6 Mass change of natrolite in th e dehydration and rehydration with a heating/cooling rate of 5 K/min under a humid condition (PH2O 12 mbar).............59 3-7 Comparison of present and previous Cp data for both hydrated and dehydrated natrolite..................................................................................................60 3-8 The heat capacity of hydration in na trolite as a function of temperature.................61

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ix Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science THERMODYNAMICS OF DEHYDRATION AND HYDRATION IN NATROLITE AND ANALCIME By Jie Wang August 2006 Chair: Philip S. Neuhoff Major Department: Geological Sciences Zeolites are framework aluminosilicates w ith open channels containing molecular water and extra-framework cations. Their distin ctive crystal structures endow them with high cation-exchange capacities and molecu lar sieve capabilities, which are widely applied in water softening, catalysis and wast ewater treatment. Thermodynamic data are essential to determine the stability of zeo lites and evaluate their paragenesis. Reversible dehydration of intracrystalline water in zeolites is an important consideration for assessing thei r stability, particularly at elevated temperatures and pressures. Derivation of thermodynamic prope rties of dehydration and rehydration from phase equilibria requires prior knowledge of the heat capacity of hydration ( Cp,r) in order to reduce the number of unknown variable s. However, experimental determination of Cp,r is difficult because measurements at elevated temperature often contain contributions from the enthalpy of dehydrati on. Statistical-mechanical reasoning is often used to suggest that Cp,r is independent of temperatur e, permitting application of Cp,r

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x determined at relatively low temperatures wh ere dehydration is not an issue. We focused our study on natrolite (Na2Al2Si3O10H2O) and analcime (NaAlSi2O6H2O), two common rock-forming zeolites that are chemi cally and structurally less complex than other zeolites due to the presence of only one extraframework cation (Na+) and one crystallographically-distinct wa ter site. In this study, we have directly measured heat capacities (Cp) of hydrated and dehydrated zeoli tes by the simultaneous differential scanning calorimetric (DSC) and thermogr avimetric analysis (TGA) system. The temperature dependence of enthalpies of hydration ( Hhyd) in analcime and natrolite was determined by the newly developed isotherm al immersion technique. The temperature dependence of Hhyd provides an alternative means of assessing Cp,r. The results obtained by these approach es show that the behaviors of Cp,r are different for different zeolites. In natrolite, Cp,r determined from Hhyd between 373 and 473 K is independent of temperature, but is s ubstantially larger than determined by direct measurements of Cp. This implies the presence of an excess heat capacity of mixing due to the solvus behavior of natr olite in solid solution. In the case of analcime, the situation is more complicated. In the lo wer temperature range (< 463 K), Cp,r, determined from Hhyd, is relatively insensitive to the temper ature and in agreement with direct Cp measurements, whereas in the highe r temperature range (> 463 K), Cp,r decreases and increases rapidly with increas ing temperature, indicating a phase transition. Coupled determination of Hhyd and Cp,r as a function of temperat ure provides important and fundamental insights into the thermodynamics of dehydration and rehydration in zeolites, and should aid quantitative prediction of the water content of zeolites as a function of temperature, pressure, and the chemical potential of H2O in geologic systems.

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1 CHAPTER 1 INTRODUCTION Minerals that contain molecular water with in their structures are modally-important at and near the earths surface. These mi neral hydrates (e.g., oxyhydroxides, sulfates, carbonates, clay minerals, zeolites) are importa nt reservoirs for water in earths crust. Many of these phases are also important natu rally-occurring nanomaterials (e.g., Banfield and Zhang, 2001). A common feature of some hydrat e minerals is the ability to reversibly dehydrate in response to changes in temperatur e, pressure, and the chemical potential of H2O, a process that has significant implications for assessing the water content of the crust and the stability of the minerals them selves (e.g., Bish and Carey, 2001). Therefore, the stability and chemical behavior of hydrates are critical consider ations for assessing the behavior of low-temperature geochemical systems. This study focuses on zeolites, which, in addition to being geologically important phases in low temperature environments (e.g., Coombs et al., 1959; Hay and Sheppard, 2001; Neuhoff et al., 2000), serve as important model systems for th e study of dehydration by mineral hydrates because they have well-delineated crystal st ructures (e.g., Armbruster and Gunter, 2001; Meier et al., 2001) and are readily manipulat ed experimentally (e.g., Bish and Carey, 2001). Understanding the formation and stability of zeolites in geologic systems is limited in part by an insufficient unde rstanding of the dehydration be havior of these minerals. Partial dehydration (i.e., compositional change s) during calorimetric or phase equilibrium experiments complicates derivation of therm odynamic properties for these minerals (e.g.,

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2 Carey, 1993; Helgeson et al., 1978). Thus, de veloping experimental techniques that provide for quantitative assessment of th e thermodynamic consequences of partial dehydration is critical for eval uating zeolites stability relati ons. In addition, the stability of zeolites with respect to other aluminosilic ates is dependent on their hydration state as hydrated zeolites are often st able at earth surf ace conditions with respect to dense aluminosilicate assemblages and free water, but their dehydrated equivalents are not (e.g., Shim et al. 1999). In fact, accurate prediction of the partial dehydration of zeolites can be key to understanding their stability in ge ologic environments (e.g., Neuhoff and Bird, 2001). Dehydration also has a dramatic effect on the ion exchange properties of zeolites (Fridriksson et al., 1999; Bish and Ca rey, 2001). Complicating the thermodynamic description of these proce sses is the fact that many zeolites contain multiple, energetically distinct wate r sites (Fridriksson et al., 2003; Fialips et al., 2005). This study applies new experimental tec hniques for assessing the thermodynamic properties of dehydration reacti ons in zeolites, focusing on th e natural zeolites analcime and natrolite. Through detailed laboratory anal ysis this study attempts to answer three specific questions: 1. Are the heat capacities of dehydr ation reactions in zeolites invariant with respect to temperature, as previously suggested? 2. Do experimental measurements of th e bulk heat capacities of hydrated and dehydrated zeolites permit accu rate assessment of the temperature dependence of the heat of dehydration determined calo rimetrically or through equilibrium observations? 3. What are the water contents of analcime and natrolite in the geologic settings in which they occur? In the pages to follow, we comment on pr oblems in four chapters. Chapter 1 (this chapter) provides background material rele vant to this study. Chapter 2 presents

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3 experimental determinations of the temperat ure dependence of the heat of hydration and heat capacity of hydration in an alcime. Chapter 3 is a simila r study of natrolite. Chapter 4 compares the results for analcime and natro lite and discusses the br oader implication of this study. Background Studies Mineralogical Nature of Zeolites Zeolites are framework aluminosilicates whose structure is characterized by a framework of Siand Al-cente red tetrahedra arranged to form 2-10 channels that contain water molecules and ex traframework (charge-balanc ing) cations (Gottardi and Galli, 1985). The general chemical formula for natural zeolites is (Li, Na, K)a(Mg, Ca, Sr, Ba)d[Al(a+2d)Sin-(a+2d)O2n]mH2O where the portion in square brackets represen ts the framework and rest of the species reside within the channels (e.g., Gottardi and Galli, 1985). While modification of the framework composition requires dissolution and precipitation of the mineral, the extraframework cations are readily exchangeable (e.g., Newell and Rees, 1983) and water molecules can be reversibly removed from th e structure at elevated temperatures (e.g., van Reeuwijk, 1974). Figure 1-1 depicts the crystal structure of two representative rock-forming zeolites, natrolite and laumontite. The tetrahedral fram ework sites in zeolites are occupied by Si4+ and Al3+ (with minor substitution of Fe3+). The extraframework cations balance the net negative charge that exists on the framework due to trivalent Al in the tetrahedral sites. The coordination number and bonding of the ex traframework cations vary significantly between different zeolites, among sites in a given zeolite, and between ions (Armbruster and Gunter, 2001). The coordinati on spheres of these ions ar e made up of a combination

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4 of framework and water oxygens. The integral presence of water within the crystal structures of zeolites has been known sin ce their discovery (Cronstedt, 1756). Water comprises 8-25% of the mass of zeolites under ambient conditions. In some zeolites like natrolite, there is only one cr ystallographically distinct wate r site (Fig. 1-1a); while in some zeolites like laumontite, water is dist ributed among several crystallographically distinct sites (Fig. 1-1b). No te that water molecules occupy four distinct sites in laumontite, labled W1, W2, W5 and W8 (Artio li and Sthl, 1993). Sites W2 and W8 are part of the coordina tion sphere of the Ca2+ extraframework ion. Site W5 is hydrogen bonded only to W2 and W8, whereas W1 is hydrogen bonded to both framework oxygens and waters on W2 and W8 (Armbruster and Kohler, 1992). Sequential loss of water molecules from distinct sites appears to be a common phenomenon in zeolites (e.g., Armbruster, 1993; Cruciani et al., 2003). Loss of water can lead to pronounced changes in the structures of the zeolite framework and the positions of extraframework cations. Most zeolites also exhibit contraction and/or collapse of the tetrah edral framework during dehydration. Zeolites produce abundant water upon h eating under atmospheric conditions. A common method of assessing the progra de dehydration of zeolites is by thermogravimetric analysis (TGA), in which ma ss loss (due to dehydration) of a zeolite sample is monitored as a function of temp erature. Some zeolites, like analcime and natrolite, dehydrate continuous ly with increasing temperat ure (Fig. 1-2a, corresponding to Fig. 1-1a), yet others like laumontite a nd heulandite show distinct steps in their dehydration behavior (Fig. 1-2b, corresponding to Fig. 1-1b) (e.g., Alberti and Vezzalini,

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5 1984). These differing behaviors reflect the stru ctural and energetic properties of water molecules within the structures. Occurrence of Zeolites Zeolites occur in a wide variety of e nvironments, including two major types of occurrences: 1) macroscopic and microscopic crys tals, often in veins, fractures, and vugs within plutonic and volcanic rocks and their metamorphosed equivalents; 2) submicroscopic crystals, commonly distributed in vitroclastic sediments which have undergone diagenetic or low-grade metamor phic processes (Passaglia and Sheppard, 2001). Zeolites can also occur during reactions of aqueous fluids with marine sediments (e.g., Boles and Coombs, 1977), saline lake sediments (e.g., Hay and Moiola, 1963), volcanic tuffs (e.g., Hay and Sheppard, 2001) and soils (e.g., Baldar and Whittig, 1968). Hydrothermal systems often produce zeolites as well with complex parageneses caused by overprinting of diagenetic and metamorphi c occurrences (Gottardi, 1989; Neuhoff et al., 1997). Zeolite stability is a sensitive f unction of pressure, temperature and fluid composition, making them useful indicators of the physical and chemical conditions associated with petroleum resources (e .g., Iijima, 2001), geothermal resources (e.g., Kristmannsdttir and Tmasson, 1978) and basa lt-hosted ore deposits (e.g., Stoiber and Davidson, 1959) in earths crust. Zeolites and associated authigenic clay minerals can significantly reduce the porosity and permeab ility of hydrocarbon reservoir rocks, and their presence is often regarded as an economic basement of exploration for oil and gas (e.g., McCulloh et al., 1973; McCulloh and Stewart, 1979). In addition, they are frequently considered as pa ssive barriers in radioactiv e waste repositories both as sorptive barriers to radionuclide migrati on and consumption of thermal energy (e.g., Carey and Bish, 1996; Bish et al., 2003).

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6 Industry Application of Zeolites The high cation-exchange capacities and mo lecular sieve capabilities make zeolites widely applied in industry, e.g., water soft ening, catalysis and wastewater treatment (Mumpton, 1977). The relatively large energy storage density of zeolites makes them employed in energy storage and heat pump technologies, where their use instead of activated alumina or silica gel can result in significant reduction of storage weight (e.g., Shigeishi et al., 1979; Scarmozzino et al., 1980; Gopal et al., 1982; Selvidge and Miaoulis, 1990). In agriculture, natural zeolites have been used as soil conditioners, carriers for insecticides and herbicides, reme diation agents in contaminated soils, slowrelease fertilizers, and dietary supplements in animal nutrition because of their capability of cation exchange, adsorption and their abund ance in near surface, sedimentary deposits (Ming, 1985, 1987, 1988; Ming and Mumpton, 1989; Boettinger and Graham, 1995). The unique cation-exchange capabilities of zeolites can be used to remove dissolved cations that affect human and animal health (e.g., NH4 +) from water by exchanging with biologically acceptable cations such as Na+, K+, Mg2+, Ca2+ or H+ (Neveu et al., 1985; Xu, 1990; Pabalan and Bertetti, 1999). The su rface area of a zeolite -rich rock is ~10 m2/g, much bigger than that of quartz sand (~0.01 m2/g), thus the filtration efficiency of a sand bed can be increased by mixing porous zeolit ic rock with it (Grigorieva et al., 1988; Galindo et al., 2000). Zeolites are being used in more and more new technologies, and these potential applications provide numerous possibilities to impr ove the environment. Thermodynamics of Dehydration in Zeolites The response of zeolites to changes in te mperature and water vapor pressure is a very important aspect of their behavior and structural changes. De tailed studies of the structural effects accompanying dehydration pr ocesses allow evaluation of the changes in

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7 environmental conditions that ultimately l ead to structural mo dification or breakdown (Bish and Carey, 2001). Alberti and Vezzalin i (1984) divided zeolites into three categories according to their thermal stabili ties. Those with: 1) reversible dehydration accompanied by rearrangement of the extraf ramework cations and residual water molecules (e.g., chabazite, analcime and morden ite); 2) complete or nearly complete reversible dehydration accompanied by a large distortion of the framework and significant decrease in unit-cell volume (e.g., na trolite, mesolite and laumontite); and 3) irreversible dehydration accompanied by irre versible changes in the framework (e.g., heulandite, barrerite and stilbite). Equilibrium between a zeolite and water va por can be represented by a reaction of the form ZnH2O = Z + n H2Ovapor (1-1) where ZnH2O and Z are homologous hydrated (wat er sites occupied) and dehydrated (water sites vacant) components of a zeolite and n is the number of moles of H2O in the fully hydrated zeolite. Evaluation of the hydr ation state of zeolites as a function of temperature and pressure requi res knowledge of the thermodynamic properties, such as Gibbs energy of reaction ( Gr,T,P), enthalpy of reaction ( Hr,T,P) and entropy of reaction ( Sr,T,P), as well as the relationship between activity and composition. Four basic approaches have been applied in order to determine the therm odynamic properties of zeolite dehydration reactions (cf. Bish and Carey, 2001). The first method involves explicit calculation of Gr,T,P from the known properties of substances in the reaction. For instance, volum e of reaction ( Vr) can be directly calculated if the molar volumes of Z n H2O and Z have been determined. This met hod has been applied to calculation of Sr

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8 and heat capacity of reaction ( Cp,r) as well from the results of heat capacity (Cp) measurements for homologous hydrates and de hydrated zeolites (Carey, 1993; Ransom and Helgeson, 1994; Neuhoff et al., 2000). In addition, Hr can be determined from the heats of formation of homologous hydrated and dehydrated zeoli tes determined via thermochemical cycles from heat of so lution data (e.g., Johnson et al., 1982, 1983; Ogorodova et al., 1996). The sec ond approach is transposed temperature drop calorimetry (e.g., Navrotsky et al., 1994; Kiseleva et al., 1996, 1997; Shim et al., 1999) in which Hr is determined via a thermochemical cycle involving dehydration of the zeolite at high temperatures (usually ~973 K) and the heat contents of the h ydrated and dehydrated zeolites and water. The third method, immersi on calorimetry, involves direct calorimetric measurement of Hr as the dehydrated zeolite is imme rsed in water or humid gas (e.g., Barrer and Cram, 1971; Coughlan and Carroll, 1 976; Carey and Bish, 1997; Muller et al., 1998; Petrova et al., 2001). The last method in volves fitting equili brium observations of the water content of a zeolite as a function of temperature and water fugacity, usually determined thermogravimetrically (e.g., Care y and Bish, 1996; Fridriksson et al., 2003) or by pressure titration tec hniques (Wilkin and Barnes, 1999) Non-linear regression of phase equilibrium observations to the th ermodynamic relations can determine the thermodynamic properties at standard conditions ( Gr,Tref,Pref, Hr,Tref,Pref, and Sr,Tref,Pref), but generally requires prior knowledge of Cp,r in order to reduce the number of unknown variables (Carey and Bish, 1996). Heat Capacities of Hydrous Zeolites Determination of the temperature dependence of Cp of hydrous minerals at superambient conditions, particularly those that dehydrate continuous ly with temperature such as zeolites, presents considerable experimental obstacles. Calorimetric

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9 measurements of Cp by adiabatic, drop, or differen tial scanning calorimetry (DSC) on these materials will inevitably incl ude contributions not only from Cp but also the heat of dehydration. Consequently, there is a paucity of reliable data for Cp of hydrate minerals, which presents considerable complicatio ns for assessing the magnitudes of the Cp. There are two basic approaches that have been taken to address this issue: 1) adjust Cp measured by drop calorimetry for the heat of dehydration determined separately (e.g., Johnson et al., 1982, 1983); and 2) assume that Cp,r is independent of temperature (Barrer, 1978; Carey, 1993), allowing Cp,r to be estimated from Cp for the hydrous and anhydrous phase determined at relatively lo w temperatures where the dehydration is not an issue. The first approach ignores potenti al exothermic effects in the calorimetric measurements as the zeolite rehydrates during the drop, potentially leading to overestimation of Cp for hydrous phases (Carey, 1993). Th e second approach is based on statistical-mechanical argu ments that sorption of H2O into a zeolite should lead to an increase in Cp for this component over that in the vapor phase as a consequence of the loss of translational and rotational degrees of freedom to vibrational modes within the zeolite structure (Barrer, 1978). While this is certainly true, and borne out by some experimental data for Cp,r for complete dehydration of some hydrates (Carey, 1993), there is a paucity of experimental data nece ssary to test this m odel. In this study, we measured the temperature dependent heat by isothermal immersion experiments and observed some contradictions with the sta tistical-mechanical model including secondorder phase transition (Johns on et al., 1982; Neuhoff and Bird, 2001) and excess heat capacity (Basler and Lechert, 1972).

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10 Calorimetric Technique All the calorimetric measurements were performed on the Netzsch STA 449C Jupiter simultaneous thermal analysis system at the University of Florida. A schematic depiction of this system is shown in Fig. 1-3. The core component is a vacuum-tight, liquid nitrogen cooled furnace enclosing a sample carrier housing an electrode for measurement of temperature differences between the sample and a reference pan, generating a heat flux DSC signal and whic h is connected to a microbalance for thermogravimetric analysis. With respect to th e present study, an esse ntial aspect of this setup is that the DSC and T GA signals are recorded simultaneously, which allows the DSC signal to be interpreted directly in term s of water loss or gain to the sample as measured by TGA. The temperature range for the furnace is 120 to 1050 K; temperature can be maintained within this range to a precision of better than 1 K and for dynamic analysis can be run at contro lled heating or cooling rates of up to 50 K/minute. Samples as large as 5 g and as small as 1 mg can be investigated, although sample sizes of the order of a few 10s of mg are opt imal. Mass changes as small as 0.1 g can be detected. Calorimetric precision varies with experi mental conditions and the amount of heat involved; instrument specifications call for Cp precision on the order of 2.5% and heat of reaction precision on the order of 3%. Precision obtained for the experiments proposed in our study is described further below. Temperature and caloric calibration is performed by measuring the melting temperatures and heat s of fusion of high purity metal standards and Cp of sapphire disks (cf. Hhne et al., 1990 ; Sarge et al., 1994; Sabbah et al., 1999). Temperature, DSC, and TGA signals are se nt via cable to a personal computer for recording and data analysis.

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11 Experimental conditions can be controlled with respect to not only temperature but also pressure. Experiments can be run under cl osed system conditions at total pressures ranging from atmospheric to vacuums of 10-4 Torr, although typical operation involves a flowing gas atmosphere at atmospheric pr essure. Gas composition and flow rate are controlled externally via mass flow controller s. Gases used in the experiments are ultrahigh purity He (for subamb ient temperatures) and N2 (for Cp measurements and measurements under controlled humidity conditions). Humidity is generated by bubbling N2 through distilled water at a constant temperature, w ith water vapor pressure (PH2O) varied by mixing H2O-saturated and dry N2 gas in different propor tions. Humidity on the outflow end of the furnace is constantly monitored and recorded by a Sable Systems RH100 flow-through dewpoint/relative humid ity analyzer (1% accuracy in PH2O down to 1 Pa).

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12 (a) (b) Figure 1-1. View of the crystal structures of natrolite (a) (after Peacor, 1973) and laumontite (b) (after Fridrikss on et al., 2003) projected along the c axis. Natrolite has only one water site, whereas laumontite has four water sites: W1, W2, W5 and W8.

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13 Figure 1-2. Thermogravimetric analysis (TGA) curves depicting the change in mass with increasing temperature of natrolite (a) a nd laumontite (b). Natrolite contains a single water site and exhibits one c ontinuous dehydration curve, whereas laumontite has several water sites that dehydrate under different conditions as reflected in the inflections observe d in the mass loss as a function of temperature.

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14 Figure 1-3. Schematic representation of the simultaneous DSC/TGA system. Dashed curves represent gas lines; solid curves represent data transfer cables between the instruments and the computer.

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15 CHAPTER 2 ENTHALPY OF HYDRA TION IN ANALCIME Introduction Analcime, nominally NaAlSi2O6H2O, is one of the most common rock-forming zeolites. It appears to be stable over a cons iderable range of temperature and pressure conditions, thus it occurs in a very wide ra nge of geologic settings Analcime generally forms during low grade metamorphism of pl utonic and volcanic rocks, as a product of reaction between saline solutions and sediments in alka line lakes (e.g., Hay and Moiola, 1963; Hay, 1966; Coombs and Whetten, 1967; Iijima and Harada, 1968; Surdam and Sheppard, 1978), and as phenocrysts in al kalic igneous rocks (e.g., Wilkinson and Hensel, 1994). Although recent progress has b een made in assessing the stability of analcime in low-temeprature environments (e.g., Neuhoff et al., 2004), phase equilibria observations at elevated temper ature and pressure are often in consistent with each other (cf. Thompson, 1973). In large part, this appear s to be due to complications arising from solid solution in analcime, particularly with respect to its variable water content due to progressive dehydration with increasing te mperature (e.g., Helgeson et al., 1978) Dehydration of analcime is a relatively simple one step process because water molecules occupy only one crystallographically distinct site (Fig. 1-1, Mazzi and Galli, 1978). Several authors have studied the hydration-dehydration thermodynamics of analcime (e.g., King, 1955; King and Weller, 19 61; Robie et al., 1979; Helgeson et al., 1978; Johnson et al., 1982; Ogorodova et al., 1996; Bish and Carey, 2001). Application of these data to predicting the hydration stat e of analcime is complicated by a lack of

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16 understanding of the temperature dependence of these properties, which largely relies on assessing of the change in heat capacity brought about by dehydration. Experimental determinations of the heat capacities (Cp) of the hydrated and de hydrated forms of these minerals (e.g., King, 1955; King and Weller, 19 61; Pankratz, 1968; Johnson et al., 1982) are complicated because the calorimetric measurements at superambient temperatures generally include contri butions not only from Cp but also the heat of dehydration. In the absence of reliable data, an assumption that Cp of dehydration in analcime is independent of temperature (Barrer, 1978; Carey, 1993) wa s made based on the statistical-mechanical arguments that much of the difference in Cp between absorbed water and the gas phase is caused by the loss of some translational a nd rotational degrees of freedom to weak vibrational modes within the zeolite structure. Although the Cp results of some stable zeolites near room temperat ure are consistent with th is assumption (Carey, 1993), no other approaches have been used to test it. In the present investigati on, the isothermal heats of hydration in analcime were measured over a range of temperature under c onstant water vapor pressure, and a general behavior of the heat capacity of hydration ( Cp,r) for analcime dehydration was obtained. In addition, the heat capacities of hydrated and dehydrated anal cime were determined as a function of temperature by differential scanni ng calorimetry (DSC), and the results were used to calculate Cp,r based on the statistical-mechanic al model. Comparison of these two methods is used to evaluate the appropriateness of the assumption that Cp,r is not a function of temperature.

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17 Methods Sample and Characterization The sample of analcime was collected from a zeolite-facies metabasalt outcrop at Maniilat on the island of Qeqertarsuaq in West Greenland (Neuhoff et al., 2003), and prepared from a 1.5 cm euhedral crystal of opaque analcime. Separates of analcime were hand picked, ground in an agate mortar, and sieved to a 20-40 m size fraction. Phase identity and purity were confirmed by X-ra y powder diffraction. A split of this sample was previously used and characterized in the 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) study of Neuhoff et al. (2003; Sample ANA002). Electron probe microanalysis indicated a composition of (NaAl)0.95Si2.05O6.024H2O, although the 29Si MAS NMR results indicate a slightly less Si-rich composition of (NaAl)0.97Si2.03O6.015H2O. The latter value was chosen as more representative of the bulk composition (cf. Neuhoff et al., 2004) and c onsistent with the water content. Water content of the sample was determined by th ermogravimetric heating to 1023 K after the equilibration with a room temperature atmo sphere of 50% relative humidity (RH), and the mass loss was measured to be 8.29% of the total sample. This value is close to water content calculated from the compositions determined by 29Si MAS NMR, 8.32% (cf. Neuhoff et al., 2004). Absorption Calorimetry Heats of hydration as a function of te mperature were determined using an isothermal DSC-based immersion tec hnique on the Netzsch STA 449C Jupiter simultaneous DSC-thermogravimetric analysis (T GA) system at the University of Florida as described by Neuhoff and Wa ng (2006). This approach combines the benefits of DSC and gas absorption calorimetry. Twenty to 30 mg of hydrated analcime were placed into a

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18 Pt-Rh crucible for each run. The sample was dehydrated by scanning heating from 298 to 873 K at the rate of 15 K/min and then allowed to cool to the experimental temperature. A purge of dry N2 was maintained at a flow rate of 50 ml/min during this period. After equilibration (20-40 min.) at this temperature under dry N2 until both DSC and TGA baselines stabilized, the gas st ream was changed to humid N2 which was generated by bubbling ultrapure N2 gas through a saturated NaCl so lution. In order to reduce the change of DSC baseline, the flow rate of humid N2 was maintained at 30 ml/min and the corresponding water vapor pressu re in the furnace was ~12 mbar. Under this condition the sample was allowed to react until the DSC trace became relatively flat. Repeated experiments on one analcime sample gave virt ually identical results, indicating that the sorption capacity of analcime was not affected by dehydration and hydration. Consequently, some of the data at different temperatures were measured from the same sample aliquot. During the experiment, the sample of dehydrated analcime could only reabsorb less than 5% of its mass, as opposed to ~9.1% mass gain for complete rehydration. Therefore, under cu rrent water vapor pressure we can only directly measure partial molar enthalpies of hydration ( hhyd) for analcime. Heat Capacity Measurements The heat capacities of hydrated and de hydrated analcime were also determined by DSC also on the simultaneous DSC-TGA system Each experiment consisted of four separate runs: 1) determination of background and baseline by measurement of an empty crucible; 2) DSC measurement of a standard (sapphire); 3) scanning heating of hydrated analcime and 4) scanning h eating of dehydrated analcime. A sample of approximately 27 mg was packed into a covered Pt-Rh crucible before step 3 and kept to the end of the experiment. Data were collected in the ra nge of 298 to 873 K at a scanning rate of 15

PAGE 29

19 K/min. A purge of dry N2 was used during the experiment to keep the relative humidity below 1%, and the gas flow was maintained at ~30 ml/min using mass flow controllers. The furnace was heated to 873 K in each run a nd then cooled back to room temperature by liquid N2 before the next step. During heati ng, hydrated analcime lost mass with increasing temperature and finally becam e completely dehydrated. The resulting dehydrated analcime was kept in an environment of dry N2 to avoid rehydration during cooling to room temperature and then measured by scanning heating again. Heat capacity of the sample as a function of temperature was calculated by std p b std b s s std pC DSC DSC DSC DSC m m C, (2-1) where ms is the mass of sample, mstd is the mass of standard (27.314 mg), DSCs, DSCstd and DSCb are for sample, standard and baseline respectively. Tripli cate calorimetric measurements were conducted and averaged. Ba sed on previous measurements in this lab the precision was taken to be 1% of Cp. Results Enthalpy of Hydration in Analcime An example of TGA and DSC response of analcime during immersion in water vapor at 403 K is shown in Fig. 2-2. It is observed that the hydration of analcime is initially relatively rapid (as shown by the peak in the first derivative of the TGA signal, dTGA) and then decays exponentially. After about 120 min. the reaction slowed to a point where the DSC signal had decayed to near baseline level and essentially invariant with respect to time even though sa mple could keep on absorbing H2O if the experiment continued. The slow rate of the reaction is also reflected in the near -zero value of dTGA.

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20 It can be seen in Fig. 2-2 that the DS C and dTGA data are strongly correlated. These data are plotted against each other in Fig. 2-3, and the slope of the linear regression is proportional to hhyd. Consequently, the partial molar enthalpies of hydration can be calculated by the equation hhyd = k (dDSC/dm) (MWH 2 O) (2-2) where k is the caloric calibration factor (in mW /V), dDSC/dm is the slope in Fig. 2-3, and MWH 2 O is the molecular weight of water. The y-intercept of the regression represents the DSC signal when dTGA = 0; i.e., the baselin e at the end of reaction. The partial molar enthalpy of hydration in analcime calculated by this method has relatively large error because of the oscillation of DSC and dTGA signals. Using the position of the baseline derived by linear regression of the DSC a nd TGA signal provides a nother approach to determine the hhyd. Cumulative, baseline-corrected DSC response plotted against cumulative mass of absorbed H2O also leads to a linear depende nce, the slope of which is also equivalent to hhyd (Fig. 2-4). The results of partial molar enthalpy at te n different temperatur es calculated by both methods are listed in Table 2-1. Most of the values of hhyd1 and hhyd2 at the same temperature are close to each other (within 1.5% difference), illustrating the general repeatability of this method. For some temperatures the difference between hhyd1 and hhyd2 is relatively big; this may result from the uncertainty of the baseline caused by the oscillation of the signals. The temperature dependence of hhyd are illustrated in Fig. 2-5. The errors include the part from standard deviation of the values and that from the calorimetric calibration (1% of the results). It can be seen that within the analysis error

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21 the hhyd at different temper atures have little change ex cept the one at 528 K, which may indicate a trend of abrupt decrease and increase of hhyd with increasing temperature. Heat Capacities of Hydrated and Dehydrated Analcime Figure 2-6 shows TGA and DSC traces of hydrated and dehydrated analcime obtained during scanning heating. Dehydration of analcime is accompanied by a mass loss from ~350 K to 743 K, which is also in dicated by the positive inflection of the DSC curve. The TGA curve is continuous a nd suggests only one stage of dehydration, consistent with previous observations that on ly one energetically distinct water site is presented in analcime (e.g., van Reeuwijk, 1974; Bish and Carey, 2001) and the crystal chemical considerations listed above. Unlik e the TGA trace for hydrated analcime, there is no mass change for dehydrated analcime during scanning heating, showing that the sample has been completely dehydrated in the first run. Calculated Cp for hydrated and dehydrated analci me are compared in Fig. 2-7. The heat capacity of hydrated analcime is relativ ely insensitive to the temperature below ~399 K, but increases quickly after that becau se of the contribution of dehydration. For dehydrated analcime the heat measuremen t only includes the contribution from Cp, so the trace of Cp increases very slowly with increasing temperature, and in a nearly linear fashion. Discussion Comparison of Present Results with Previous Studies Low-temperature (below 350 K) Cp measurements on analcime have been reported previously (King, 1955; King and Weller, 1961; Johnson et al., 1982). The data of lowtemperature Cp were directly measured by adiabati c calorimetry. In addition, Johnson et al. (1982) also assessed the Cp for analcime above 350 K by drop calorimetry. The

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22 present results of Cp of hydrated analcime below 350 K are in good agreement with those from Johnson et al. (1982). For instance, at 298 K the difference between the two values is only ~0.6%. However, above 350 K, Cp measured in the present study is higher than those of Johnson et al. (1982) because of the strong dehydration effect on the DSC signal. As for the Cp of dehydrated analcime, the present re sults are closer to those from Johnson et al. (1982) at low temper atures than at high temper atures, but are generally. Examination of the method that Johnson et al. (1982) used to determine the Cp of dehydrated analcime suggests th at the cause is that their sample was not completely anhydrous. The enthalpy of hydration in analcime at 298 K has been determined by several authors using different methods (Johnson et al., 1982; Ogorodova et al., 1996; Barany, 1962; Bish and Carey, 2001), and the result s are listed in Table 2-3. Especially, Ogorodova et al. (1996) found that enthalpy of formation of analcime is linearly dependent on the degree of hydration in an alcime, indicating that there are no excess contributions to the enthalpy across the so lid solution between hydr ated and dehydrated analcime. Therefore, it is reasonable to a ssume that the integral molar enthalpy of hydration ( Hhyd) should equal hhyd at the same temperature. In general, present values of Hhyd within error increase with increasing temperature below 463 K and the difference between the values of 403 K and 463 K is relatively small (Table 2-1, Fig. 25). The result of Hhyd at 298 K interpreted from present data is in good agreement with previous studies. Temperature Dependence of Heat of Hydration The enthalpies of hydration in analcime shown in Fig. 2-5 as a function of temperature can be divided into two re gions. At temperatur es below ~470 K, hhyd is

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23 relatively insensitive to the temperature. In this range of temperature hhyd generally increases with increasing temperature. At temperatures above ~470 K, the abnormal value of hhyd at 528 K may indicate that a dramatic change occurs to hhyd. It decreases abruptly at ~470 K and continues to drop rapi dly with increasing temperature. At even higher temperatures hhyd starts to increase again with increasing temperature. The temperature dependence of hhyd can be used to assess the magnitude of Cp,r via the relationship p hyd r pT h C ) (, (2-3) The measurement of Cp for hydrated analcime in this study is greatly affected by the contribution from dehydration above ~350 K, whereas this effect is smaller in the drop calorimetric method according to the da ta of Johnson et al (1982). As for the dehydrated analcime, our samples were run und er a completely dry condition, so our Cp data for dehydrated analcime is reliable. Considering the similar compositions of the analcime samples used in the experiments of Johnson et al. (1982) and this study, we combine the Cp data of hydrated analcime from J ohnson et al. (1982) and our data of dehydrated analcime to estimate the Cp,r for analcime hydration. Equilibrium between dehydrated and hydrat ed analcime can be represented by a hydration of the form (NaAl)0.97Si2.03O6 + 1.015 H2Ovapor = (NaAl)0.97Si2.03O6.015H2O (2-4) Thus, heat capacity of hydration can be calculated by the following equation Cp,r = (Cp,ana Cp,deana 1.015Cp,H2O ) / 1.015 (2-5) where Cp,ana and Cp,deana are the heat capacities of hydr ated and dehydrated analcime, respectively; Cp,H2O is the heat capacity of water vapor taken from Robie and Hemingway

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24 (1995). The results of Cp,r derived from our data and the combined data are both plotted in Fig. 2-8 as a function of temperature. Note that both curves have a relatively flat part in the relatively low temperature range (e.g., 298-350 K for results of our data and 351430 K for results of combined data), in whic h dehydration effect is much smaller than that above 430 K to the Cp measurement. In this region, Cp,r is insensitive to temperature and the two results have simila r magnitudes, 12-15 J/molK (about 1.5R-2R, R is the gas constant, equals 8.314 J/molK). Th is is quite similar to the value obtained by linear regression of hhyd below 470 K, which indiates a value of Cp,r of 17.1 12.4 J/molK (Fig. 2-9). Above 470 K, the decrease and then increase in hhyd with increasing temperature indicates that Cp,r becomes negative and then positive again. This is indicative of a phase transition in hydrated analcime. This behavior is not apparent in Cp,r calculated from Cp data, as the transition is masked by the effects of dehydration that lead to erroneously large values of Cp for hydrated analcime. Behavior of Heat Capacity of Hydration Heat capacities of hydrated zeo lites are difficult to measure, especially at relatively high temperatures. In the absence of the reliabl e data for the hydrations of zeolites, Barrer (1978) discussed a statistical-mechanical m odel for heat capacity of molecular gases absorbed in zeolites to simplify the calculation of Cp,r. This model argues that the sorption of H2O into a zeolite should lead to an increase in Cp for this component because of the loss of some translational and rotati onal degrees of freedom to week vibrational modes within the zeolite structure (Carey, 1993). The difference in Cp between absorbed H2O and gas phase can be determined by the nu mber of vibrational modes gained by the absorbed species. With this model, the heat capacity of the absorbed species increases by

PAGE 35

25 R/2 for each saturated vibrational mode gaine d, and the heat capacity difference between absorbed and free H2O is limited between zero and 3R (R is the gas constant, 8.314 J/molK) (Bish and Carey, 2001). Based on th e statistical-mechanical model, an assumption can be made that Cp,r of hydration in zeolites is independent of temperature and the value is n*R/2 (0 n 6). In the case of analcime, the Cp,r calculated from combined data is almost constant in the lo w temperature range, for instance, 350-430 K, in which the dehydration effect can be ignore d. That means the heat capacity of hydration in analcime is independent of temperature and the magnitude is approximately 1.5R-2R. Thus, it appears that the statis tical-mechanical model is valid in this temperature range. The enthalpies of hydration at five differe nt temperatures between 373 and 463 K show an almost linear relationship w ith temperature, indicating that Cp,r in this region is generally independent of temperatur e. In addition, the magnitude of Cp,r determined by this approach is approximately 2R, whic h agrees with that of the calorimetric Cp,r within error. Enthalpies of hydration measured above 463 K show clear evidence of a phase transition in analcime. Although the limited resolution afforded by the hhyd measurements precludes a rigorous assessment of the nature of this transition, the general shape implied by the results in Fig. 2-5 indi cates that this transition is probably a -type, second order transition. This kind of phase transition has previously been found in wairakite (Neuhoff and Keddy, 2004) at slig htly lower temperatures, in which Cp,r becomes negative after the transition (dehydr ation precludes assessm ent of the behavior at higher temperatures). It a ppears that almost identical be havior occurs in analcime. Neither the calorimetric results nor the sta tistical-mechanical model predict this phase

PAGE 36

26 transition, underscoring the need for measuremen ts such as those conducted in this study. If more data of hhyd are available, an empirical equation for Cp,r may be extrapolated by the first derivative of hhyd.

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27 Table 2-1. Isothermal immersion calorimetric data for analcime. T (K) Sample mass (mg) Duration1 (min) % H2O uptake2 hhyd1 3 (kJ/mol) hhyd2 4 (kJ/mol) hhyd,ave 5 (kJ/mol) Error (kJ/mol) 403 22.93 73 1.40 -86.29 -86.45 -85.69 1.02 403 25.67 97 1.60 -85.61 -85.15 403 29.60 95 1.62 -85.39 -85.27 417 25.67 82 1.80 -86.07 -85.91 417 27.46 83 1.94 -85.44 -85.19 417 29.13 110 2.00 -83.93 -84.20 -85.12 1.23 432 22.93 110 2.40 -85.35 -84.94 432 25.67 100 2.38 -85.14 -84.80 432 29.13 100 2.59 -84.77 -84.87 -84.98 0.88 446 22.91 150 3.14 -85.55 -84.77 446 22.93 69 2.25 -84.70 -83.62 446 29.14 74 2.35 -86.93 -86.46 446 29.13 115 2.63 -85.71 -85.61 -85.42 1.35 463 22.93 79 2.76 -84.75 -84.12 463 29.14 84 2.81 -84.85 -84.47 463 29.13 100 2.83 -86.30 -85.78 463 29.60 108 3.08 -86.26 -85.81 -85.29 1.20 470 25.67 110 2.98 -86.51 -85.99 470 27.76 260 3.70 -83.95 -83.75 -85.05 1.64 490 27.76 120 3.29 -85.93 -86.18 490 29.14 110 3.03 -87.91 -88.00 490 29.13 67 2.64 -87.87 -88.21 -87.35 1.34 499 29.14 100 2.92 -87.73 -87.11 499 30.99 110 3.02 -87.7 -87.15 -87.42 0.94 528 29.13 105 2.68 -89.16 -89.07 528 29.14 110 2.49 -89.64 -88.82 -89.17 0.96 548 25.02 116 2.05 -88.68 -87.70 548 29.13 130 2.05 -88.08 -87.24 548 33.06 120 1.97 -87.97 -87.26 -87.82 1.03 1Duration of immersion portion of experiment used in data regresion. 2Mass of the H2O (in percentage) absorbed. 3Partial molar enthalpy of hydrat ion in analcime calculated by DSC and dTGA. 4Partial molar enthalpy of hydrati on in analcime calculated from cumulative, baseline-corrected DSC and mass of absorbed H2O. 5Average partial molar enthalpy of hydration in analcime, calculated by averaging values of hhyd1 and hhyd2 at the same temperature.

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28 Table 2-2. Heat capacities of hydrated and de hydrated analcime, steam, and hydration of analcime at differe nt temperatures. T (K) Cp, an 1 (J/molK) Cp, an 2 (J/molK) Cp, dean 1 (J/molK) Cp, H2O 3 (J/molK) Cp, r 4 (J/mol-H2O/K) Cp, r 5 (J/mol-H2O/K) 298 210.44 162.32 33.59 13.81 300 211.01 162.95 33.59 13.77 310 213.54 165.92 33.58 13.33 320 216.74 168.47 33.59 13.97 330 219.42 171.25 33.61 13.85 340 222.37 173.70 33.64 14.31 350 225.43 223.11 175.96 33.69 15.05 12.77 360 228.29 225.25 178.56 33.74 15.25 12.26 370 231.09 227.67 180.98 33.80 15.56 12.20 380 234.20 230.13 183.12 33.88 16.45 12.45 390 237.62 232.63 185.45 33.95 17.45 12.53 400 241.94 235.16 187.77 34.04 19.33 12.64 410 246.84 237.71 190.30 34.13 21.58 12.59 420 252.23 240.30 192.94 34.22 24.19 12.43 430 258.35 242.90 194.82 34.32 28.27 13.05 440 264.89 245.53 196.51 34.42 32.96 13.88 450 272.64 248.18 198.58 34.53 38.44 14.34 460 281.53 250.85 200.60 34.63 45.10 14.88 470 291.85 253.54 202.34 34.75 53.44 15.69 480 304.33 256.24 204.54 34.86 63.46 16.08 490 318.08 258.95 207.08 34.97 74.39 16.13 500 332.91 261.68 208.75 35.09 87.23 17.05 510 350.21 264.42 210.21 35.21 102.72 18.20 520 368.18 267.17 211.76 35.33 118.77 19.26 530 386.85 269.93 213.41 35.46 135.41 20.23 540 880.35 272.70 215.72 35.58 619.22 20.56 550 429.12 275.48 217.98 35.71 172.32 20.95 560 452.40 278.27 220.04 35.83 193.09 21.53 570 478.75 281.06 222.03 35.96 216.97 22.21 580 509.19 283.87 223.13 36.09 245.75 23.75 590 547.49 286.67 225.03 36.21 281.49 24.52 600 594.89 289.49 225.81 36.34 327.29 26.40 610 656.84 292.31 227.57 36.47 386.46 27.31 620 733.15 295.14 228.74 36.60 460.35 28.81 630 811.90 297.97 229.89 36.73 536.68 30.34 640 880.35 300.81 230.09 36.87 603.78 32.80 1Cp, an and Cp, dean are heat capacities of hydrated a nd dehydrated analcime collected in this study. 2Heat capacity of hydrated analci me from Johnson et al (1982). 3Heat capacity of H2O(g) from Robie and Hemingway (1995). 4Heat capacity of hydration in analcime calculated by Cp, an 1 and Cp, dean 1. 5Heat capacity of hydration in analcime calculated by Cp, an 2 and Cp, dean 1.

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29 Table 2-3. Enthalpy of hydration in analcime. Composition Method1 Temperature (K) Hhyd (kJ/mol) (NaAl)0.96Si2.04O6H2O HF 298.15 -84.9 4.02 (NaAl)0.95Si2.05O6H2O TTD 298.15 -85.7 1.93 (NaAl)0.96Si2.04O6H2O HF 298.15 -73.9 4.44 NaAlSi2O6H2O PE 298.15 -80.45 Not reported PE 569.15 -83.7 4.06 Not reported PE 660.15 -86.6 4.06 1Methods: HF: determination of enthalpies of formation of hydrated and dehydrated homologs by HF solution calorimetry; TTD: transposed temperature drop calorimetry; PE: retrieval from phase equilibrium observations. 2Johnson et al. (1982) 3Ogorodova et al. (1996) 4Calculated from data of Barany (1962) as recal culated by Johnson et al. (1982) 5Retrieved by Bish and Carey (2001) from observations of Balgord and Roy (1973) 6van Reeuwijk (1974)

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30 Figure 2-1. Crystal structure of analcime viewed down the b crystallographic axis (after Mazzi and Galli, 1978). The framework of Siand Alcentered tetrahedra are surrounded by four oxygens. The big s pheres denote the positions of Na+ ions. The small spheres are the positions of water molecules.

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31 Figure 2-2. Example isothermal immersi on experiment on analcime at 403K. Simultaneously-recorded TGA and DSC signa ls for analcime as a function of temperature. The first derivative of the TG curve is given by the curve labeled dTGA. Region of the gray box denotes initial equilibration of sample at experimental temperature under dry N2. The rest of the experiment was conducted in the presence of a flow of humidified N2.

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32 R2 = 0.9941 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.0000.0050.0100.0150.020dTGA (mg/min)DSC (mw)hhyd = -85.39 kJ/mol Figure 2-3. Plot of DSC versus dTGA for the e xperimental results shown in Fig. 2-2. The slope of the linear regression show n in the figure is proportional to hhyd.

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33 R2 = 1 -2.5 -2 -1.5 -1 -0.5 0 0.000.100.200.300.400.50H2O absorbed (mg)Heat flow (mw)hhyd = -85.27 kJ/mol Figure 2-4. Heat evolved during absorption of water into analcime as a function of mass absorbed. The slope of the linear equation can be transformed to hhyd.

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34 Figure 2-5. Enthalpy of hydration in analcime as a function of temperature. The value of hhyd at 528 K is abnormal, may imply a trend that hhyd decreases and then increases rapidly with increasing temper ature, indicating a phase transition. -92.0 -90.0 -88.0 -86.0 -84.0 -82.0 -80.0 -78.0 280330380430480530580T (K) hhyd (kJ/mol) C p, r 2 R Johnson et al. (1982) Ogorodova et al. (1996) van Reeuwijk (1974)

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35 Figure 2-6. The mass change (a) and DSC response (b) of hydrated and dehydrated analcime collected at a scanning rate of 15 K/min under ultrapure N2.

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36 Figure 2-7. Experimental Cp data as a function of te mperature for hydrated and dehydrated analcime.

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37 0 8.314 16.628 24.942 33.256 41.57 49.884 290340390440490540590640T (K) Cp,r (J/molK)0.0R 1.0R 2.0R 3.0R 4.0R 5.0R 6.0R Figure 2-8. The heat capacity of hydration in analcime as a function of temperature. The dotted line depicts the results of Cp,r derived from data collected in this study, and the solid line represents the Cp,r calculated by Cp data of hydrated analcime from Johnson et al. (1982) and Cp data of dehydrated analcime from this study. R is the gas constant.

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38 Figure 2-9. Partial molar enthalpies of hydration in analcime vs. temperature. The slope of the linear regression provides a magnitude for Cp,r.

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39 CHAPTER 3 EXCESS HEAT CAPACITY IN NATROLITE HYDRATION Introduction Natrolite occurs in nature as an esse ntially stoichiometric mineral (composition Na2Al2Si3O10H2O; Gottardi and Galli, 1985) and has one of the best-defined crystal structures of any zeolite, especially with respect to H2O (in fact, it was the first zeolite structure refined; Pauling, 1930; Peacor, 1973; Artioli et al ., 1984; Gottardi and Galli, 1985; Joswig and Baur, 1995). The framework of natrolite is composed of Siand Alcentered tetrahedra. The arrangem ent of Si and Al in the tetr ahedral sites is variable but tends to be largely ordered (e.g., Alberti et al., 1995; Neuhoff et al., 2002). Channels within the structure contain two Na+ ions and two H2O molecules per ten framework oxygens oriented in zigzag chains (Meier, 1960; Alberti and Vezzalini, 1981) (Fig. 3-1; Peacor, 1973). Each Na+ ion is six coordinated to f our framework oxygens and two H2O molecules (Line and Kearley, 1998). Natrolite occurs in a wide range of ge ologic environments, including soils (e.g., Ming and Allen, 2001), zeolite facies metaba sites (Walker, 1960), and pegmatites around alkaline intrusions (e.g., Ande rsen et al., 1990). Despite the widespread occurrence of natrolite, its stability in ge ologic environments is poorly known. Previous studies mostly focused on the crystal structure (e.g., Peacor, 1973; Alberti et al., 1982; Joswig and Baur, 1995; Sapiga and Sergeev, 2001) and order/diso rder (Si, Al) dist ribution (e.g., Alberti and Vezzalini, 1981; Hesse, 1983; Andersen et al., 1990; Ross et al., 1992; Neuhoff et al., 2002) of natrolite. Only a few works have explicitly consider ed the thermodynamic

PAGE 50

40 stability of natrolite (e.g., Johnson et al., 1983; Vucelic and Vucelic, 1985; Paukov et al., 2002; Neuhoff and Keddy, 2004). Because natro lite occurs over a wide range of temperature and pressure, temperature (and pressure) dependent processes such as disordering and dehydration are important conc erns in assessing the stability of this mineral. This is especially true because dehydr ation of natrolite leads to a large decrease in unit cell volume and a different space group (Peacor, 1973; Alberti and Vezzalini, 1983; Joswig and Baur, 1995; Baur and Joswi g, 1996). After dehydration is complete, the rotation of the chains of Si and Al terahedr a occurs simultaneously with the contraction of the lattice. Although natrolit e can be completely rehydrated at low temperature, this process is complicated by the presence of hysteresis during cycled dehydration (heating) and rehydration (cooling). Unlike many zeolite s, for which dehydrat ion and rehydration are completely reversible, rehydration in natrolit e, occurs at lower temperatures than does dehydration at a given chemical poten tial of water (van Reeuwijk, 1974). The present study provides new insight into the thermodynamics of the dehydration process in natrolite. Heats of hydration as a function of temperature and the heat capacity of reaction are determined independently by differential scanning calorimetric (DSC) methods. Comparison of the results of thes e measurements reveals the existence of excess heat capacities and enthal pies of hydration, which in la rge part explain the cause of hysteresis in the dehydrati on behavior of this mineral. Methods Sample and Characterization The sample of natrolite was previously described and characterized by Neuhoff et al. (2002; sample NAT001). It was collected as veins within a me tabasaltic tectonic inclusion at the famous Dallas Gem Mine be nitoite and neptunite locality, San Benito

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41 County, California. Phase pure separates were hand picked, ground in an agate mortar, and sieved to a 20-40 m size fraction. Sample identifica tion and purity were confirmed by X-ray powder diffraction. The composition was determined by electron probe microanalysis at Stanford University to be essentially stoichiometric (Na2Al2Si3O10 n H2O). Water content of the sample was determined in this study by thermogravimetric heating to 1023 K after th e equilibration with a room temperature atmosphere of 50 % relative humidity (RH). Th e mass loss is about 9.49% of total sample mass, very close to the ideal water content of natrolite (9.48%), and the water content taken to be 2 moles of water per formula unit. Heat Capacity Measurements The heat capacities of hydrated and dehydr ated natrolite were determined by DSC with the Netzsch STA 449C Jupiter simulta neous DSC-thermogravimetric analysis (TGA) system at the University of Florida. Each experiment consisted of four separate runs: 1) determination of background and base line by measurement of an empty crucible; 2) DSC measurement of a sta ndard (sapphire); 3) scanning heating of hydrated natrolite and 4) scanning heating of dehydrated natro lite. A sample of approximately 27 mg was packed into a covered Pt-Rh cr ucible before step 3 and rema ined until completion of the experiment. Data were collected in the range of 298 to 733 K (higher temperatures cause an irreversible structure change for natrolite; Joswig and Ba ur, 1995) at a scanning rate of 15 K/min. A purge of dry N2 was used during the experiment to keep the relative humidity below 1%, and the gas flow was ma intained at ~30 ml/min using mass flow controllers. The furnace was heated to 733 K in each run and then cooled back to room temperature by liquid N2 for next step. During heating, hyd rated natrolite lost mass with increasing temperature and finally becam e completely dehydrated. The resulting

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42 dehydrated natrolite was kept in an environment of dry N2 to avoid rehydration during cooling to room temperature a nd then measured by scanning heating again. Heat capacity of the sample as a function of temperature was calculated by std p b std b s s std pC DSC DSC DSC DSC m m C, (3-1) where ms is the mass of sample, mstd is the mass of standard (27.314 mg), DSCs, DSCstd and DSCb are for sample, standard and baseline respectively. Triplicate calorimetric measurements were conducted and averaged, and the error of the result is within 1% of the average value. Absorption Calorimetry Heats of hydration as a function of te mperature were determined using an isothermal DSC-based immersion techni que described by Neuhoff and Wang (2006). This approach combines the benefits of DS C and gas absorption calorimetry. Twenty to 30 mg of hydrated natrolite was placed into the Pt-Rh crucible for each run. The sample was dehydrated by scanning heating from 298 to 733 K at the rate of 15 K/min and then allowed to cool to the experiment al temperature. A purge of dry N2 was maintained at flow rate of 50 ml/min during this period. After equilibration (20-40 min) under dry gas at this temperature until both DSC and TGA baselines stabilized, the gas stream was changed to humid N2 which was generated by bubbling N2 through saturated NaCl solution. In order to reduce th e change of DSC baseline, the flow rate of humid N2 was maintained at 30 ml/min and the corresponding water vapor pressure in the furnace was ~12 mbar. Under this condition the sample was allowed to react until the DSC and TGA baselines stabilized again. Experiments were re peated on the same sample but indicated a

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43 progressive loss of hydration capacity and decrease in hydration heat. Consequently, a fresh aliquot of sample was used for each experiment. Results Heat Capacities of Hydrated and Dehydrated Natrolite The TGA and DSC traces of hydrated a nd dehydrated natrolite obtained during scanning heating are shown in Fig. 3-2. It can be seen that natrolite starts dehydrating at a relatively high temperature, ~450 K, and dehydr ation occurs over a fa irly short range of temperature (450-673 K). The TGA trace also suggests that there is only one energetically distinct water site in natrolite because the curve is continuous and does not show any inflections. In addition, only one peak is observed in the DSC and first derivative of TGA (dTGA) signals, consistent with this interpretation. The TGA curve of dehydrated natrolite shows no mass change during the scanning heating, which also indicates that the sample had been fully de hydrated in the first r un and the condition for the experiment was dry enough to a void rehydration of the sample. Heat capacities of hydrated and dehydrated natrolite calculated from the DSC in Fig. 3-2b are shown in Table 3-1. Figure 3-3 depicts the Cp data of both hydrated and dehydrated natrolite determined by DSC data from the same experiment from 320 to 680 K. The heat capacity of dehydrat ed natrolite is relatively in sensitive to the temperature. Calculated Cp for hydrated natrolite exhibits a strong positive inflection at ~450 K reflecting excess heat associated with dehydration of the sample. Enthalpy of Hydration in Natrolite An example of TGA and DSC response of natrolite during immersion in water vapor at 412 K is shown in Fig. 3-4. Note th at after the atmosphere changed, but before the onset of rehydration, the DSC baseline does not change. The marked changes in both

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44 TGA and DSC signal shortly after the atmos phere changed reflect absorption of water vapor by the sample. The mass of the sample in creases as it absorbs more water, and the absorption rate is essentially constant for most of the reaction. Rehydr ation of natrolite is relatively rapid, with the w hole process only taking about 110 minutes. Once hydration is complete, the baseline of TGA and DSC becomes stable again. The heat flow of the natrolite hydrati on is proportional to the area under the DSC curve, which allows a calculation of the enthalpy of hydration ( Hhyd) by the equation: Hhyd = -18.015 A / k mgain (3-2) where A is the area under the DSC curve, mgain is the mass gain in the rehydration and k is the caloric calibration f actor. The results of Hhyd for natrolite at four different temperatures are listed in Table 3-2 and shown in Fig. 3-5. The values of Hhyd at the same temperature are very close (<0.5% diffe rence), and the errors include the part from standard deviation of the values and that from the calorimetric calibration (1% of the results), which have been investigated to be within 1.5%. It can be seen in Table 3-2 that the de gree of rehydration mainly depends on the temperature. The masses of water absorb ed during the rehydration at the same temperature are similar, except one at 412 K with a mass gain of about 9.34%, which may result from an abnormally higher water vapor pressure. Rehydrat ions of natrolite were also attempted at higher temperatur es. Generally, the degree of rehydration increases with decreasing temperature below ~432 K, even though the difference between the mass gains during the rehydration at differe nt temperatures is small (with only about 0.5% difference from 323 to 432 K). However, at temperatures above ~432 K the degree of rehydration decreases rapidly with incr easing temperature. Th e fraction of water

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45 absorbed during the rehydrati on drops by about 1.4% from 432 to 482 K. The sample of dehydrated natrolite can not rehydrate when the temperature reaches 522K (the fraction of water aborbed <0.1%). This behavior is also manifested by the mass change of natrolite in the scanning hea ting (with a heating/cooling ra te of 5 K/min) dehydrationrehydration experiment (Fig. 3-6). The whol e process is run under a humid condition with a constant water vapor pressure (~12 mbar). Note that the rehydration does not commence until the temperature decreases to ab out 513 K, similar to the results of the isothermal immersion experiments in whic h natrolite could not rehydrate above 522 K. Discussion Comparison with Previous Results Johnson et al. (1983) have measured the Cp of natrolite from 5 to 350 K by adiabatic calorimetric method, and Dr ebushchak (1990) determined the Cp of dehydrated natrolite by DSC from room te mperature to 800 K. In the present study we measured the Cp for both hydrated and dehydrated natrolit e from 143 to 703 K, which covers some temperature ranges in the previous studies. The data presented here for hydrated and dehydrated natrolite are the aver age values of the results of three measurements starting from 143 K. These data are compared to prev iously published result s in Fig. 3-7. It can be seen that the results of pr esent study agree well with th ose of Johnson et al. (1983) and Drebushchak (1990). For instance, at 298.15 K Cp of hydrated natrolit e measured in this study differs within 0.5% of th at determined by Johnson et al. (1983), and the difference between the Cp of dehydrated natrolite in this stud y and that of Drebushchak (1990) is about 0.7%. However, Cp data for dehydrated natrolite in the present study exhibits a small peak near 500 K, indicating a small, reversible phase transition, th at is not present in the data of Drebushchak (1990).

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46 The various determinations of Hhyd from the literature are listed in Table 3-2, and no correlations between Hhyd and temperature can be observed from these values. Compared to these values, the result of Hhyd at 451 K in this study agrees within error with that of Guliev et al. (1989) determined by immersion calorimetry at a similar temperature. In a ddition, the value of Hhyd in this study increases with increasing temperature, showing that the hydration of natrolite becomes less energetic at higher temperatures, which is consistent with thermodynamic theory. According to the theory and the similar difference between the vaules of Hhyd in this study, the Hhyd at 298.15 K is interpreted to be similar to the one of Kiseleva et al. (1997) determined by transposed temperature drop calor imetry. The other values of Hhyd for natrolite reported in the literature were determined at signifi cantly different temperat ures; however, all of the data are similar in magnitude (and mostly within error) of the re sults from this study. Heat Capacity of Hydration in Natrolite Hydration of natrolite can be represented by the reaction Na2Al2Si3O10 + 2 H2Ovapor = Na2Al2Si3O10H2O (3-3) dehydrated natrolite natroltie for which the standard heat capacity change ( Cp,r) is given by Cp,r = Cp,nat Cp,denat Cp,steam (3-4) where Cp,nat and Cp,denat are the heat capacities of hydr ated and dehydrated natrolite, respectively; Cp,steam is the heat capacity of water vapor. The results of present study provide two independent means of assessing the magnitude of Cp,r. The first is by direct calculation from measured Cp for hydrated and dehydrated natrolite along with Cp,steam (Robie and Hemingway, 1995). The second is by no ting that the heat ca pacity is defined as the differential change in heat with respect to temperature

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47 p hyd r p,) T H ( C (3-5) thus the first derivative of calculated values of Hhyd with respect to temperature also provides a means of assessing Cp,r. Values of Cp,r consistent with the Cp results listed in Table 3-1 and shown in Fig. 3-3 are given in Table 3-1 and illustra ted in Fig. 3-8. It can be seen that Cp,r increases steadily from value of ~4 J/mol-H2O/K at 298.15 K to ~16.2 J/mol-H2O/K at 360 K. Above 360 K until the sample starts to dehydrate at about 420 K, Cp,r is relatively constant. The temperature dependence of Cp,r between room temperature and 360 K is indicative of a phase transition occurring in this region. Similar tr ansitions have been noted in water molecules in synthetic zeo lites A and X (Vucelic and Vucelic, 1985; Basler and Lechert, 1972) in which water mo lecules appear to change their state of motion over a protracted range of temperature, similar to a gl ass transition. This behavior is in contrast to the relatively sharp, -type transitions observed in water molecules in laumontite (Neuhoff and Bird, 2001) and wairakite (Neuhoff and Keddy, 2004). The relative temperature insensitivity of Cp,r above 360 K is consistent with statistical-mechanical model of the behavior of confined water molecules. Carey (1993), following Barrer (1978) used st atistical-mechanical reasoni ng to suggest that the difference in Cp between absorbed H2O and the gas phase is caused by the loss of translational and rotational degrees of free dom to weak vibrati onal modes within the zeolite structure. Thus, the heat capacity of hydration in zeolites re flects the transition between translational/rotational and vibr ational modes. With this model, the Cp,r increases by 0.5 R for every vibrational mode gained by the absorbed species, up to a

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48 maximum of 3R (Bish and Carey, 2001). The he at capacity of hydration for natrolite in our study is relatively insensitive to the temperature between 373 and 403 K, and the value is almost constant, ~16.24 J/mol-H2O/K. According to the statistical-mechanical model, it is reasonable to suppose that the Cp,r of hydration in natr olite is about 2R. In contrast, the temperature dependence of Hhyd data in Table 3-2 and Fig. 3-5 indicate a significantly higher value of Cp,r. Linear regression of these data indicates an average value of Cp,r = 68.0 22.7 J/mol-H2O/K. This value is nearly four times the value determined by direct measurement of Cp for the phases in the reaction. Although the limited range of temperature and number of data points preclude ri gorous analysis of the temperature dependence of Cp,r, the essentially linear trend of these data suggests that this property is relatively temperat ure-invariant over this temperature range. The discrepancy between Cp,r determined directly from Cp measurements of individual phases and that assessed via equation 3-5 indicat es that there is an excess contribution to Cp of mixing in this region. Such a contribu tion would not be detected in direct Cp measurements (and for reasons cited below can not be assessed for partially hydrated natrolite), further underscoring the need for direct measurem ents of heat capacities and enthalpies of hydration in zeoli tes. The presence of excess Cp of mixing in the solid solution between natrolite and dehydrated natr olite indicates that there must also be excess properties in the integral properties of Cp of mixing, namely the Gibbs energy, enthalpy, and entropy of mixing. Nature of the Natrolite-Dehydrated Natrolite Solid Solution The behavior of natrolite observed in th is study suggests that the solid solution between its hydrated and dehydrated forms beha ves in a fundamentally different fashion from that often observed in other zeolites. In many zeolites, dehydration is fully

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49 reversible, with similar water content observe d under identical conditions of temperature, pressure, and the chemical potential of H2O during both dehydration and rehydration (e.g., Balgord and Roy, 1973; Carey and Bish, 1996; Fialips et al., 2005). In natrolite, however, it appears that significant hysteres is is observed between water contents achieved during hydration and dehydration. We suggest that this phenomenon is the result of solvus behavior of natrolite in th is solid solution, which is consistent with the behavior of natrolite during re hydration as well as the ener getics of natrolite hydration discussed above. As shown in Fig. 3-6 that a strong hysteres is effect occurs after the dehydration of natrolite, dehydrated natrolit e could not follow the dehydrat ion path and the rehydration is initiated not until about 130 K below the end-point of dehydration. The strong hysteresis is partly due to th e high heating/cooling rate (5 K/min), for instance, with a heating/cooling rate of 2 K/min the rehydrati on begins about 50 K below the end-point of dehydration. Neimark et al. (2000) modeled hysteresis in water ab sorption in nanopores as a solvus, and found that the hysteresis was related to the pore st ructure characterization of the material. However, even when the de hydrated natrolite starts absorbing water at about 513 K, the trace of mass change is still quite different from th at of dehydration. The rehydration of natrolite should have reached equilibrium below 513 K as indicated by the isothermal immersion experiments. That m eans the hysteresis is probably present at equilibrium, and this is also supported by the evidence that limited degree of rehydration is possible above 473 K. Similar hysteretic phe nomena were observed in the W1 site in laumontite (Fridriksson et al., 2003) and smec tite (Tamura et al., 2000) that are clearly associated with the presence of two coexisting phases.

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50 The rate of hydration in natr olite also shows a different feature from many other zeolites. The hydration rate of most zeolit es depends on the degree of hydration (the concentration of reactant; e.g., analcime, Fig. 2-2). However, in natrolite the TGA signal increases monotonically during hydration until leveling off after reaction was complete (Neuhoff and Wang, 2006). Thus, th e rate of hydration in natrolite is independent of the hydration degree, and the hydrat ion of natrolite is a zero order reaction. This can be explained by the solvus beha vior of natrolite. During th e hydration of natrolite, two immiscible solutions (hydrated and dehydrated na trolite) coexist as a mechanical mixture, so the concentration of dehydrated natrolite remains constant during the whole process. The phenomena of hysteresis and zero order kinetics are not observed in many zeolites, such as chabazite (van Reeuwijk, 1974), mordenite, analcime and K-clinoptilolite (Bish and Carey, 2001), etc. Consequently, we belie ve that the special characters of hydration in natrolite (like excess heat capacity, hysteresi s effect and zero order kinetics) are mainly attributed to the solvus behavior of natrolite in solid solution.

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51 Table 3-1. Heat capacities of hydrated and de hydrated natrolite, st eam, and hydration of natrolite at different temperatures. T (K) Cp, an (J/molK) Cp, dean (J/molK) Cp, H2O (J/molK) Cp, r (J/mol-H2O/K) 298.15 360.37 285.30 33.59 3.94 300 362.00 286.29 33.59 4.27 310 370.62 290.89 33.58 6.28 320 379.48 295.25 33.59 8.53 330 388.27 299.33 33.61 10.87 340 396.55 303.40 33.64 12.93 350 403.94 307.80 33.69 14.38 360 410.16 312.17 33.74 15.26 370 415.54 316.43 33.80 15.75 380 420.39 320.53 33.88 16.05 390 424.95 324.59 33.95 16.23 400 429.74 328.34 34.04 16.67 410 434.48 332.18 34.13 17.03 420 439.87 336.13 34.22 17.65 430 446.04 340.06 34.32 18.67 440 453.23 344.13 34.42 20.13

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52 Table 3-2. Isothermal immersion cal orimetric data for natrolite. T (K) Sample mass (mg) Duration1 (min) Mass gain2 (%) Hhyd (kJ/molH2O) Hhyd, ave 3 (kJ/molH2O) Error (kJ/molH2O) 412 23.97 121 9.18 -97.16 -97.37 0.99 412 25.41 100 9.17 -97.50 412 25.48 95 9.34 -97.44 432 24.4 87 9.18 -95.74 432 24.24 83 9.24 -95.49 432 29.76 117 9.21 -95.84 -95.69 0.97 451 29.44 127 9.17 -94.70 451 23.6 114 9.15 -94.55 451 27.56 109 9.14 -94.77 -94.67 0.95 472 29.34 130 8.96 -93.21 472 27.45 126 8.93 -93.21 472 25.73 118 8.94 -93.16 -93.19 0.93 1Duration of immersion portion of experiment used in data regression. 2Mass of the H2O (in percentage) absorbed in that period. 3The average value of Hhyd, as the integral enthalpy of hydration in natrolite.

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53 Table 3-3. Enthalpy of h ydration in natrolite. Composition Method1 Temperature (K) Hhyd (kJ/mol-H2O) (NaAl)2Si2O10H2O TTD 298.15 -101.7 3.62 (NaAl)2Si2O10H2O PE 900 -1083 Not reported PE 684.15 -102.9 4.04 Not reported DSC 623.15 -89.15 (NaAl)2Si2O10H2O IM 453.15 -100.0 5.06 1Methods: TTD: transposed temperature drop calorimetry; PE: retrieval from phase equilibrium observations; DSC: calculated fr om scanning DSC measurement; IM: heat of immersion in water. 2Kiseleva et al. (1997) 3Hey (1932) 4van Reeuwijk (1974) 5van Reeuwijk (1972) 6Guliev et al. (1989)

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54 Figure 3-1. Crystal structur es of natrolite down the c crystallographic ax is. (after Peacor, 1973). The framework of Siand Alcen tered tetrahedra are surrounded by four oxygens. The big spheres denote the positions of Na+ ions. The small spheres are the positions of water molecules.

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55 Figure 3-2. The mass change (a) and DSC response (b) of hydrated and dehydrated collected at a scanning rate of 15 K/min under ultrapure N2.

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56 Figure 3-3. Heat capacities of hydrated and dehydrated natrolite as a function of temperature from 320 to 680 K, which are calculated from the DSC data collected in the same sca nning heating experiment.

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57 Figure 3-4. Example immersion calorimetric expe riment on natrolite conducted at 412 K. Simultaneously-recorded TGA and DSC signa ls for natrolite as a function of temperature. The first derivative of the TG curve is given by the curve labeled dTGA. Region of the gray box denotes initial equilibration of sample at experimental temperature under dry N2. The rest of the experiment was conducted in the presence of a flow of humidified N2.

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58 Figure 3-5. The enthalpy of dehydration in na trolite as a function of temperature. The slope of the linear regr ession is the value of Cp,r. -106 -103 -100 -97 -94 -91 290330370410450490T (K) Hhyd (kJ/mol)Kiseleva et al. (1997) Guliev et al. (1989) This study Cp,r = 68.0 22.7 J/mol-H2O/K R2 = 0.9936

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59 88 90 92 94 96 98 100 102 280340400460520580640700T (K)Mass change (%) Figure 3-6. Mass change of natrolite in the dehydration and rehydration with a heating/cooling rate of 5 K/min under a humid condition (PH2O 12 mbar). A significant hysteresis occurs in the rehydration of natrolite.

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60 0 100 200 300 400 500 600 02004006008001000T (K)Cp (J/molK) This study Johnson et al. (1983) Drebushchak (1990)Hydrated natrolite Dehydrated natrolite Figure 3-7. Comparison of present and previous Cp data for both hydrated and dehydrated natrolite. Our data are the average values of Cp from three different measurements starting from 143 K.

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61 Figure 3-8. The heat capacity of hydration in natrolite as a function of temperature.

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62 CHAPTER 4 CONCLUSION Comparison of the Results of Analcime and Natrolite Heat capacities of hydration directly measured by DSC technique for analcime and natrolite behave differently with increasing temperature. However, they both have a temperature range in which Cp,r is relatively insensitive to the temperature, 298 to 430 K for analcime and 370 to 410 K for natrolite. The data in these ranges conform to the statistical-mechanical model which argues that Cp,r is related to the weak vibrational modes for absorbed H2O compared with rotational and translational freedom in the vapor-phase molecule and it is independent of temperature (Carey, 1993). However, enthalpies of hydration in analcime and natr olite determined by an isothermal DSC-based immersion technique at differe nt temperatures illustrate two phenomena that complicate application of the statisti cal mechanical model for Cp,r. In natrolite, a linear relationship exists between Hhyd and temperature, indicating a temperature independent Cp,r. However, the Cp,r for natrolite hydration calculated by this approach is about 68.0 22.7 J/molK, nearly four times the value determined by the model. A temperature independent excess heat capacity is suggested to exist in the hydration of natrolite. This excess heat capacity is probably related to the solvus behavior in the natrolite-dehydrated natrolite solid solution. In analcime, Hhyd between 298 and 470 K is relatively insensitive to the temperature and implies a value of Cp,r of approximately 2R. This result is consistent with that of the calorimetric Cp,r, showing the applicability of statistical-mechanical model for analcime in the relatively low temperature range.

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63 However, the abrupt change of Hhyd above 470 K suggests a phase transition which has a significant effect on Cp,r, and can even make the Cp,r become negative. The thermodynamics of dehydration and re hydration in rock-forming zeolites are different even for the simple-structure materi als like analcime and na trolite. The behavior of Cp,r is more complicated as a function of temp erature than expected by the statisticalmechanical model. That model may be appl icable for certain zeolites under some low temperature conditions (e.g., 298.15 K), but it is probably invalid for the zeolites containing phase transition during dehydr ation and rehydration. The enthalpy of hydration in zeolites determined by the isothe rmal immersion technique is critical for understanding these processes. Heuristic Outcomes of This Study This study generated two unparalleled datase ts on the temperature dependence of the heat of hydration in zeolites that both complement existing data and enhance our understanding of this important geochemical process. The therm odynamic properties of the zeolites studied here provi de a better understanding of the stability and reactivity of these materials in the earths crust. In addition, this study al so provides important data for assessing the reactivity of these minerals and for devel oping new industrial applications for them. For instance, the thermodynamics of ion exchange by zeolites (which are a critical process in radioactive waste disposal wastewater treatment, and soil remediation; e.g., Kallo, 2001; Pabalan and Bertetti, 2001; Ming and Allen, 2001) is largely a function of the change in hydration state accompanying this process. This study also has a direct impact on the study of other hydrate minerals especially hydrated nanomaterials. The methods and insights gained from this study can be used to develop new investigations of

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64 the relationship between hydrat ion/dehydration processes and nanophase stability in other mineral systems. Future Work There are still several problems left unr esolved in this study, for example, the reason that lead to the phase transition in the hydration of analcime, the behavior of Cp,r in the higher/lower range of temperature fo r analcime hydration, th e real cause of the excess heat capacity for natro lite dehydration. More experime nts will be done to get the specific data to address these problems. Future work will also include the thermodynamics of dehydration and rehydrati on on the complex zeolites like chabazite, wairakite etc. Furthermore, some geologic obs ervations of the temp erature and pressure conditions under which zeolites (e.g., analcime, natrolite) form in nature will be done to compare the results determined in the lab with these observations. This will be significant to the development of the techniques in our study.

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65 LIST OF REFERENCES Alberti, A., Cruciani, G. and Daura, I. ( 1995) Order-disorder in natrolite-group minerals. European Journal of Mineralogy 7 501-508. Alberti, A., Pongiluppi, D. and Vezzalini, G. (1982) The crystal-chemistry of natrolite, mesolite and scolecite. Neues jahrbuch Fur Mineralogie-Abhandlungen 143 231248. Alberti, A., and Vezzalini, G. (1981) A part ially disordered natrolite: Relationships between cell parameters and Si-Al distribution. Acta Crystallographica B37 781788. Alberti, A., and Vezzalini, G. (1983) How the structure of natrolite is modified through the heating-induced dehydration. Neues jahrbuch Fur Mi neralogie-Monatshefte 3 135-144. Alberti, A., and Vezzalini, G. (1984) T opological changes in dehydrated zeolites: Breaking of T-O-T bridges. Pp. 834-841 in: Proceedings of the 6th International Conference on Zeolites (A. Bisio and D.H. Olson, ed itors). Butterworths, Guildford, U.K. Andersen, E.K., Andersen, I.G.K. and PlougSor ensen, G., (1990) Disord er in natrolites: structure determinations of three disordered natrolites and one lithium-exchanged disordered natrolite. European Journal of Mineralogy 2 799-807. Armbruster, T. (1993) Dehydfration mechanism of clinoptilolite a nd heulandite-singlecrystal X-ray study of Na-poor, Ca-rich, K -rich, Mg-rich clinop tilolite at 100 K. American Mineralogist 78 260-264. Armbruster, T., and Gunter, M.E. (2001) Crysta l structures of natura l zeolites. Pp. 1-68 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, editors). Reviews in Mi neralogy and Geochemistry 45 Mineralogical Society of America and Geochemical Society. Armbruster, T., and Kohler, T. (1992) Rea nd dehydration of laumontite: a single-crystal X-ray study at 100 K. Neues Jahrbuch Fur Mineralogie Monatshefte 9 385-397. Artioli, G., Smith, J.V. and Kvich, A. (1984) Neutron diffraction study of natrolite, Na2Al2Si3O10H2O, at 20 K. Acta Crystallographica Section C-Crystal Structure Communications 40 1658-1662.

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66 Artioli, G., and Sthl, K. (1993) Fully hydrated laumontite-A structure study by flat-plate and capillary powder diffraction techniques. Zeolites 13 249-255. Baldar, N.A., and Whittig, L.D. (1968) Occu rrence and synthesis of soil zeolites. Soil Science Society of America Proceedings 32 235. Balgord, W.D., and Roy, R. (1973) Crystal chem ical relationships in the analcite family. II. Influence of temperature and PH2O on structure. Molecular Sieves 16 189-199. Banfield, J.F., and Zhang, H. (2001) Nanocry stals in the environment. Pp. 1-59 in: Nanoparticles and the environment (J.F. Banfield and A. Navrotsky, editors). Reviews in Mineralogy and Geochemistry 44 Mineralogical Society of America. Barany, R. (1962) Heats and free energies of formation of some hydrated and anhydrous sodiumand calcium-aluminum silicates. U.S. Bureau of Mines Report of Investigations 5900. Barrer, R.M., and Cram, P.J. (1971) Heats of immersion of outgassed ion-exchanged zeolites. Pp. 105-131 in: Molecular Sieve Zeolites-II Advances in Chemistry Series 102 American Chemical Society, Washington, D.C. Barrer, R.M. (1978) Zeolites and clay minerals as sorbents and molecular sieves Academic Press, London, 497 pp. Basler, W.D., and Lechert, H. (1972) Molar heat measurements on adsorbed water in zeolites linde-13-X. Zeitschrift Fur Physikalische Chemie-Frankfurt 78 199-204. Baur, W.H., and Joswig, W. (1996) The phase s of natrolite occu rring during dehydration and rehydration studied by single crystal X-ray diffraction methods between room temperature and 923 K. Neues Jahrbuch Fur Mineralogie-Monatshefte 4 171-187. Bish, D.L., Vaniman, D.T., Chipera, S.J. a nd Carey, J.W. (2003) The distribution of zeolites and their effect on the performance of a nuclear waste repository at Yucca Mountain, Nevada, U.S.A. American Mineralogist 88 1889-1902. Bish, D.L., and Carey, J.W. (2001) Thermal behavior of natural zeolites. Pp. 403 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, editors). Reviews in Mine ralogy and Geochemistry 45 Mineralogical Society of America and Geochemical Society. Boettinger, J.L., and Graham, R.C. (1995) Ze olite occurrence in soil environments: An updated review. Pp. 23-27 in: Natural Zeolites : Occu rrence, Properties, Use. International Committee on Natural Zeolites, Brockport, New York. Boles, J.R., and Coombs, D.S. (1977) Zeolite fa cies alteration of sandstones in southland Syncline, New Zealand. American Journal of Science 277 982-1012.

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67 Carey, J.W. (1993) The heat capacity of hydrous cordierite above 295 K. Physics and Chemistry of Minerals 19 578-583. Carey, J.W., and Bish, D.L. (1996), Equilibrium in the clinoptilolite-H2O system. American Mineralogist 81 952-962. Carey, J.W., and Bish, D.L. (1997) Calori metric measurement of the enthalpy of hydration of clinoptilolite. Clays and Clay Minerals 45 814-825. Coombs, D.S., Ellis, A.J., Fyfe, W.S. and Ta ylor, A.M. (1959) The zeolite facies, with comments on the interpretation of hydrothermal syntheses. Geochimica et Cosmochimica Acta 17 53-107. Coombs, D.S., and Whetten, J.T. (1967) Com position of analcime from sedimentary and burial metamorphic rocks. Geological Society of American Bulletin 78 269-292. Coughlan, B., and Carroll, W.M. (1976) Water in ion-exchanged L, A, X, and Y zeolites: A heat of immersion and thermogravimetric study. Journal of the Chemical Society, Faraday Transactions I 72 2016-2030. Cronstedt, A.F. (1756) Ron och beskrifning om en obekant brg art, som kallas zeolites. Kongl. Vetenskaps Acad. Handl. Stockholm 17 120-123. Cruciani, G., Martucci, A., a nd Meneghini, C. (2003) Dehydration dynamics of epistilbite by in situ time resolved synhrotron powder diffraction. European Journal of Mineralogy 15 257-266. Drebushchak, V.A. (1990) Calorimetric studi es on dehydrated zeolites: natrolite, heulandite, chabazite, and mordenite. Geochemistry International 5 123-130. Fialips, C.I., Carey, W.J., and Bish, D.L. (2005) Hydration-dehydr ation behavior and thermodynamics of chabazite. Geochimica et Cosmochimica Acta 69 2293-2308. Fridriksson, Th., Neuhoff, P.S. and Bird, D.K. (1999) Ion exchange equilibria between heulandite and geothermal fluids. Geological Society of Am erica Abstracts with Programs 31 A27. Fridriksson, Th., Carey, J.W., Bish, D.L., Ne uhoff, P.S. and Bird, D.K. (2003) Hydrogenbonded water in laumontite II: Experiment al determination of site-specific thermodynamic properties of hydrati on of the W1 and W5 sites. American Mineralogist 7 1060-1072. Galindo Jr.C., Ming, D.W., Morgan, A. and Pick ering, K. (2000) Use of Ca-saturated clinoptilolite for ammonium from NASAs advanced life support wastewater system. Pp. 363-371 in: Natural Zeolites for the Third Millennium (C. Colella and F.A. Mumpton, editors). De Frede Editore, Naples, Italy.

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68 Gopal, R., Hollebone, B.R., Langford, C.H. and Shigeishi, R.A. (1982) The rates of solar energy storage and retrieval in a zeolite-water system. Solar Energy 28 424-424. Gottardi, G. (1989) The genesis of zeolites. European Journal of Mineralogy 1 479-487. Gottardi, G., and Galli, E. (1985) Natural Zeolites Springer-Verlag, Berlin, 409 pp. Grigorieva, L.V., Salata, O.V., Kolesn ikov, V.G. and Malakhova, L.A. (1988) Effectiveness of the sorptive and coagulat ional removal of enteric bacteria and viruses from water. Khim Teknol Vody 10 458-461. Guliev, T.M., Isirikyan, A.A., Mirzai, D.I. and Serpinskii, V.V. (1989) Energy of rehydration of natrol ite and scolecite. Bulletin of the Academy of Sciences of the USSR Division of Chemical Scienc e, 37 1308-1310. Hay, R.L., and Moiola, R.J. (1963) Authigen ic silicate minerals in Searles Lake, California. Sedimenology 2 312-332. Hay, R.L. (1966) Zeolites and zeolitic reactions in sedimentary rocks. Geological Society of America Special Paper 85 130. Hay, R.L., and Sheppard, R.A. (2001) Occurrenc es of zeolites in se dimentary rocks: An overview. Pp. 217-234 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, editors). Re views in Mineralogy and Geochemistry 45 Mineralogical Society of Amer ica and Geochemical Society. Helgeson, H.C., Delany, J.M., Nesbitt, H. W. and Bird, D.K. (1978) Summary and critique of the thermodynamic prope rties of rock-forming minerals. American Journal of Science 278A 1-229. Hesse, K.F. (1983) Refinement of a partially disordered natrolite, Na2Al2Si3O10H2O. Zeitschrift Fur Kristallographite 163 69-74. Hey, M.H. (1932) Studies on the zeolites. Pa rt III. Natrolite and metanatrolite. Mineralogical Magazine 23 243-289. Hhne, G.W.H., Cammenga, H.K. and Eysel, W. (1990) The temperature calibration of scanning calorimeters. Thermochimica Acta 160 1-12. Iijima, A., and harada, K. (1968) Authigenic zeolites in zeolitic palagonite tuff on Oahu, Hawaii. American Mineralogist 54 182-197. Iijima, A. (2001) Zeolites in petr oleum and natural gas reservoirs. Reviews in Mineralogy & Geochemistry 45 347-402. Johnson, G.K., Flotow, H.E., OHare, P.A. G. and Wise, W.S. (1982) Thermodynamic studies of zeolites: analcime and dehydrated analcime. American Mineralogist 67 736-748.

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69 Johnson, G.K., Flotow, H.E., OHare, P.A. G. and Wise, W.S. (1983) Thermodynamic studies of zeolites: Natrolite, msolite, and scolecite. American Mineralogist 68 1134-1145. Joswig, W., and Baur, W.H. (1995) The ex treme collapse of a framework of NAT topology: the crystal structure of metanatr olite (dehydrated natrolite) at 548 K. Neues jahrbuch Fur Mineralogie-Monatshefte 1 26-38. Kallo, D. (2001) Applications of natural zeolit es in water and wastewater treatment. Pp. 519-550 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, editors). Reviews in Mineralogy and Geochemistry 45 Mineralogical Society of America and Geochemical Society. King, E.G. (1955) Low temperature heat cap acity and entropy at 298.16 K of analcite. Journal of the American Chemical Society 77 2192. King, E.G., and Weller, W.W. (1961) Low temperature heat capacity and entropy at 298.15 K of some sodiumand calcium-aluminum silicates. U.S. Bureau of Mines Report of Investigations 5855. Kiseleva, I., Navrotsky, A., Belitsky, I.A. and Fursenko, B.A. (1996) Thermochemistry of natural potassium sodium calcium le onhardite and its cation exchanged forms. American Mineralogist 81 668-675. Kiseleva, I.A., Ogorodova, L.P., Melchakova, L.V., Belitsky, I.A. and Fursenko, B.A. (1997) Thermochemical investigat ion of natural fibrous zeolites. European Journal of Mineralogy 9 327-332. Kristmannsdttir, H., and Tmasson, J. (1978) Zeolite zones in geothermal areas of Iceland. Pp. 277 in: Natural Zeolite Occurrence, Properties and Use (L.B. Sand and F.M. Mumpton, editors). Pergamon Press, Oxford, UK. Line, C.M.B., and Kearley, G.J. (1998) The libra tional and vibrational spectra of water in natrolite, Na2Al2Si3O10H2O compared with abinitio calculations. Chemical Physics 234 207. Mazzi, F., and Galli, E. (1978) Is each analcime different. American Mineralogist 63 448-460. McCulloh T.H., Cashman S.H. and Stewar t R.J. (1973) Diagnetic baselines for interpretive reconstructions of maximum burial depths and paleotemperatures in clastic sedimentary rocks. Pp. 18-46 in: A Symposium in Geochemistry: Low Temperature Metamorphism of Kerogen and Clay Minerals Pacific Sect SEPM, Los Angeles. McCulloh T.H., and Stewart R.J. (1979) S ubsurface laumontite crystallization and porosity destruction in Neogene sedimentary basins. Geological Society of America Abstracts with Programs 11 475.

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70 Meier, W.M. (1960) The crystal structure of natrolite. Z Kristallogr 113 430-444. Meier, W.M., Olson, D. and Baerlocher, C. (2001) Atlas of zeolite structure types Elsevier, Amsterdam, 302 pp. Ming, D.W. (1985) Chemical and crystalline pr operties of clinoptilolite in South Texas soils. PhD dissertation, Texas A&M Univ ersity, College Station, Texas, 257 pp. Ming, D.W. (1987) Quantitative determinati on of clinoptilolite in soils by a cationexchange capacity method. Clays & Clay Minerals 35 463-468. Ming, D.W. (1988) Occurrence and weathering of zeolites in soil environments. Pp. 699715 in: Occurrence, Properties, and U tilization of Natural Zeolites (D. Kall and H.S. Sherry, editors). Akadm iai Kiad, Budapest, Hungary. Ming, D.W., and Allen, E.R. (2001) Use of na tural zeolites in agr onomy, horticulture and environmental soil remediation. Pp. 619-654 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, ed itors). Reviews in Mineralogy and Geochemistry 45 Mineralogical Society of Amer ica and Geochemical Society. Ming, D.W., and Mumpton, F.A. (1989) Zeolites in soils. Pp. 873-911 in: Minerals in Soil Environments (J.B. Dixon and S.B. Weed, edito rs). Soil Science Society of America, Madison, Wisconsin. Muller, J.C.M., Hakvoort, G. and Jansen, J.C. (1998) DSC and TG study of water adsorption and desorption on zeolite NaA-Po wder and attached as a layer on metal. Journal of Thermal Analysis and Calorimetry 53 449-466. Mumpton, F.A. (1977) Utilization of natural zeolites. Pp. 177-204 in: Mineralogy and Geology of Natural Zeolites (F.A. Mumpton, editor). Reviews in Mineralogy 4 Mineralogical Society of America. Navrotsky, A., Rapp, R.P., Smelik, E., Burnle y, P., Circone, S., Chai, L., Bose, K. and Westrich, H.R. (1994) The behavior of H2O and CO2 in high-temperature lead borate solution calorimetry of volatile-bearing phases. American Mineralogist 79 10991109. Neimark, A.V., Ravikovitch, P.I. and Vis hnyakov, A. (2000) Adsorption hysteresis in nanopores. Physical Review E 62 R1493-R1496. Neuhoff, P.S., and Bird, D.K. (2001) Partial dehydration of lamontite: Thermodynamic constraints and petrogenetic implications. Mineralogical Magazine 65 59-70. Neuhoff, P.S., Fridriksson, Th. and Bird, D.K. (2000) Zeolite parageneses in the North Atlantic igneous province: Implications for geotectonics and groundwater quality of basaltic crust. International Geology Review 42 15-44.

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71 Neuhoff, P.S., Hovis, G., Balassone, G. and Stebbins, J. (2004) Thermodynamic properties of analcime solid solutions. American Journal of Science 304 21-65. Neuhoff, P.S., and Keddy, S.M. (2004) Heat capac ity of intra-crystal line water molecules in zeolites. Geological Society of America Abstracts with Programs 36 260. Neuhoff, P.S., Kroeker, S., Du, L., Frid riksson, Th. And Stebbins, J. (2002) Order/disorder in natr olite group zeolites: A 29S i and 27Al MAS NMR study. American Mineralogist 87 1307-1320. Neuhoff, P.S., Stebbins, J.F. a nd Bird, D.K. (2003) Si-Al diso rder and solid solutions in analcime, chabazite and wairakite. American Mineralogist 88 410-423. Neuhoff, P.S., and Wang, J. (2006) Isothermal measuremen t of heats of hydration in zeolites by simultaneous thermogravimetry and differential scanning calorimetry. Submitted to Clays and Clay Minerals Neuhoff, P.S., Watt, W.S., Bird, D.K. and Pe dersen, A.K. (1997) Timing and structural relations of Regional Zeolite Zones in Ba salts of the East Greenland continental Margin. Geology 25 803-806. Neveu, A., Gaspard, M., Blanchard, G., and Martin, G. (1985) Intracrystalline selfdiffusion of ions in clinoptilolite ammonia and sodium-cations studies. Water Research 19 611-618. Newell, P.A., and Rees, L.V.C. (1983) Ion-ex change and cation site locations in zeolite L. Zeolites 3 22-27. Ogorodova, L.P., Kiseleva, I.A., Mel.chakova, L.V., Belitskii, I.A. and Fursenko, B.A. (1996) Enthalpies of formation a nd dehydration of natural analcime. Geochemistry International 34 980-984. Pabalan, R.T., and Bertetti, E.P. (1999) Experimental and m odeling study of ion exchange between aqueous solutions a nd the zeolite mineral clinoptilolite. Journal of Solution Chemistry 28 367-393. Pabalan, R.T., and Bertetti, E.P. (2001) Catio n-exchange properties of natural zeolties. Pp. 453-518 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, editors). Reviews in Mineralogy and Geochemistry 45 Mineralogical Society of Amer ica and Geochemical Society. Passaglia, E., and Sheppard, R.A. (2001) Th e crystal chemistry of zeolites. Pp. 69 in: Natural Zeolites: Occurrence, Properties, Applications (D.L. Bish and D.W. Ming, editors). Reviews in Mi neralogy and Geochemistry 45 Mineralogical Society of America and Geochemical Society.

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72 Pankratz, L.B. (1968) High temperature heat contents and entr opies of dehydrated analcite, kaliophilite, and leucite. U.S. Bureau of Mines Report of Investigations 7073. Paukov, I.E., Kovalevskaya, Y.A., Seretkin, Y.V. and Belitskii, I.A. (2002) The thermodynamic properties and structure of potassium-substituted natrolite in the phase transition region. Russian Journal of Physical Chemistry 76 1406-1410. Pauling, L. (1930) The structure of some sodium and calcium aluminosilicates. Procedings of the Natio nal Academy of Sciences 16 453. Peacor, D.R. (1973) High-temperature, single-crystal X-ray study of natrolite. American Mineralogist 58 676-680. Petrova, N., Mizota, T. and Fujiwara, K. (2001) Hydration heats of zeolites for evaluation of heat exchangers. Journal of Thermal Analysis and Calorimetry 64 157-166. Ransom, B., and Helgeson, H.C. (1994) Estimati on of the standard molal heat capacities, entropies, and volumes of 2:1 clay minerals. Geochimica et Cosmochimica Acta 58 4537-4547. Ross, M., Flohr, M.J.K. and Ross, D.R. ( 1992) Crystalline soluti on series and orderdisorder within the natrolite. American Mineralogist 77 685-703. Robie, R.A., and Hemingway, B.S. (1995) Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 pascals) pressure and at higher temperatures. United States Geological Survey Bulleti n, 2131 461. Robie, R.A., Hemingway, B.S. and Fisher, J.R. (1979) Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 Pascals) pressure and at higher temperatures. U.S. Geological Survey Bulletin 1452 456. Sabbah, R., Xu-wu, A., Chickos, J.S., Planas Leito, M.L., Roux, M.V. and Torres, L.A. (1999) Reference materials for calorime try and differential thermal analysis. Thermochimica Acta 331 93-204. Sapiga, A.V., and Sergeev, N.A. (2001) NMR investigation of natrolite structure. Crystal Research and Technology 36 875-883. Sarge, S.M., Gmelin, E., Hhne, G.W.H., Ca mmenga, H.K., Hemminger, W. and Eysel, W. (1994) The caloric calibrati on of scanning calorimeters. Thermochimica Acta 247 129-168. Scarmozzino, R., Aiello, R. and Santucci, A. (1980) Chabazitic tuff for thermal storage. Solar Energy 24 415-416. Selvidge, M., and Miaoulis, I.N. (1990) Eval uation of reversible hydration reactions for use in thermal energy storage. Solar Energy 44 173-178.

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73 Shigeishi, R.A., Langford C.H. and Holleborne B.R. (1979) Solar energy storage using chemical potential changes associated with drying of zeolites. Solar Energy 23 489495. Shim, S.H., Navrotsky, A., Gaffney, T.R. and Macdougall, J.E. (1999) Chabazite: Energetics of hydration, enthalpy of forma tion and effect of cations on stability. American Mineralogist 84 1870-1822. Stoiber, R.E., and Davidson, E.S. (1959) Am ygdule mineral zoning in the Portage Lake Lava Series, Michigan Copper District. Econoomic Geology 54 1250-1277. Surdam, R.C., and Sheppard, R.A. (1978) Zeol ites in saline, alkaline-lake deposits. Pp. 145-174 in: Natural Zeolites: Occurrence, Properties, Use (L.B. Sand and F.A. Mumpton, editors). Pergamon Press, Elmsford, New York. Tamura, K., Yamada, H. and Nakazawa, H. (2000) Stepwise hydr ation of high-quality synthetic smectite with various cations. Clays and Clay Minerals 48 400-404. Thompson, A.B. (1973) Analcime: Free energy from hydrothermal data Implications for phase equilibria and thermodynami c quantities for phases in NaAlO2-SiO2-H2O. American Mineralogist 58 277-286 van Reeuwijk, L.P. (1972) High-temperature phases of zeolites of the natrolite group. American Mineralogist 57 499-510. van Reeuwijk, L.P. (1974) The thermal dehydr ation of natural zeo lites. Dissertation, Wageningen, Netherlands, 88 pp. Vucelic, V., and Vucelic, D (1985) Heat cap acities of water on zeolites. Pp. 475-480 in: Zeolites (B. Drzaj, S. Hocevar and S. Pej ovnik, Editors). Elsevier, Amsterdam. Walker, G.P.L. (1960) Zeolite z ones and dike distribution in relation to the structure of basalts of eastern Iceland. Journal of Geology 68 515-528. Wilkin, R.T., and Barnes, H.L. (1999) Ther modynamics of hydration of Naand Kclinoptilolite to 300 C. Physics and Chemistry of Minerals 26 468-476. Wilkinson, J.F.G., and Hensel, H.D. (1994) Nephelines and analcimes in some alkaline igneous rocks. Contributions to Mi neralogy and Petrology 118 79-91. Xu, G. (1990) Use of natural zeolite fo r ammonia in distilled water production. Shuichuji Jishu 16 456-499.

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74 BIOGRAPHICAL SKETCH Jie Wang was born in Ezhou, Hubei Province, China, in January 1981. He lived in with his parents and attended elementary and high schools in that small town until entering the University of Science & Tec hnology of China in 1999. He graduated with a B.S. in earth and space sciences in 2004. In August of 2004 he entered the University of Florida and continued his graduate study in the Department of Geological Sciences. He worked under Dr. Philip Neuhoffs direc tion and studied the thermodynamics of dehydration and rehydration in zeolites. Upon co mpletion of masters study, he will keep on working with Dr. Neuhoff for his Ph.D. degree.


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Copyright Date: 2008

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THERMODYNAMICS OF DEHYDRATION AND HYDRATION
IN NATROLITE AND ANALCIME















By

JIE WANG


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

Jie Wang















ACKNOWLEDGMENTS

Funding for this project was provided by an NSF-EAR grant to Dr. Philip Neuhoff

at the University of Florida. First, I gratefully acknowledge Dr. Neuhoff for his support

during the past two years in my study and living in United States. His patience and advice

helped me overcome a lot of difficulties and finally finish this project. I would like to

thank my committee (Dr. Mike Perfit and Dr. Jon Martin) and Dr. Guerry McClellan for

their support and guidance. I would also like to thank Laura Ruhl and Scott Keddy for

their assistance with sample preparation, Jane Gustavson and Gokce Atalan for their help

in my research. Special thanks also go to Ryan Francis, Shawn Malone, Susanna Blair,

Derrick Newkirk and all my other friends for their help with my spoken and written

English. Finally, I would like to thank my parents for all their love and support

throughout my life.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TABLES .................. ............. ...... ........... .......... ....... vi

L IST O F F IG U R E S .... ...... ................................................ .. .. ..... .............. vii

ABSTRACT .............. .......................................... ix

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

Background Studies .................. .................................... ................ .3
O ccurrence of Z eolites......... ............................................................ ................ .5
Industry A application of Z eolites............................................................................. 6
Thermodynamics of Dehydration in Zeolites ............................................................6
Heat Capacities of Hydrous Zeolites ...................... .................8
C alorim etric T echniqu e ...................................................................... ..................10

2 ENTHALPy OF HYDRATION IN AnalCIME ............................... ................ 15

In tro d u ctio n ...................................... ................................................ 15
M methods ................. ........................................17
Sample and Characterization.................. ......... ........................... 17
A bsorption C alorim etry .................................................................. ............... 17
H eat Capacity M easurem ents .................................................... ...... ........ 18
R e su lts ............................... ......................... ................. ................ 1 9
Enthalpy of Hydration in Analcime .......................... .......... .... ........... 19
Heat Capacities of Hydrated and Dehydrated Analcime................................. 21
D iscu ssion ................... .... ............. ............. ............................... ....... .. 2 1
Comparison of Present Results with Previous Studies ........................................21
Temperature Dependence of Heat of Hydration ...............................................22
Behavior of Heat Capacity of Hydration..........................................................24

3 EXCESS HEAT CAPACITY IN NATROLITE HYDRATION ............................39

In tro d u ctio n ...................................... ................................................ 3 9
M eth o d s ..............................................................................4 0










Sample and Characterization.................. ......... ........................... 40
H eat Capacity M easurem ents ................................................... ....... ........ 41
Absorption Calorimetry .................... ............................... 42
R esu lts ....................................... ... ..... .......................... ..................... 4 3
Heat Capacities of Hydrated and Dehydrated Natrolite..................... ......... 43
Enthalpy of Hydration in Natrolite ................. ....... ..... .................... ............... 43
D discussion .............. ..... ... .......................... 45
Comparison with Previous Results......................... ................................45
Heat Capacity of Hydration in Natrolite .............. ................................46
Nature of the Natrolite-Dehydrated Natrolite Solid Solution..............................48

4 C O N C L U SIO N ......... ...................................................................... ......... .. ..... .. 62

Comparison of the Results of Analcime and Natrolite.............................................62
Heuristic Outcomes of This Study.................. ..................................63
F u tu re W o rk ...................................................... ................ 6 4

LIST OF REFEREN CE S ........................................ ........................... ............... 65

B IO G R A PH IC A L SK E TCH ..................................................................... ..................74


































v
















LIST OF TABLES


Table p

2-1 Isothermal immersion calorimetric data for analcime............... ...............27

2-2 Heat capacities of hydrated and dehydrated analcime, steam, and hydration of
analcim e at different tem peratures. ........................................ ....... ............... 28

2-3 Enthalpy of hydration in analcim e. .......................................... .........................29

3-1 Heat capacities of hydrated and dehydrated natrolite, steam, and hydration of
natrolite at different tem peratures. ........................................ ....................... 51

3-2 Isothermal immersion calorimetric data for natrolite................... ........ ........52

3-3 Enthalpy of hydration in natrolite ...................................................... ..................53















LIST OF FIGURES


Figure page

1-1 View of the crystal structures of natrolite (a) (after Peacor, 1973) and laumontite
(b) (after Fridriksson et al., 2003) projected along the c axis .............................12

1-2 Thermogravimetric analysis (TGA) curves depicting the change in mass with
increasing temperature of natrolite (a) and laumontite (b).................................... 13

1-3 Schematic representation of the simultaneous DSC/TGA system...........................14

2-1 Crystal structure of analcime viewed down the b crystallographic axis (after
M azzi and G alli, 1978) .......... .... .......................... ........ ..... .. .. ............ 30

2-2 Example isothermal immersion experiment on analcime at 403K...........................31

2-3 Plot of DSC versus dTGA for the experimental results shown in Fig. 2-2..............32

2-4 Heat evolved during absorption of water into analcime as a function of mass
ab so rb e d .......................................................................... 3 3

2-5 Enthalpy of hydration in analcime as a function of temperature .............................34

2-6 The mass change (a) and DSC response (b) of hydrated and dehydrated analcime
collected at a scanning rate of 15 K/min under ultrapure N2 ..............................35

2-7 Experimental Cp data as a function of temperature for hydrated and dehydrated
a n a lc im e .......................................................................... 3 6

2-8 The heat capacity of hydration in analcime as a function of temperature ...............37

2-9 Partial molar enthalpies of hydration in analcime vs. temperature..........................38

3-1 Crystal structures of natrolite down the c crystallographic axis. (after Peacor,
1 9 7 3 ) ................................ ......... ................................................. 5 4

3-2 The mass change (a) and DSC response (b) of hydrated and dehydrated collected
at a scanning rate of 15 K/min under ultrapure N2...........................................55

3-3 Heat capacities of hydrated and dehydrated natrolite as a function of temperature
from 320 to 680 K ...................... ...................... .................... .. ..... 56









3-4 Example immersion calorimetric experiment on natrolite conducted at 412 K.......57

3-5 The enthalpy of dehydration in natrolite as a function of temperature....................58

3-6 Mass change of natrolite in the dehydration and rehydration with a
heating/cooling rate of 5 K/min under a humid condition (PH20o12 mbar).............59

3-7 Comparison of present and previous Cp data for both hydrated and
dehydrated natrolite .................. ................................................. 60

3-8 The heat capacity of hydration in natrolite as a function of temperature ...............61















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

THERMODYNAMICS OF DEHYDRATION AND HYDRATION
IN NATROLITE AND ANALCIME

By

Jie Wang

August 2006

Chair: Philip S. Neuhoff
Major Department: Geological Sciences

Zeolites are framework aluminosilicates with open channels containing molecular

water and extra-framework cations. Their distinctive crystal structures endow them with

high cation-exchange capacities and molecular sieve capabilities, which are widely

applied in water softening, catalysis and wastewater treatment. Thermodynamic data are

essential to determine the stability of zeolites and evaluate their paragenesis.

Reversible dehydration of intracrystalline water in zeolites is an important

consideration for assessing their stability, particularly at elevated temperatures and

pressures. Derivation of thermodynamic properties of dehydration and rehydration from

phase equilibria requires prior knowledge of the heat capacity of hydration (ACp,r) in

order to reduce the number of unknown variables. However, experimental determination

of ACp,r is difficult because measurements at elevated temperature often contain

contributions from the enthalpy of dehydration. Statistical-mechanical reasoning is often

used to suggest that ACp,r is independent of temperature, permitting application of ACp,r









determined at relatively low temperatures where dehydration is not an issue. We focused

our study on natrolite (Na2Al2Si3010o2H20) and analcime (NaAlSi206-H20), two

common rock-forming zeolites that are chemically and structurally less complex than

other zeolites due to the presence of only one extraframework cation (Na ) and one

crystallographically-distinct water site. In this study, we have directly measured heat

capacities (Cp) of hydrated and dehydrated zeolites by the simultaneous differential

scanning calorimetric (DSC) and thermogravimetric analysis (TGA) system. The

temperature dependence of enthalpies of hydration (AHhyd) in analcime and natrolite was

determined by the newly developed isothermal immersion technique. The temperature

dependence of AHhyd provides an alternative means of assessing ACp,r.

The results obtained by these approaches show that the behaviors of ACp,r are

different for different zeolites. In natrolite, ACp,r determined from AHhyd between 373 and

473 K is independent of temperature, but is substantially larger than determined by direct

measurements of Cp. This implies the presence of an excess heat capacity of mixing due

to the solvus behavior of natrolite in solid solution. In the case of analcime, the situation

is more complicated. In the lower temperature range (< 463 K), ACp,r, determined from

AHhyd, is relatively insensitive to the temperature and in agreement with direct Cp

measurements, whereas in the higher temperature range (> 463 K), ACp,r decreases and

increases rapidly with increasing temperature, indicating a phase transition. Coupled

determination of AHhyd and ACp,r as a function of temperature provides important and

fundamental insights into the thermodynamics of dehydration and rehydration in zeolites,

and should aid quantitative prediction of the water content of zeolites as a function of

temperature, pressure, and the chemical potential of H20 in geologic systems.














CHAPTER 1
INTRODUCTION

Minerals that contain molecular water within their structures are modally-important

at and near the earth's surface. These mineral hydrates (e.g., oxyhydroxides, sulfates,

carbonates, clay minerals, zeolites) are important reservoirs for water in earth's crust.

Many of these phases are also important naturally-occurring nanomaterials (e.g., Banfield

and Zhang, 2001). A common feature of some hydrate minerals is the ability to reversibly

dehydrate in response to changes in temperature, pressure, and the chemical potential of

H20, a process that has significant implications for assessing the water content of the

crust and the stability of the minerals themselves (e.g., Bish and Carey, 2001). Therefore,

the stability and chemical behavior of hydrates are critical considerations for assessing

the behavior of low-temperature geochemical systems. This study focuses on zeolites,

which, in addition to being geologically important phases in low temperature

environments (e.g., Coombs et al., 1959; Hay and Sheppard, 2001; Neuhoff et al., 2000),

serve as important model systems for the study of dehydration by mineral hydrates

because they have well-delineated crystal structures (e.g., Armbruster and Gunter, 2001;

Meier et al., 2001) and are readily manipulated experimentally (e.g., Bish and Carey,

2001).

Understanding the formation and stability of zeolites in geologic systems is limited

in part by an insufficient understanding of the dehydration behavior of these minerals.

Partial dehydration (i.e., compositional changes) during calorimetric or phase equilibrium

experiments complicates derivation of thermodynamic properties for these minerals (e.g.,









Carey, 1993; Helgeson et al., 1978). Thus, developing experimental techniques that

provide for quantitative assessment of the thermodynamic consequences of partial

dehydration is critical for evaluating zeolites stability relations. In addition, the stability

of zeolites with respect to other aluminosilicates is dependent on their hydration state as

hydrated zeolites are often stable at earth surface conditions with respect to dense

aluminosilicate assemblages and free water, but their dehydrated equivalents are not (e.g.,

Shim et al. 1999). In fact, accurate prediction of the partial dehydration of zeolites can be

key to understanding their stability in geologic environments (e.g., Neuhoff and Bird,

2001). Dehydration also has a dramatic effect on the ion exchange properties of zeolites

(Fridriksson et al., 1999; Bish and Carey, 2001). Complicating the thermodynamic

description of these processes is the fact that many zeolites contain multiple,

energetically distinct water sites (Fridriksson et al., 2003; Fialips et al., 2005).

This study applies new experimental techniques for assessing the thermodynamic

properties of dehydration reactions in zeolites, focusing on the natural zeolites analcime

and natrolite. Through detailed laboratory analysis this study attempts to answer three

specific questions:

1. Are the heat capacities of dehydration reactions in zeolites invariant with respect to
temperature, as previously suggested?

2. Do experimental measurements of the bulk heat capacities of hydrated and
dehydrated zeolites permit accurate assessment of the temperature dependence of
the heat of dehydration determined calorimetrically or through equilibrium
observations?

3. What are the water contents of analcime and natrolite in the geologic settings in
which they occur?

In the pages to follow, we comment on problems in four chapters. Chapter 1 (this

chapter) provides background material relevant to this study. Chapter 2 presents









experimental determinations of the temperature dependence of the heat of hydration and

heat capacity of hydration in analcime. Chapter 3 is a similar study of natrolite. Chapter 4

compares the results for analcime and natrolite and discusses the broader implication of

this study.

Background Studies

Mineralogical Nature of Zeolites

Zeolites are framework aluminosilicates whose structure is characterized by a

framework of Si- and Al-centered tetrahedra arranged to form 2-10 A channels that

contain water molecules and extraframework (charge-balancing) cations (Gottardi and

Galli, 1985). The general chemical formula for natural zeolites is

(Li, Na, K)a(Mg, Ca, Sr, Ba)d[Al(a+2d)Sin-(a+2d)02n1]mH20

where the portion in square brackets represents the framework and rest of the species

reside within the channels (e.g., Gottardi and Galli, 1985). While modification of the

framework composition requires dissolution and precipitation of the mineral, the

extraframework cations are readily exchangeable (e.g., Newell and Rees, 1983) and water

molecules can be reversibly removed from the structure at elevated temperatures (e.g.,

van Reeuwijk, 1974).

Figure 1-1 depicts the crystal structure of two representative rock-forming zeolites,

natrolite and laumontite. The tetrahedral framework sites in zeolites are occupied by Si4+

and Al3+ (with minor substitution of Fe3+). The extraframework cations balance the net

negative charge that exists on the framework due to trivalent Al in the tetrahedral sites.

The coordination number and bonding of the extraframework cations vary significantly

between different zeolites, among sites in a given zeolite, and between ions (Armbruster

and Gunter, 2001). The coordination spheres of these ions are made up of a combination









of framework and water oxygens. The integral presence of water within the crystal

structures of zeolites has been known since their discovery (Cronstedt, 1756). Water

comprises 8-25% of the mass of zeolites under ambient conditions. In some zeolites like

natrolite, there is only one crystallographically distinct water site (Fig. 1-la); while in

some zeolites like laumontite, water is distributed among several crystallographically

distinct sites (Fig. 1-1b). Note that water molecules occupy four distinct sites in

laumontite, labled W1, W2, W5 and W8 (Artioli and Stahl, 1993). Sites W2 and W8 are

part of the coordination sphere of the Ca2+ extraframework ion. Site W5 is hydrogen

bonded only to W2 and W8, whereas W1 is hydrogen bonded to both framework oxygens

and waters on W2 and W8 (Armbruster and Kohler, 1992). Sequential loss of water

molecules from distinct sites appears to be a common phenomenon in zeolites (e.g.,

Armbruster, 1993; Cruciani et al., 2003). Loss of water can lead to pronounced changes

in the structures of the zeolite framework and the positions of extraframework cations.

Most zeolites also exhibit contraction and/or collapse of the tetrahedral framework during

dehydration.

Zeolites produce abundant water upon heating under atmospheric conditions. A

common method of assessing the prograde dehydration of zeolites is by

thermogravimetric analysis (TGA), in which mass loss (due to dehydration) of a zeolite

sample is monitored as a function of temperature. Some zeolites, like analcime and

natrolite, dehydrate continuously with increasing temperature (Fig. 1-2a, corresponding

to Fig. 1-la), yet others like laumontite and heulandite show distinct steps in their

dehydration behavior (Fig. 1-2b, corresponding to Fig. 1-1b) (e.g., Alberti and Vezzalini,









1984). These differing behaviors reflect the structural and energetic properties of water

molecules within the structures.

Occurrence of Zeolites

Zeolites occur in a wide variety of environments, including two major types of

occurrences: 1) macroscopic and microscopic crystals, often in veins, fractures, and vugs

within plutonic and volcanic rocks and their metamorphosed equivalents; 2)

submicroscopic crystals, commonly distributed in vitroclastic sediments which have

undergone diagenetic or low-grade metamorphic processes (Passaglia and Sheppard,

2001). Zeolites can also occur during reactions of aqueous fluids with marine sediments

(e.g., Boles and Coombs, 1977), saline lake sediments (e.g., Hay and Moiola, 1963),

volcanic tuffs (e.g., Hay and Sheppard, 2001) and soils (e.g., Baldar and Whittig, 1968).

Hydrothermal systems often produce zeolites as well with complex parageneses caused

by overprinting of diagenetic and metamorphic occurrences (Gottardi, 1989; Neuhoff et

al., 1997). Zeolite stability is a sensitive function of pressure, temperature and fluid

composition, making them useful indicators of the physical and chemical conditions

associated with petroleum resources (e.g., Iijima, 2001), geothermal resources (e.g.,

Kristmannsd6ttir and T6masson, 1978) and basalt-hosted ore deposits (e.g., Stoiber and

Davidson, 1959) in earth's crust. Zeolites and associated authigenic clay minerals can

significantly reduce the porosity and permeability of hydrocarbon reservoir rocks, and

their presence is often regarded as an economic basement of exploration for oil and gas

(e.g., McCulloh et al., 1973; McCulloh and Stewart, 1979). In addition, they are

frequently considered as passive barriers in radioactive waste repositories both as

sorptive barriers to radionuclide migration and consumption of thermal energy (e.g.,

Carey and Bish, 1996; Bish et al., 2003).









Industry Application of Zeolites

The high cation-exchange capacities and molecular sieve capabilities make zeolites

widely applied in industry, e.g., water softening, catalysis and wastewater treatment

(Mumpton, 1977). The relatively large energy storage density of zeolites makes them

employed in energy storage and heat pump technologies, where their use instead of

activated alumina or silica gel can result in significant reduction of storage weight (e.g.,

Shigeishi et al., 1979; Scarmozzino et al., 1980; Gopal et al., 1982; Selvidge and

Miaoulis, 1990). In agriculture, natural zeolites have been used as soil conditioners,

carriers for insecticides and herbicides, remediation agents in contaminated soils, slow-

release fertilizers, and dietary supplements in animal nutrition because of their capability

of cation exchange, adsorption and their abundance in near surface, sedimentary deposits

(Ming, 1985, 1987, 1988; Ming and Mumpton, 1989; Boettinger and Graham, 1995). The

unique cation-exchange capabilities of zeolites can be used to remove dissolved cations

that affect human and animal health (e.g., NH4+) from water by exchanging with

biologically acceptable cations such as Na+, K+, Mg2+, Ca2 or H+ (Neveu et al., 1985;

Xu, 1990; Pabalan and Bertetti, 1999). The surface area of a zeolite-rich rock is -10

m2/g, much bigger than that of quartz sand (-0.01 m2/g), thus the filtration efficiency of a

sand bed can be increased by mixing porous zeolitic rock with it (Grigorieva et al., 1988;

Galindo et al., 2000). Zeolites are being used in more and more new technologies, and

these potential applications provide numerous possibilities to improve the environment.

Thermodynamics of Dehydration in Zeolites

The response of zeolites to changes in temperature and water vapor pressure is a

very important aspect of their behavior and structural changes. Detailed studies of the

structural effects accompanying dehydration processes allow evaluation of the changes in









environmental conditions that ultimately lead to structural modification or breakdown

(Bish and Carey, 2001). Alberti and Vezzalini (1984) divided zeolites into three

categories according to their thermal stabilities. Those with: 1) reversible dehydration

accompanied by rearrangement of the extraframework cations and residual water

molecules (e.g., chabazite, analcime and mordenite); 2) complete or nearly complete

reversible dehydration accompanied by a large distortion of the framework and

significant decrease in unit-cell volume (e.g., natrolite, mesolite and laumontite); and 3)

irreversible dehydration accompanied by irreversible changes in the framework (e.g.,

heulandite, barrerite and stilbite).

Equilibrium between a zeolite and water vapor can be represented by a reaction of

the form

Z-nH20 = Z + n H2Ovapor (1-1)

where Z-nH20 and Z are homologous hydrated (water sites occupied) and dehydrated

(water sites vacant) components of a zeolite and n is the number of moles of H20 in the

fully hydrated zeolite. Evaluation of the hydration state of zeolites as a function of

temperature and pressure requires knowledge of the thermodynamic properties, such as

Gibbs energy of reaction (AGr,T,P), enthalpy of reaction (AHr,T,P) and entropy of reaction

(ASr,T,P), as well as the relationship between activity and composition. Four basic

approaches have been applied in order to determine the thermodynamic properties of

zeolite dehydration reactions (cf. Bish and Carey, 2001). The first method involves

explicit calculation of AGr,T,P from the known properties of substances in the reaction.

For instance, volume of reaction (AVr) can be directly calculated if the molar volumes of

Z*nH20 and Z have been determined. This method has been applied to calculation of ASr









and heat capacity of reaction (ACp,r) as well from the results of heat capacity (Cp)

measurements for homologous hydrates and dehydrated zeolites (Carey, 1993; Ransom

and Helgeson, 1994; Neuhoff et al., 2000). In addition, AHr can be determined from the

heats of formation of homologous hydrated and dehydrated zeolites determined via

thermochemical cycles from heat of solution data (e.g., Johnson et al., 1982, 1983;

Ogorodova et al., 1996). The second approach is transposed temperature drop calorimetry

(e.g., Navrotsky et al., 1994; Kiseleva et al., 1996, 1997; Shim et al., 1999) in which AHr

is determined via a thermochemical cycle involving dehydration of the zeolite at high

temperatures (usually -973 K) and the heat contents of the hydrated and dehydrated

zeolites and water. The third method, immersion calorimetry, involves direct calorimetric

measurement of AHr as the dehydrated zeolite is immersed in water or humid gas (e.g.,

Barrer and Cram, 1971; Coughlan and Carroll, 1976; Carey and Bish, 1997; Muller et al.,

1998; Petrova et al., 2001). The last method involves fitting equilibrium observations of

the water content of a zeolite as a function of temperature and water fugacity, usually

determined thermogravimetrically (e.g., Carey and Bish, 1996; Fridriksson et al., 2003)

or by pressure titration techniques (Wilkin and Barnes, 1999). Non-linear regression of

phase equilibrium observations to the thermodynamic relations can determine the

thermodynamic properties at standard conditions (AGor,Tref,Pref, AHor,Tref,Pref, and

ASor,Tref,Pref), but generally requires prior knowledge of ACp,r in order to reduce the

number of unknown variables (Carey and Bish, 1996).

Heat Capacities of Hydrous Zeolites

Determination of the temperature dependence of Cp of hydrous minerals at

superambient conditions, particularly those that dehydrate continuously with temperature

such as zeolites, presents considerable experimental obstacles. Calorimetric









measurements of Cp by adiabatic, drop, or differential scanning calorimetry (DSC) on

these materials will inevitably include contributions not only from Cp but also the heat of

dehydration. Consequently, there is a paucity of reliable data for Cp of hydrate minerals,

which presents considerable complications for assessing the magnitudes of the Cp.

There are two basic approaches that have been taken to address this issue: 1) adjust

Cp measured by drop calorimetry for the heat of dehydration determined separately (e.g.,

Johnson et al., 1982, 1983); and 2) assume that ACp,r is independent of temperature

(Barrer, 1978; Carey, 1993), allowing ACp,r to be estimated from Cp for the hydrous and

anhydrous phase determined at relatively low temperatures where the dehydration is not

an issue. The first approach ignores potential exothermic effects in the calorimetric

measurements as the zeolite rehydrates during the drop, potentially leading to

overestimation of Cp for hydrous phases (Carey, 1993). The second approach is based on

statistical-mechanical arguments that sorption of H20 into a zeolite should lead to an

increase in Cp for this component over that in the vapor phase as a consequence of the

loss of translational and rotational degrees of freedom to vibrational modes within the

zeolite structure (Barrer, 1978). While this is certainly true, and borne out by some

experimental data for ACp,r for complete dehydration of some hydrates (Carey, 1993),

there is a paucity of experimental data necessary to test this model. In this study, we

measured the temperature dependent heat by isothermal immersion experiments and

observed some contradictions with the statistical-mechanical model including second-

order phase transition (Johnson et al., 1982; Neuhoff and Bird, 2001) and excess heat

capacity (Basler and Lechert, 1972).









Calorimetric Technique

All the calorimetric measurements were performed on the Netzsch STA 449C

Jupiter simultaneous thermal analysis system at the University of Florida. A schematic

depiction of this system is shown in Fig. 1-3. The core component is a vacuum-tight,

liquid nitrogen cooled furnace enclosing a sample carrier housing an electrode for

measurement of temperature differences between the sample and a reference pan,

generating a heat flux DSC signal and which is connected to a microbalance for

thermogravimetric analysis. With respect to the present study, an essential aspect of this

setup is that the DSC and TGA signals are recorded simultaneously, which allows the

DSC signal to be interpreted directly in terms of water loss or gain to the sample as

measured by TGA. The temperature range for the furnace is 120 to 1050 K; temperature

can be maintained within this range to a precision of better than 1 K and for dynamic

analysis can be run at controlled heating or cooling rates of up to 50 K/minute. Samples

as large as 5 g and as small as 1 mg can be investigated, although sample sizes of the

order of a few 10's of mg are optimal. Mass changes as small as 0.1 tg can be detected.

Calorimetric precision varies with experimental conditions and the amount of heat

involved; instrument specifications call for Cp precision on the order of 2.5% and heat of

reaction precision on the order of 3%. Precision obtained for the experiments proposed in

our study is described further below. Temperature and caloric calibration is performed by

measuring the melting temperatures and heats of fusion of high purity metal standards

and Cp of sapphire disks (cf Hohne et al., 1990; Sarge et al., 1994; Sabbah et al., 1999).

Temperature, DSC, and TGA signals are sent via cable to a personal computer for

recording and data analysis.









Experimental conditions can be controlled with respect to not only temperature but

also pressure. Experiments can be run under closed system conditions at total pressures

ranging from atmospheric to vacuums of 10-4 Torr, although typical operation involves a

flowing gas atmosphere at atmospheric pressure. Gas composition and flow rate are

controlled externally via mass flow controllers. Gases used in the experiments are ultra-

high purity He (for subambient temperatures) and N2 (for Cp measurements and

measurements under controlled humidity conditions). Humidity is generated by bubbling

N2 through distilled water at a constant temperature, with water vapor pressure (PH20)

varied by mixing H20-saturated and dry N2 gas in different proportions. Humidity on the

outflow end of the furnace is constantly monitored and recorded by a Sable Systems RH-

100 flow-through dewpoint/relative humidity analyzer (1% accuracy in PH20 down to 1

Pa).






















































; L- -I o; ; o
(b)
Figure 1-1. View of the crystal structures of natrolite (a) (after Peacor, 1973) and
laumontite (b) (after Fridriksson et al., 2003) projected along the c axis.
Natrolite has only one water site, whereas laumontite has four water sites: W1,
W2, W5 and W8.







13


a

100


98


96


4 94-


92


90

300 350 400 450 500 550 600 650 700

T(K)

b

100

98

96

94

92

C 90

88

86-

84

300 400 500 600 700 800 900
T (K)
Figure 1-2. Thermogravimetric analysis (TGA) curves depicting the change in mass with
increasing temperature of natrolite (a) and laumontite (b). Natrolite contains a
single water site and exhibits one continuous dehydration curve, whereas
laumontite has several water sites that dehydrate under different conditions as
reflected in the inflections observed in the mass loss as a function of
temperature.
























rir Lt i


MASS
rF nwI


NETZSCH TA449C SIN/AL
CONTROLLER SIMULTANEOUS DSC/TGA
Figure 1-3. Schematic representation of the simultaneous DSC/TGA system. Dashed
curves represent gas lines; solid curves represent data transfer cables between
the instruments and the computer.














CHAPTER 2
ENTHALPY OF HYDRATION IN ANALCIME

Introduction

Analcime, nominally NaAlSi206-H2O, is one of the most common rock-forming

zeolites. It appears to be stable over a considerable range of temperature and pressure

conditions, thus it occurs in a very wide range of geologic settings. Analcime generally

forms during low grade metamorphism of plutonic and volcanic rocks, as a product of

reaction between saline solutions and sediments in alkaline lakes (e.g., Hay and Moiola,

1963; Hay, 1966; Coombs and Whetten, 1967; lijima and Harada, 1968; Surdam and

Sheppard, 1978), and as phenocrysts in alkalic igneous rocks (e.g., Wilkinson and

Hensel, 1994). Although recent progress has been made in assessing the stability of

analcime in low-temeprature environments (e.g., Neuhoff et al., 2004), phase equilibria

observations at elevated temperature and pressure are often inconsistent with each other

(cf. Thompson, 1973). In large part, this appears to be due to complications arising from

solid solution in analcime, particularly with respect to its variable water content due to

progressive dehydration with increasing temperature (e.g., Helgeson et al., 1978)

Dehydration of analcime is a relatively simple one step process because water

molecules occupy only one crystallographically distinct site (Fig. 1-1, Mazzi and Galli,

1978). Several authors have studied the hydration-dehydration thermodynamics of

analcime (e.g., King, 1955; King and Weller, 1961; Robie et al., 1979; Helgeson et al.,

1978; Johnson et al., 1982; Ogorodova et al., 1996; Bish and Carey, 2001). Application

of these data to predicting the hydration state of analcime is complicated by a lack of









understanding of the temperature dependence of these properties, which largely relies on

assessing of the change in heat capacity brought about by dehydration. Experimental

determinations of the heat capacities (Cp) of the hydrated and dehydrated forms of these

minerals (e.g., King, 1955; King and Weller, 1961; Pankratz, 1968; Johnson et al., 1982)

are complicated because the calorimetric measurements at superambient temperatures

generally include contributions not only from Cp but also the heat of dehydration. In the

absence of reliable data, an assumption that Cp of dehydration in analcime is independent

of temperature (Barrer, 1978; Carey, 1993) was made based on the statistical-mechanical

arguments that much of the difference in Cp between absorbed water and the gas phase is

caused by the loss of some translational and rotational degrees of freedom to weak

vibrational modes within the zeolite structure. Although the Cp results of some stable

zeolites near room temperature are consistent with this assumption (Carey, 1993), no

other approaches have been used to test it.

In the present investigation, the isothermal heats of hydration in analcime were

measured over a range of temperature under constant water vapor pressure, and a general

behavior of the heat capacity of hydration (ACp,r) for analcime dehydration was obtained.

In addition, the heat capacities of hydrated and dehydrated analcime were determined as a

function of temperature by differential scanning calorimetry (DSC), and the results were

used to calculate ACp,r based on the statistical-mechanical model. Comparison of these

two methods is used to evaluate the appropriateness of the assumption that ACp,r is not a

function of temperature.









Methods

Sample and Characterization

The sample of analcime was collected from a zeolite-facies metabasalt outcrop at

Maniilat on the island of Qeqertarsuaq in West Greenland (Neuhoff et al., 2003), and

prepared from a 1.5 cm euhedral crystal of opaque analcime. Separates of analcime were

hand picked, ground in an agate mortar, and sieved to a 20-40 tm size fraction. Phase

identity and purity were confirmed by X-ray powder diffraction. A split of this sample

was previously used and characterized in the 29Si magic angle spinning nuclear magnetic

resonance (MAS NMR) study of Neuhoff et al. (2003; Sample ANA002). Electron probe

microanalysis indicated a composition of (NaAl)0.95Si2.0506-1.024H20, although the 29Si

MAS NMR results indicate a slightly less Si-rich composition of

(NaAl)0.97Si2.0306-1.015H20. The latter value was chosen as more representative of the

bulk composition (cf Neuhoff et al., 2004) and consistent with the water content. Water

content of the sample was determined by thermogravimetric heating to 1023 K after the

equilibration with a room temperature atmosphere of 50% relative humidity (RH), and

the mass loss was measured to be 8.29% of the total sample. This value is close to water

content calculated from the compositions determined by 29Si MAS NMR, 8.32% (cf

Neuhoff et al., 2004).

Absorption Calorimetry

Heats of hydration as a function of temperature were determined using an

isothermal DSC-based immersion technique on the Netzsch STA 449C Jupiter

simultaneous DSC-thermogravimetric analysis (TGA) system at the University of Florida

as described by Neuhoff and Wang (2006). This approach combines the benefits of DSC

and gas absorption calorimetry. Twenty to 30 mg of hydrated analcime were placed into a









Pt-Rh crucible for each run. The sample was dehydrated by scanning heating from 298 to

873 K at the rate of 15 K/min and then allowed to cool to the experimental temperature.

A purge of dry N2 was maintained at a flow rate of 50 ml/min during this period. After

equilibration (20-40 min.) at this temperature under dry N2 until both DSC and TGA

baselines stabilized, the gas stream was changed to humid N2 which was generated by

bubbling ultrapure N2 gas through a saturated NaCl solution. In order to reduce the

change of DSC baseline, the flow rate of humid N2 was maintained at 30 ml/min and the

corresponding water vapor pressure in the furnace was -12 mbar. Under this condition

the sample was allowed to react until the DSC trace became relatively flat. Repeated

experiments on one analcime sample gave virtually identical results, indicating that the

sorption capacity of analcime was not affected by dehydration and hydration.

Consequently, some of the data at different temperatures were measured from the same

sample aliquot. During the experiment, the sample of dehydrated analcime could only

reabsorb less than 5% of its mass, as opposed to -9.1% mass gain for complete

rehydration. Therefore, under current water vapor pressure we can only directly measure

partial molar enthalpies of hydration (Ahhyd) for analcime.

Heat Capacity Measurements

The heat capacities of hydrated and dehydrated analcime were also determined by

DSC also on the simultaneous DSC-TGA system. Each experiment consisted of four

separate runs: 1) determination of background and baseline by measurement of an empty

crucible; 2) DSC measurement of a standard (sapphire); 3) scanning heating of hydrated

analcime and 4) scanning heating of dehydrated analcime. A sample of approximately 27

mg was packed into a covered Pt-Rh crucible before step 3 and kept to the end of the

experiment. Data were collected in the range of 298 to 873 K at a scanning rate of 15









K/min. A purge of dry N2 was used during the experiment to keep the relative humidity

below 1%, and the gas flow was maintained at -30 ml/min using mass flow controllers.

The furnace was heated to 873 K in each run and then cooled back to room temperature

by liquid N2 before the next step. During heating, hydrated analcime lost mass with

increasing temperature and finally became completely dehydrated. The resulting

dehydrated analcime was kept in an environment of dry N2 to avoid rehydration during

cooling to room temperature and then measured by scanning heating again. Heat capacity

of the sample as a function of temperature was calculated by

mstd DSCs DSCb 2-
Cp =- p, std (2-1)
C x xC 1)
Sm DSCsd, DSCb p

where ms is the mass of sample, mstd is the mass of standard (27.314 mg), DSCs, DSCstd

and DSCb are for sample, standard and baseline respectively. Triplicate calorimetric

measurements were conducted and averaged. Based on previous measurements in this lab

the precision was taken to be 1% of Cp.

Results

Enthalpy of Hydration in Analcime

An example of TGA and DSC response of analcime during immersion in water

vapor at 403 K is shown in Fig. 2-2. It is observed that the hydration of analcime is

initially relatively rapid (as shown by the peak in the first derivative of the TGA signal,

dTGA) and then decays exponentially. After about 120 min. the reaction slowed to a

point where the DSC signal had decayed to near baseline level and essentially invariant

with respect to time even though sample could keep on absorbing H20 if the experiment

continued. The slow rate of the reaction is also reflected in the near-zero value of dTGA.









It can be seen in Fig. 2-2 that the DSC and dTGA data are strongly correlated.

These data are plotted against each other in Fig. 2-3, and the slope of the linear regression

is proportional to Ahhyd. Consequently, the partial molar enthalpies of hydration can be

calculated by the equation

Ahhyd = k (dDSC/dm) (MWH2o) (2-2)

where k is the caloric calibration factor (in mW/iV), dDSC/dm is the slope in Fig. 2-3,

and MWH20 is the molecular weight of water. The y-intercept of the regression represents

the DSC signal when dTGA = 0; i.e., the baseline at the end of reaction. The partial molar

enthalpy of hydration in analcime calculated by this method has relatively large error

because of the oscillation of DSC and dTGA signals. Using the position of the baseline

derived by linear regression of the DSC and TGA signal provides another approach to

determine the Ahhyd. Cumulative, baseline-corrected DSC response plotted against

cumulative mass of absorbed H20 also leads to a linear dependence, the slope of which is

also equivalent to Ahhyd (Fig. 2-4).

The results of partial molar enthalpy at ten different temperatures calculated by both

methods are listed in Table 2-1. Most of the values of Ahhyd1 and Ahhyd2 at the same

temperature are close to each other (within 1.5% difference), illustrating the general

repeatability of this method. For some temperatures the difference between Ahhyd1 and

Ahhyd2 is relatively big; this may result from the uncertainty of the baseline caused by the

oscillation of the signals. The temperature dependence of Ahhyd are illustrated in Fig. 2-5.

The errors include the part from standard deviation of the values and that from the

calorimetric calibration (1% of the results). It can be seen that within the analysis error









the Ahhyd at different temperatures have little change except the one at 528 K, which may

indicate a trend of abrupt decrease and increase of Ahhyd with increasing temperature.

Heat Capacities of Hydrated and Dehydrated Analcime

Figure 2-6 shows TGA and DSC traces of hydrated and dehydrated analcime

obtained during scanning heating. Dehydration of analcime is accompanied by a mass

loss from -350 K to 743 K, which is also indicated by the positive inflection of the DSC

curve. The TGA curve is continuous and suggests only one stage of dehydration,

consistent with previous observations that only one energetically distinct water site is

presented in analcime (e.g., van Reeuwijk, 1974; Bish and Carey, 2001) and the crystal

chemical considerations listed above. Unlike the TGA trace for hydrated analcime, there

is no mass change for dehydrated analcime during scanning heating, showing that the

sample has been completely dehydrated in the first run.

Calculated Cp for hydrated and dehydrated analcime are compared in Fig. 2-7. The

heat capacity of hydrated analcime is relatively insensitive to the temperature below -399

K, but increases quickly after that because of the contribution of dehydration. For

dehydrated analcime the heat measurement only includes the contribution from Cp, so the

trace of Cp increases very slowly with increasing temperature, and in a nearly linear

fashion.

Discussion

Comparison of Present Results with Previous Studies

Low-temperature (below 350 K) Cp measurements on analcime have been reported

previously (King, 1955; King and Weller, 1961; Johnson et al., 1982). The data of low-

temperature Cp were directly measured by adiabatic calorimetry. In addition, Johnson et

al. (1982) also assessed the Cp for analcime above 350 K by drop calorimetry. The









present results of Cp of hydrated analcime below 350 K are in good agreement with those

from Johnson et al. (1982). For instance, at 298 K the difference between the two values

is only -0.6%. However, above 350 K, Cp measured in the present study is higher than

those of Johnson et al. (1982) because of the strong dehydration effect on the DSC signal.

As for the Cp of dehydrated analcime, the present results are closer to those from Johnson

et al. (1982) at low temperatures than at high temperatures, but are generally.

Examination of the method that Johnson et al. (1982) used to determine the Cp of

dehydrated analcime suggests that the cause is that their sample was not completely

anhydrous.

The enthalpy of hydration in analcime at 298 K has been determined by several

authors using different methods (Johnson et al., 1982; Ogorodova et al., 1996; Barany,

1962; Bish and Carey, 2001), and the results are listed in Table 2-3. Especially,

Ogorodova et al. (1996) found that enthalpy of formation of analcime is linearly

dependent on the degree of hydration in analcime, indicating that there are no excess

contributions to the enthalpy across the solid solution between hydrated and dehydrated

analcime. Therefore, it is reasonable to assume that the integral molar enthalpy of

hydration (AHhyd) should equal Ahhyd at the same temperature. In general, present values

of AHhyd within error increase with increasing temperature below 463 K and the

difference between the values of 403 K and 463 K is relatively small (Table 2-1, Fig. 2-

5). The result of AHhyd at 298 K interpreted from present data is in good agreement with

previous studies.

Temperature Dependence of Heat of Hydration

The enthalpies of hydration in analcime shown in Fig. 2-5 as a function of

temperature can be divided into two regions. At temperatures below -470 K, Ahhyd is









relatively insensitive to the temperature. In this range of temperature Ahhyd generally

increases with increasing temperature. At temperatures above -470 K, the abnormal

value of Ahhyd at 528 K may indicate that a dramatic change occurs to Ahhyd. It decreases

abruptly at -470 K and continues to drop rapidly with increasing temperature. At even

higher temperatures Ahhyd starts to increase again with increasing temperature. The

temperature dependence of Ahhyd can be used to assess the magnitude of ACp,r via the

relationship


ACr =( ) (2-3)


The measurement of Cp for hydrated analcime in this study is greatly affected by

the contribution from dehydration above -350 K, whereas this effect is smaller in the

drop calorimetric method according to the data of Johnson et al (1982). As for the

dehydrated analcime, our samples were run under a completely dry condition, so our Cp

data for dehydrated analcime is reliable. Considering the similar compositions of the

analcime samples used in the experiments of Johnson et al. (1982) and this study, we

combine the Cp data of hydrated analcime from Johnson et al. (1982) and our data of

dehydrated analcime to estimate the ACp,r for analcime hydration.

Equilibrium between dehydrated and hydrated analcime can be represented by a

hydration of the form

(NaAl)0.97Si2.0306 + 1.015 H2Ovapor = (NaAl)0.97Si2.0306-1.015H20 (2-4)

Thus, heat capacity of hydration can be calculated by the following equation

ACp,r = (Cp,ana Cp,deana 1.015Cp,H20) / 1.015 (2-5)

where Cp,ana and Cp,deana are the heat capacities of hydrated and dehydrated analcime,

respectively; Cp,H2o is the heat capacity of water vapor taken from Robie and Hemingway









(1995). The results of ACp,r derived from our data and the combined data are both plotted

in Fig. 2-8 as a function of temperature. Note that both curves have a relatively flat part

in the relatively low temperature range (e.g., 298-350 K for results of our data and 351-

430 K for results of combined data), in which dehydration effect is much smaller than

that above 430 K to the Cp measurement. In this region, ACp,r is insensitive to

temperature and the two results have similar magnitudes, 12-15 J/molK (about 1.5R-2R,

R is the gas constant, equals 8.314 J/molK). This is quite similar to the value obtained by

linear regression of Ahhyd below 470 K, which indiates a value of ACp,r of 17.1 + 12.4

J/molK (Fig. 2-9).

Above 470 K, the decrease and then increase in Ahhyd with increasing temperature

indicates that ACp,r becomes negative and then positive again. This is indicative of a

phase transition in hydrated analcime. This behavior is not apparent in ACp,r calculated

from Cp data, as the transition is masked by the effects of dehydration that lead to

erroneously large values of Cp for hydrated analcime.

Behavior of Heat Capacity of Hydration

Heat capacities of hydrated zeolites are difficult to measure, especially at relatively

high temperatures. In the absence of the reliable data for the hydrations of zeolites, Barrer

(1978) discussed a statistical-mechanical model for heat capacity of molecular gases

absorbed in zeolites to simplify the calculation of ACp,r. This model argues that the

sorption of H20 into a zeolite should lead to an increase in Cp for this component because

of the loss of some translational and rotational degrees of freedom to week vibrational

modes within the zeolite structure (Carey, 1993). The difference in Cp between absorbed

H20 and gas phase can be determined by the number of vibrational modes gained by the

absorbed species. With this model, the heat capacity of the absorbed species increases by









R/2 for each saturated vibrational mode gained, and the heat capacity difference between

absorbed and free H20 is limited between zero and 3R (R is the gas constant, 8.314

J/molK) (Bish and Carey, 2001). Based on the statistical-mechanical model, an

assumption can be made that ACp,r of hydration in zeolites is independent of temperature

and the value is n*R/2 (0
combined data is almost constant in the low temperature range, for instance, 350-430 K,

in which the dehydration effect can be ignored. That means the heat capacity of hydration

in analcime is independent of temperature and the magnitude is approximately 1.5R-2R.

Thus, it appears that the statistical-mechanical model is valid in this temperature range.

The enthalpies of hydration at five different temperatures between 373 and 463 K show

an almost linear relationship with temperature, indicating that ACp,r in this region is

generally independent of temperature. In addition, the magnitude of ACp,r determined by

this approach is approximately 2R, which agrees with that of the calorimetric ACp,r within

error.

Enthalpies of hydration measured above 463 K show clear evidence of a phase

transition in analcime. Although the limited resolution afforded by the Ahhyd

measurements precludes a rigorous assessment of the nature of this transition, the general

shape implied by the results in Fig. 2-5 indicates that this transition is probably a K-type,

second order transition. This kind of phase transition has previously been found in

wairakite (Neuhoff and Keddy, 2004) at slightly lower temperatures, in which ACp,r

becomes negative after the transition (dehydration precludes assessment of the behavior

at higher temperatures). It appears that almost identical behavior occurs in analcime.

Neither the calorimetric results nor the statistical-mechanical model predict this phase






26


transition, underscoring the need for measurements such as those conducted in this study.

If more data of Ahhyd are available, an empirical equation for ACp,r may be extrapolated

by the first derivative of Ahhyd.










Table 2-1. Isothermal immersion calorimetric data for analcime.


T (K) Sample
T (K)
mass (mg)
403 22.93
403 25.67
403 29.60


417
417
417

432
432
432

446
446
446
446

463
463
463
463

470
470

490
490
490

499
499


Duration1
(min)
73
97
95

82
83
110

110
100
100

150
69
74
115

79
84
100
108


% H20
uptake2
1.40
1.60
1.62

1.80
1.94
2.00

2.40
2.38
2.59

3.14
2.25
2.35
2.63

2.76
2.81
2.83
3.08


25.67
27.46
29.13

22.93
25.67
29.13

22.91
22.93
29.14
29.13

22.93
29.14
29.13
29.60

25.67
27.76

27.76
29.14
29.13

29.14
30.99

29.13
29.14

25.02
29.13
33.06


Ahhydl3
(kJ/mol)
-86.29
-85.61
-85.39

-86.07
-85.44
-83.93

-85.35
-85.14
-84.77

-85.55
-84.70
-86.93
-85.71

-84.75
-84.85
-86.30
-86.26


Ahhyd24
(kJ/mol)
-86.45
-85.15
-85.27

-85.91
-85.19
-84.20

-84.94
-84.80
-84.87

-84.77
-83.62
-86.46
-85.61

-84.12
-84.47
-85.78
-85.81


-86.51 -85.99
-83.95 -83.75


-85.93
-87.91
-87.87


-86.18
-88.00
-88.21


-87.73 -87.11
-87.7 -87.15

-89.16 -89.07
-89.64 -88.82


-88.68
-88.08
-87.97


-87.70
-87.24
-87.26


Ahhyd,ave5
(kJ/mol)
-85.69


Error
(kJ/mol)
1.02


-85.12 1.23



-84.98 0.88




-85.42 1.35




-85.29 1.20


-85.05



-87.35



-87.42


1.64



1.34



0.94


-89.17 0.96


-87.82


1.03


'Duration of immersion portion of experiment used in data regresion. 2Mass of the H20
(in percentage) absorbed. 3Partial molar enthalpy of hydration in analcime calculated by
DSC and dTGA. 4Partial molar enthalpy of hydration in analcime calculated from
cumulative, baseline-corrected DSC and mass of absorbed H20. 5Average partial molar
enthalpy of hydration in analcime, calculated by averaging values of Ahhyd1 and Ahhyd2 at
the same temperature.


2.98
3.70

3.29
3.03
2.64

2.92
3.02

2.68
2.49

2.05
2.05
1.97










Table 2-2. Heat capacities of hydrated and dehydrated analcime, steam, and hydration of
analcime at different temperatures.
1 3 45
T (K) Cp, an Cp, an2 Cp, dean Cp, H203 ACp, r4 AC, r5
(J/molK) (J/molK) (J/molK) (J/molK) (J/mol-H20/K) (J/mol-H20/K)


298
300
310
320
330
340
350
360
370
380
390
400
410
420
430
440
450
460
470
480
490
500
510
520
530
540
550
560
570
580
590
600
610
620
630
640


210.44
211.01
213.54
216.74
219.42
222.37
225.43
228.29
231.09
234.20
237.62
241.94
246.84
252.23
258.35
264.89
272.64
281.53
291.85
304.33
318.08
332.91
350.21
368.18
386.85
880.35
429.12
452.40
478.75
509.19
547.49
594.89
656.84
733.15
811.90
880.35


223.11
225.25
227.67
230.13
232.63
235.16
237.71
240.30
242.90
245.53
248.18
250.85
253.54
256.24
258.95
261.68
264.42
267.17
269.93
272.70
275.48
278.27
281.06
283.87
286.67
289.49
292.31
295.14
297.97
300.81


162.32
162.95
165.92
168.47
171.25
173.70
175.96
178.56
180.98
183.12
185.45
187.77
190.30
192.94
194.82
196.51
198.58
200.60
202.34
204.54
207.08
208.75
210.21
211.76
213.41
215.72
217.98
220.04
222.03
223.13
225.03
225.81
227.57
228.74
229.89
230.09


33.59
33.59
33.58
33.59
33.61
33.64
33.69
33.74
33.80
33.88
33.95
34.04
34.13
34.22
34.32
34.42
34.53
34.63
34.75
34.86
34.97
35.09
35.21
35.33
35.46
35.58
35.71
35.83
35.96
36.09
36.21
36.34
36.47
36.60
36.73
36.87


13.81
13.77
13.33
13.97
13.85
14.31
15.05
15.25
15.56
16.45
17.45
19.33
21.58
24.19
28.27
32.96
38.44
45.10
53.44
63.46
74.39
87.23
102.72
118.77
135.41
619.22
172.32
193.09
216.97
245.75
281.49
327.29
386.46
460.35
536.68
603.78


12.77
12.26
12.20
12.45
12.53
12.64
12.59
12.43
13.05
13.88
14.34
14.88
15.69
16.08
16.13
17.05
18.20
19.26
20.23
20.56
20.95
21.53
22.21
23.75
24.52
26.40
27.31
28.81
30.34
32.80


'Cp, an and Cp, dean are heat capacities of hydrated and dehydrated analcime collected in
this study. 2Heat capacity of hydrated analcime from Johnson et al (1982). Heat capacity
of H20(g) from Robie and Hemingway (1995). 4Heat capacity of hydration in analcime
calculated by Cp, an and Cp, dean. 5Heat capacity of hydration in analcime calculated by
Cp, an2 and Cp, dean 1









Table 2-3. Enthalpy of hydration in analcime.

Composition Method Temperature AHhyd
(K) (kJ/mol)

(NaAl)0.96Si2.o406-H20 HF 298.15 -84.9 4.02

(NaAl)0.95Si2.0506-H20 TTD 298.15 -85.7 + 1.93

(NaAl)0.96Si2.o406-H20 HF 298.15 -73.9 4.44

NaAlSi206-H20 PE 298.15 -80.45

Not reported PE 569.15 -83.7 4.06

Not reported PE 660.15 -86.6 4.06
Methods: HF: determination of enthalpies of formation of hydrated and dehydrated
homologs by HF solution calorimetry; TTD: transposed temperature drop calorimetry;
PE: retrieval from phase equilibrium observations.
2Johnson et al. (1982)
3Ogorodova et al. (1996)
4Calculated from data of Barany (1962) as recalculated by Johnson et al. (1982)
Retrieved by Bish and Carey (2001) from observations of Balgord and Roy (1973)
6van Reeuwijk (1974)









































Figure 2-1. Crystal structure of analcime viewed down the b crystallographic axis (after
Mazzi and Galli, 1978). The framework of Si- and Al- centered tetrahedra are
surrounded by four oxygens. The big spheres denote the positions ofNa+ ions.
The small spheres are the positions of water molecules.










0,6

_ 0,5
0)

OA



0.2
E ^,


100


120


0,03



0.02 >



0.01



0.00


(D
0.0 .
os


-0.5


-1 0O


-1.5


140


time (min)
Figure 2-2. Example isothermal immersion experiment on analcime at 403K.
Simultaneously-recorded TGA and DSC signals for analcime as a function of
temperature. The first derivative of the TG curve is given by the curve labeled
dTGA. Region of the gray box denotes initial equilibration of sample at
experimental temperature under dry N2. The rest of the experiment was
conducted in the presence of a flow of humidified N2.


-start of humid atmosphere









0.6

0.4 -

0.2 Ahhyd = -85.39 kJ/mol

0 R2 = 0.9941

E -0.2

0 -0.4
o0
-0.6

-0.8

-1 I I
0.000 0.005 0.010 0.015 0.020

dTGA (mg/min)
Figure 2-3. Plot of DSC versus dTGA for the experimental results shown in Fig. 2-2. The
slope of the linear regression shown in the figure is proportional to Ahhyd.









0


-0.5
-0.5 hhyd = -85.27 kJ/mol

R2 1
E

0 -1.5


4) -2


-2.5
0.00 0.10 0.20 0.30 0.40 0.50

H20 absorbed (mg)

Figure 2-4. Heat evolved during absorption of water into analcime as a function of mass
absorbed. The slope of the linear equation can be transformed to Ahhyd.











AC,,r = 2R
r_----------------------------


Johnson
et al. (1982)


S-et al. (1996)
I -
I I


330


380


430
T (K)


van Reeuwijh
(1974)
I




\%


480


530


Figure 2-5. Enthalpy of hydration in analcime as a function of temperature. The value of
Ahhyd at 528 K is abnormal, may imply a trend that Ahhyd decreases and then
increases rapidly with increasing temperature, indicating a phase transition.


-78.0

-80.0


-82.0

-84.0


-86.0

-88.0

-90.0

-92.0


280


580










a


0.0-
Dehydrated analcime

-0.5


CD
-1.0-
S-.0 Hydrated analcime





-2.0-


-2.5- I I II
300 400 500 600 700 800 900

T (K)


b

14-

12

i 10

8-
0
SHydrated analcime
C 6


SDehydrated analcime

2


300 400 500 600 700 800 900

T (K)
Figure 2-6. The mass change (a) and DSC response (b) of hydrated and dehydrated
analcime collected at a scanning rate of 15 K/min under ultrapure N2.












800

700

600- Hydrated analcime

0 500

400

300-

200Dehydrated analcime
I I I I '
300 400 500 600 700 800 900

T (K)
Figure 2-7. Experimental Cp data as a function of temperature for hydrated and
dehydrated analcime.









49.884 6.0R


41.57 5.0R


E 33.256 4.0R
o5
E 24.942 3.0R


0 16.628 ,-' -2.0R


8.314 1.OR


0 0.OR
290 340 390 440 490 540 590 640
T (K)
Figure 2-8. The heat capacity of hydration in analcime as a function of temperature. The
dotted line depicts the results of ACp,r derived from data collected in this
study, and the solid line represents the ACp,r calculated by Cp data of hydrated
analcime from Johnson et al. (1982) and Cp data of dehydrated analcime from
this study. R is the gas constant.











-80-



-82-



-84-



-86-



-88-



-90-


400 410 420 430


I 0 I 0I
440 450 460


T (K)
Figure 2-9. Partial molar enthalpies of hydration in analcime vs. temperature. The slope
of the linear regression provides a magnitude for ACp,r.


AC = 17.112.4 JimolK
p,r





* a


470














CHAPTER 3
EXCESS HEAT CAPACITY IN NATROLITE HYDRATION

Introduction

Natrolite occurs in nature as an essentially stoichiometric mineral (composition

Na2Al2Si30102H20; Gottardi and Galli, 1985) and has one of the best-defined crystal

structures of any zeolite, especially with respect to H20 (in fact, it was the first zeolite

structure refined; Pauling, 1930; Peacor, 1973; Artioli et al., 1984; Gottardi and Galli,

1985; Joswig and Baur, 1995). The framework ofnatrolite is composed of Si- and Al-

centered tetrahedra. The arrangement of Si and Al in the tetrahedral sites is variable but

tends to be largely ordered (e.g., Alberti et al., 1995; Neuhoff et al., 2002). Channels

within the structure contain two Na+ ions and two H20 molecules per ten framework

oxygens oriented in zigzag chains (Meier, 1960; Alberti and Vezzalini, 1981) (Fig. 3-1;

Peacor, 1973). Each Na+ ion is six coordinated to four framework oxygens and two H20

molecules (Line and Kearley, 1998).

Natrolite occurs in a wide range of geologic environments, including soils (e.g.,

Ming and Allen, 2001), zeolite facies metabasites (Walker, 1960), and pegmatites around

alkaline intrusions (e.g., Andersen et al., 1990). Despite the widespread occurrence of

natrolite, its stability in geologic environments is poorly known. Previous studies mostly

focused on the crystal structure (e.g., Peacor, 1973; Alberti et al., 1982; Joswig and Baur,

1995; Sapiga and Sergeev, 2001) and order/disorder (Si, Al) distribution (e.g., Alberti

and Vezzalini, 1981; Hesse, 1983; Andersen et al., 1990; Ross et al., 1992; Neuhoff et al.,

2002) of natrolite. Only a few works have explicitly considered the thermodynamic









stability of natrolite (e.g., Johnson et al., 1983; Vucelic and Vucelic, 1985; Paukov et al.,

2002; Neuhoff and Keddy, 2004). Because natrolite occurs over a wide range of

temperature and pressure, temperature (and pressure) dependent processes such as

disordering and dehydration are important concerns in assessing the stability of this

mineral. This is especially true because dehydration of natrolite leads to a large decrease

in unit cell volume and a different space group (Peacor, 1973; Alberti and Vezzalini,

1983; Joswig and Baur, 1995; Baur and Joswig, 1996). After dehydration is complete, the

rotation of the chains of Si and Al terahedra occurs simultaneously with the contraction

of the lattice. Although natrolite can be completely rehydrated at low temperature, this

process is complicated by the presence of hysteresis during cycled dehydration (heating)

and rehydration (cooling). Unlike many zeolites, for which dehydration and rehydration

are completely reversible, rehydration in natrolite, occurs at lower temperatures than does

dehydration at a given chemical potential of water (van Reeuwijk, 1974).

The present study provides new insight into the thermodynamics of the dehydration

process in natrolite. Heats of hydration as a function of temperature and the heat capacity

of reaction are determined independently by differential scanning calorimetric (DSC)

methods. Comparison of the results of these measurements reveals the existence of

excess heat capacities and enthalpies of hydration, which in large part explain the cause

of hysteresis in the dehydration behavior of this mineral.

Methods

Sample and Characterization

The sample of natrolite was previously described and characterized by Neuhoff et

al. (2002; sample NAT001). It was collected as veins within a metabasaltic tectonic

inclusion at the famous Dallas Gem Mine benitoite and neptunite locality, San Benito









County, California. Phase pure separates were hand picked, ground in an agate mortar,

and sieved to a 20-40 [tm size fraction. Sample identification and purity were confirmed

by X-ray powder diffraction. The composition was determined by electron probe

microanalysis at Stanford University to be essentially stoichiometric

(Na2Al2Si3010inH20). Water content of the sample was determined in this study by

thermogravimetric heating to 1023 K after the equilibration with a room temperature

atmosphere of 50 % relative humidity (RH). The mass loss is about 9.49% of total sample

mass, very close to the ideal water content of natrolite (9.48%), and the water content

taken to be 2 moles of water per formula unit.

Heat Capacity Measurements

The heat capacities of hydrated and dehydrated natrolite were determined by DSC

with the Netzsch STA 449C Jupiter simultaneous DSC-thermogravimetric analysis

(TGA) system at the University of Florida. Each experiment consisted of four separate

runs: 1) determination of background and baseline by measurement of an empty crucible;

2) DSC measurement of a standard (sapphire); 3) scanning heating of hydrated natrolite

and 4) scanning heating of dehydrated natrolite. A sample of approximately 27 mg was

packed into a covered Pt-Rh crucible before step 3 and remained until completion of the

experiment. Data were collected in the range of 298 to 733 K (higher temperatures cause

an irreversible structure change for natrolite; Joswig and Baur, 1995) at a scanning rate of

15 K/min. A purge of dry N2 was used during the experiment to keep the relative

humidity below 1%, and the gas flow was maintained at -30 ml/min using mass flow

controllers. The furnace was heated to 733 K in each run and then cooled back to room

temperature by liquid N2 for next step. During heating, hydrated natrolite lost mass with

increasing temperature and finally became completely dehydrated. The resulting









dehydrated natrolite was kept in an environment of dry N2 to avoid rehydration during

cooling to room temperature and then measured by scanning heating again. Heat capacity

of the sample as a function of temperature was calculated by

mtd DSC, -DSCb
C = x b Cp, d (3-1)
S m DSCstd -DSCb td

where ms is the mass of sample, mstd is the mass of standard (27.314 mg), DSCs, DSCstd

and DSCb are for sample, standard and baseline respectively. Triplicate calorimetric

measurements were conducted and averaged, and the error of the result is within 1% of

the average value.

Absorption Calorimetry

Heats of hydration as a function of temperature were determined using an

isothermal DSC-based immersion technique described by Neuhoff and Wang (2006).

This approach combines the benefits ofDSC and gas absorption calorimetry. Twenty to

30 mg of hydrated natrolite was placed into the Pt-Rh crucible for each run. The sample

was dehydrated by scanning heating from 298 to 733 K at the rate of 15 K/min and then

allowed to cool to the experimental temperature. A purge of dry N2 was maintained at

flow rate of 50 ml/min during this period. After equilibration (20-40 min) under dry gas

at this temperature until both DSC and TGA baselines stabilized, the gas stream was

changed to humid N2 which was generated by bubbling N2 through saturated NaCl

solution. In order to reduce the change of DSC baseline, the flow rate of humid N2 was

maintained at 30 ml/min and the corresponding water vapor pressure in the furnace was

-12 mbar. Under this condition the sample was allowed to react until the DSC and TGA

baselines stabilized again. Experiments were repeated on the same sample but indicated a









progressive loss of hydration capacity and decrease in hydration heat. Consequently, a

fresh aliquot of sample was used for each experiment.

Results

Heat Capacities of Hydrated and Dehydrated Natrolite

The TGA and DSC traces of hydrated and dehydrated natrolite obtained during

scanning heating are shown in Fig. 3-2. It can be seen that natrolite starts dehydrating at a

relatively high temperature, -450 K, and dehydration occurs over a fairly short range of

temperature (450-673 K). The TGA trace also suggests that there is only one

energetically distinct water site in natrolite, because the curve is continuous and does not

show any inflections. In addition, only one peak is observed in the DSC and first

derivative of TGA (dTGA) signals, consistent with this interpretation. The TGA curve of

dehydrated natrolite shows no mass change during the scanning heating, which also

indicates that the sample had been fully dehydrated in the first run and the condition for

the experiment was dry enough to avoid rehydration of the sample.

Heat capacities of hydrated and dehydrated natrolite calculated from the DSC in

Fig. 3-2b are shown in Table 3-1. Figure 3-3 depicts the Cp data of both hydrated and

dehydrated natrolite determined by DSC data from the same experiment from 320 to 680

K. The heat capacity of dehydrated natrolite is relatively insensitive to the temperature.

Calculated Cp for hydrated natrolite exhibits a strong positive inflection at -450 K

reflecting excess heat associated with dehydration of the sample.

Enthalpy of Hydration in Natrolite

An example of TGA and DSC response of natrolite during immersion in water

vapor at 412 K is shown in Fig. 3-4. Note that after the atmosphere changed, but before

the onset of rehydration, the DSC baseline does not change. The marked changes in both









TGA and DSC signal shortly after the atmosphere changed reflect absorption of water

vapor by the sample. The mass of the sample increases as it absorbs more water, and the

absorption rate is essentially constant for most of the reaction. Rehydration of natrolite is

relatively rapid, with the whole process only taking about 110 minutes. Once hydration is

complete, the baseline of TGA and DSC becomes stable again.

The heat flow of the natrolite hydration is proportional to the area under the DSC

curve, which allows a calculation of the enthalpy of hydration (AHhyd) by the equation:

AHhyd = -18.015A / kmgain (3-2)

where A is the area under the DSC curve, gain is the mass gain in the rehydration and k is

the caloric calibration factor. The results of AHhyd for natrolite at four different

temperatures are listed in Table 3-2 and shown in Fig. 3-5. The values of AHhyd at the

same temperature are very close (<0.5% difference), and the errors include the part from

standard deviation of the values and that from the calorimetric calibration (1% of the

results), which have been investigated to be within 1.5%.

It can be seen in Table 3-2 that the degree of rehydration mainly depends on the

temperature. The masses of water absorbed during the rehydration at the same

temperature are similar, except one at 412 K with a mass gain of about 9.34%, which

may result from an abnormally higher water vapor pressure. Rehydrations of natrolite

were also attempted at higher temperatures. Generally, the degree of rehydration

increases with decreasing temperature below -432 K, even though the difference between

the mass gains during the rehydration at different temperatures is small (with only about

0.5% difference from 323 to 432 K). However, at temperatures above -432 K the degree

of rehydration decreases rapidly with increasing temperature. The fraction of water









absorbed during the rehydration drops by about 1.4% from 432 to 482 K. The sample of

dehydrated natrolite can not rehydrate when the temperature reaches 522K (the fraction

of water aborbed <0.1%). This behavior is also manifested by the mass change of

natrolite in the scanning heating (with a heating/cooling rate of 5 K/min) dehydration-

rehydration experiment (Fig. 3-6). The whole process is run under a humid condition

with a constant water vapor pressure (-12 mbar). Note that the rehydration does not

commence until the temperature decreases to about 513 K, similar to the results of the

isothermal immersion experiments in which natrolite could not rehydrate above 522 K.

Discussion

Comparison with Previous Results

Johnson et al. (1983) have measured the Cp of natrolite from 5 to 350 K by

adiabatic calorimetric method, and Drebushchak (1990) determined the Cp of dehydrated

natrolite by DSC from room temperature to 800 K. In the present study we measured the

Cp for both hydrated and dehydrated natrolite from 143 to 703 K, which covers some

temperature ranges in the previous studies. The data presented here for hydrated and

dehydrated natrolite are the average values of the results of three measurements starting

from 143 K. These data are compared to previously published results in Fig. 3-7. It can

be seen that the results of present study agree well with those of Johnson et al. (1983) and

Drebushchak (1990). For instance, at 298.15 K Cp of hydrated natrolite measured in this

study differs within 0.5% of that determined by Johnson et al. (1983), and the difference

between the Cp of dehydrated natrolite in this study and that of Drebushchak (1990) is

about 0.7%. However, Cp data for dehydrated natrolite in the present study exhibits a

small peak near 500 K, indicating a small, reversible X phase transition, that is not present

in the data of Drebushchak (1990).









The various determinations of AHhyd from the literature are listed in Table 3-2, and

no correlations between AHhyd and temperature can be observed from these values.

Compared to these values, the result of AHhyd at 451 K in this study agrees within error

with that of Guliev et al. (1989) determined by immersion calorimetry at a similar

temperature. In addition, the value of AHhyd in this study increases with increasing

temperature, showing that the hydration of natrolite becomes less energetic at higher

temperatures, which is consistent with thermodynamic theory. According to the theory

and the similar difference between the vaules of AHhyd in this study, the AHhyd at 298.15

K is interpreted to be similar to the one of Kiseleva et al. (1997) determined by

transposed temperature drop calorimetry. The other values of AHhyd for natrolite reported

in the literature were determined at significantly different temperatures; however, all of

the data are similar in magnitude (and mostly within error) of the results from this study.

Heat Capacity of Hydration in Natrolite

Hydration of natrolite can be represented by the reaction

Na2Al2Si3010 + 2 H2Ovapor = Na2Al2Si3010o2H20 (3-3)
dehydrated natrolite natroltie

for which the standard heat capacity change (ACp,r) is given by

ACp,r = Cp,nat Cp,denat Cp,steam (3-4)

where Cp,nat and Cp,denat are the heat capacities of hydrated and dehydrated natrolite,

respectively; Cp,steam is the heat capacity of water vapor. The results of present study

provide two independent means of assessing the magnitude of ACp,r. The first is by direct

calculation from measured Cp for hydrated and dehydrated natrolite along with Cp,steam

(Robie and Hemingway, 1995). The second is by noting that the heat capacity is defined

as the differential change in heat with respect to temperature










ACp =r -) (3-5)


thus the first derivative of calculated values of AHhyd with respect to temperature also

provides a means of assessing ACp,r.

Values of ACp,r consistent with the Cp results listed in Table 3-1 and shown in Fig.

3-3 are given in Table 3-1 and illustrated in Fig. 3-8. It can be seen that ACp,r increases

steadily from value of 4 J/mol-H20/K at 298.15 K to -16.2 J/mol-H20/K at 360 K.

Above 360 K until the sample starts to dehydrate at about 420 K, ACp,r is relatively

constant.

The temperature dependence of ACp,r between room temperature and 360 K is

indicative of a phase transition occurring in this region. Similar transitions have been

noted in water molecules in synthetic zeolites A and X (Vucelic and Vucelic, 1985;

Basler and Lechert, 1972) in which water molecules appear to change their state of

motion over a protracted range of temperature, similar to a glass transition. This behavior

is in contrast to the relatively sharp, X-type transitions observed in water molecules in

laumontite (Neuhoff and Bird, 2001) and wairakite (Neuhoff and Keddy, 2004).

The relative temperature insensitivity of ACp,r above 360 K is consistent with

statistical-mechanical model of the behavior of confined water molecules. Carey (1993),

following Barrer (1978) used statistical-mechanical reasoning to suggest that the

difference in Cp between absorbed H20 and the gas phase is caused by the loss of

translational and rotational degrees of freedom to weak vibrational modes within the

zeolite structure. Thus, the heat capacity of hydration in zeolites reflects the transition

between translational/rotational and vibrational modes. With this model, the ACp,r

increases by 0.5 R for every vibrational mode gained by the absorbed species, up to a









maximum of 3R (Bish and Carey, 2001). The heat capacity of hydration for natrolite in

our study is relatively insensitive to the temperature between 373 and 403 K, and the

value is almost constant, -16.24 J/mol-H20/K. According to the statistical-mechanical

model, it is reasonable to suppose that the ACp,r of hydration in natrolite is about 2R.

In contrast, the temperature dependence of AHhyd data in Table 3-2 and Fig. 3-5

indicate a significantly higher value of ACp,r. Linear regression of these data indicates an

average value of ACp,r = 68.0 22.7 J/mol-H20/K. This value is nearly four times the

value determined by direct measurement of Cp for the phases in the reaction. Although

the limited range of temperature and number of data points preclude rigorous analysis of

the temperature dependence of ACp,r, the essentially linear trend of these data suggests

that this property is relatively temperature-invariant over this temperature range. The

discrepancy between ACp,r determined directly from Cp measurements of individual

phases and that assessed via equation 3-5 indicates that there is an excess contribution to

Cp of mixing in this region. Such a contribution would not be detected in direct Cp

measurements (and for reasons cited below can not be assessed for partially hydrated

natrolite), further underscoring the need for direct measurements of heat capacities and

enthalpies of hydration in zeolites. The presence of excess Cp of mixing in the solid

solution between natrolite and dehydrated natrolite indicates that there must also be

excess properties in the integral properties of Cp of mixing, namely the Gibbs energy,

enthalpy, and entropy of mixing.

Nature of the Natrolite-Dehydrated Natrolite Solid Solution

The behavior of natrolite observed in this study suggests that the solid solution

between its hydrated and dehydrated forms behaves in a fundamentally different fashion

from that often observed in other zeolites. In many zeolites, dehydration is fully









reversible, with similar water content observed under identical conditions of temperature,

pressure, and the chemical potential of H20 during both dehydration and rehydration

(e.g., Balgord and Roy, 1973; Carey and Bish, 1996; Fialips et al., 2005). In natrolite,

however, it appears that significant hysteresis is observed between water contents

achieved during hydration and dehydration. We suggest that this phenomenon is the

result of solvus behavior of natrolite in this solid solution, which is consistent with the

behavior of natrolite during rehydration as well as the energetic of natrolite hydration

discussed above.

As shown in Fig. 3-6 that a strong hysteresis effect occurs after the dehydration of

natrolite, dehydrated natrolite could not follow the dehydration path and the rehydration

is initiated not until about 130 K below the end-point of dehydration. The strong

hysteresis is partly due to the high heating/cooling rate (5 K/min), for instance, with a

heating/cooling rate of 2 K/min the rehydration begins about 50 K below the end-point of

dehydration. Neimark et al. (2000) modeled hysteresis in water absorption in nanopores

as a solvus, and found that the hysteresis was related to the pore structure characterization

of the material. However, even when the dehydrated natrolite starts absorbing water at

about 513 K, the trace of mass change is still quite different from that of dehydration. The

rehydration of natrolite should have reached equilibrium below 513 K as indicated by the

isothermal immersion experiments. That means the hysteresis is probably present at

equilibrium, and this is also supported by the evidence that limited degree of rehydration

is possible above 473 K. Similar hysteretic phenomena were observed in the W1 site in

laumontite (Fridriksson et al., 2003) and smectite (Tamura et al., 2000) that are clearly

associated with the presence of two coexisting phases.









The rate of hydration in natrolite also shows a different feature from many other

zeolites. The hydration rate of most zeolites depends on the degree of hydration (the

concentration of reactant; e.g., analcime, Fig. 2-2). However, in natrolite the TGA signal

increases monotonically during hydration until leveling off after reaction was complete

(Neuhoff and Wang, 2006). Thus, the rate of hydration in natrolite is independent of the

hydration degree, and the hydration of natrolite is a zero order reaction. This can be

explained by the solvus behavior of natrolite. During the hydration of natrolite, two

immiscible solutions (hydrated and dehydrated natrolite) coexist as a mechanical mixture,

so the concentration of dehydrated natrolite remains constant during the whole process.

The phenomena of hysteresis and zero order kinetics are not observed in many zeolites,

such as chabazite (van Reeuwijk, 1974), mordenite, analcime and K-clinoptilolite (Bish

and Carey, 2001), etc. Consequently, we believe that the special characters of hydration

in natrolite (like excess heat capacity, hysteresis effect and zero order kinetics) are mainly

attributed to the solvus behavior of natrolite in solid solution.












Table 3-1. Heat capacities of hydrated and dehydrated natrolite, steam, and hydration of
natrolite at different temperatures.

T (K) Cp, an (J/molK) Cp, dean (J/molK) Cp, H20 (J/molK) ACp, r (J/mol-H20/K)


298.15
300
310
320
330
340
350
360
370
380
390
400
410
420
430
440


360.37
362.00
370.62
379.48
388.27
396.55
403.94
410.16
415.54
420.39
424.95
429.74
434.48
439.87
446.04
453.23


285.30
286.29
290.89
295.25
299.33
303.40
307.80
312.17
316.43
320.53
324.59
328.34
332.18
336.13
340.06
344.13


33.59
33.59
33.58
33.59
33.61
33.64
33.69
33.74
33.80
33.88
33.95
34.04
34.13
34.22
34.32
34.42


3.94
4.27
6.28
8.53
10.87
12.93
14.38
15.26
15.75
16.05
16.23
16.67
17.03
17.65
18.67
20.13











Table 3-2. Isothermal immersion calorimetric data for natrolite.

1 2 2 AHhyd AHhyd, ave Error
T Sample mass Duration Mass gain2 AHhyd AHhyd, ae3 Error
S(kJ/mol- (kJ/mol- (kJ/mol-
(K) (mg) (min) (%) H20) H20) H20)
H20) H20) H20)
412 23.97 121 9.18 -97.16 -97.37 0.99
412 25.41 100 9.17 -97.50
412 25.48 95 9.34 -97.44

432 24.4 87 9.18 -95.74
432 24.24 83 9.24 -95.49 -95.69 0.97
432 29.76 117 9.21 -95.84

451 29.44 127 9.17 -94.70
451 23.6 114 9.15 -94.55 -94.67 0.95
451 27.56 109 9.14 -94.77

472 29.34 130 8.96 -93.21
472 27.45 126 8.93 -93.21 -93.19 0.93
472 25.73 118 8.94 -93.16
Duration of immersion portion of experiment used in data regression. Mass of the H20
(in percentage) absorbed in that period. 3The average value of AHhyd, as the integral
enthalpy of hydration in natrolite.











Table 3-3. Enthalpy of hydration in natrolite.

Composition Method Temperature AHhyd
(K) (kJ/mol-H20)

(NaAl)2Si201io2H20 TTD 298.15 -101.7 + 3.62

(NaAl)2Si2010o2H20 PE 900 -1083

Not reported PE 684.15 -102.9 4.04

Not reported DSC 623.15 -89.15

(NaAl)2Si2010o2H20 IM 453.15 -100.0 5.06
Methods: TTD: transposed temperature drop calorimetry; PE: retrieval from phase
equilibrium observations; DSC: calculated from scanning DSC measurement; IM: heat of
immersion in water.
2Kiseleva et al. (1997)
3Hey (1932)
4van Reeuwijk (1974)
5van Reeuwijk (1972)
6Guliev et al. (1989)











































Figure 3-1. Crystal structures of natrolite down the c crystallographic axis. (after Peacor,
1973). The framework of Si- and Al- centered tetrahedra are surrounded by
four oxygens. The big spheres denote the positions ofNa+ ions. The small
spheres are the positions of water molecules.










a
0.5

0.0
Dehydrated
-0.5 natrolite

S-1.0-
E Hydrated natrolite
^ -1.5
E

-2.0

-2.5

-3.0

-3.5- -,- i ,,
300 400 500 600 700

T(K)


b
100-


80-


S60-
E
0
4 40


20
Hydrated natrolite

0 Dehydrated natrolite

300 400 500 600 700

T(K)
Figure 3-2. The mass change (a) and DSC response (b) of hydrated and dehydrated
collected at a scanning rate of 15 K/min under ultrapure N2.










3500

3000

2500

0 2000

2 1500

1000
Hydrated natrolite
500
Dehydrated natrolite
0 I I I I III
300 350 400 450 500 550 600 650 700

T(K)
Figure 3-3. Heat capacities of hydrated and dehydrated natrolite as a function of
temperature from 320 to 680 K, which are calculated from the DSC data
collected in the same scanning heating experiment.









3.0 0,04 2

2 5 r DSC
2.5 DS

TGA -
: 2.0 dTGA 0.03 C 1 D

0- 1.5


0.0002
J 1.0 /- t ( "
S0.5 oo001 -1--


start of humid atmosphere
.... _-_0.00 .-2
40 60 80 100 120 140
time (min)
Figure 3-4. Example immersion calorimetric experiment on natrolite conducted at 412 K.
Simultaneously-recorded TGA and DSC signals for natrolite as a function of
temperature. The first derivative of the TG curve is given by the curve labeled
dTGA. Region of the gray box denotes initial equilibration of sample at
experimental temperature under dry N2. The rest of the experiment was
conducted in the presence of a flow of humidified N2.










-91
ACp,r = 68.0 22.7 J/mol-H20/K
94 R2 = 0.9936 -
OQ This study
-97

-100 Kiseleva et al. Guliev et a4
Mc (1989)
I o (1997)
< -103

-106
290 330 370 410 450 490
T (K)

Figure 3-5. The enthalpy of dehydration in natrolite as a function of temperature. The
slope of the linear regression is the value of ACp,r.


























280 340 400 460 520
T (K)


580 640 700


Figure 3-6. Mass change of natrolite in the dehydration and rehydration with a
heating/cooling rate of 5 K/min under a humid condition (PH2o012 mbar). A
significant hysteresis occurs in the rehydration of natrolite.


102

100


()

.C


Cu













600

500


2 400
0
E 300

. 200


100

0


-This study
* Johnson etal. (1983)
A Drebushchak (1990)


Hydrated natrolite


A A


Dehydrated natrolite


200


400


T (K)


600


800


Figure 3-7. Comparison of present and previous Cp data for both hydrated and
dehydrated natrolite. Our data are the average values of Cp from three
different measurements starting from 143 K.


1000










30


25-


o
20-


15-


< 10


5-



280 300 320 340 360 380 400 420 440 460 480

T(K)
Figure 3-8. The heat capacity of hydration in natrolite as a function of temperature.














CHAPTER 4
CONCLUSION

Comparison of the Results of Analcime and Natrolite

Heat capacities of hydration directly measured by DSC technique for analcime and

natrolite behave differently with increasing temperature. However, they both have a

temperature range in which ACp,r is relatively insensitive to the temperature, 298 to 430 K

for analcime and 370 to 410 K for natrolite. The data in these ranges conform to the

statistical-mechanical model which argues that ACp,r is related to the weak vibrational

modes for absorbed H20 compared with rotational and translational freedom in the

vapor-phase molecule and it is independent of temperature (Carey, 1993). However,

enthalpies of hydration in analcime and natrolite determined by an isothermal DSC-based

immersion technique at different temperatures illustrate two phenomena that complicate

application of the statistical mechanical model for ACp,r. In natrolite, a linear relationship

exists between AHhyd and temperature, indicating a temperature independent ACp,r.

However, the ACp,r for natrolite hydration calculated by this approach is about 68.0 +

22.7 J/molK, nearly four times the value determined by the model. A temperature

independent excess heat capacity is suggested to exist in the hydration of natrolite. This

excess heat capacity is probably related to the solvus behavior in the natrolite-dehydrated

natrolite solid solution. In analcime, AHhyd between 298 and 470 K is relatively

insensitive to the temperature and implies a value of ACp,r of approximately 2R. This

result is consistent with that of the calorimetric ACp,r, showing the applicability of

statistical-mechanical model for analcime in the relatively low temperature range.









However, the abrupt change of AHhyd above 470 K suggests a phase transition which has

a significant effect on ACp,r, and can even make the ACp,r become negative.

The thermodynamics of dehydration and rehydration in rock-forming zeolites are

different even for the simple-structure materials like analcime and natrolite. The behavior

of ACp,r is more complicated as a function of temperature than expected by the statistical-

mechanical model. That model may be applicable for certain zeolites under some low

temperature conditions (e.g., 298.15 K), but it is probably invalid for the zeolites

containing phase transition during dehydration and rehydration. The enthalpy of

hydration in zeolites determined by the isothermal immersion technique is critical for

understanding these processes.

Heuristic Outcomes of This Study

This study generated two unparalleled datasets on the temperature dependence of

the heat of hydration in zeolites that both complement existing data and enhance our

understanding of this important geochemical process. The thermodynamic properties of

the zeolites studied here provide a better understanding of the stability and reactivity of

these materials in the earth's crust. In addition, this study also provides important data for

assessing the reactivity of these minerals and for developing new industrial applications

for them. For instance, the thermodynamics of ion exchange by zeolites (which are a

critical process in radioactive waste disposal, wastewater treatment, and soil remediation;

e.g., Kallo, 2001; Pabalan and Bertetti, 2001; Ming and Allen, 2001) is largely a function

of the change in hydration state accompanying this process. This study also has a direct

impact on the study of other hydrate minerals, especially hydrated nanomaterials. The

methods and insights gained from this study can be used to develop new investigations of









the relationship between hydration/dehydration processes and nanophase stability in other

mineral systems.

Future Work

There are still several problems left unresolved in this study, for example, the

reason that lead to the phase transition in the hydration of analcime, the behavior of ACp,r

in the higher/lower range of temperature for analcime hydration, the real cause of the

excess heat capacity for natrolite dehydration. More experiments will be done to get the

specific data to address these problems. Future work will also include the

thermodynamics of dehydration and rehydration on the complex zeolites like chabazite,

wairakite etc. Furthermore, some geologic observations of the temperature and pressure

conditions under which zeolites (e.g., analcime, natrolite) form in nature will be done to

compare the results determined in the lab with these observations. This will be significant

to the development of the techniques in our study.
















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BIOGRAPHICAL SKETCH

Jie Wang was born in Ezhou, Hubei Province, China, in January 1981. He lived in

with his parents and attended elementary and high schools in that small town until

entering the University of Science & Technology of China in 1999. He graduated with a

B.S. in earth and space sciences in 2004. In August of 2004 he entered the University of

Florida and continued his graduate study in the Department of Geological Sciences. He

worked under Dr. Philip Neuhoff s direction and studied the thermodynamics of

dehydration and rehydration in zeolites. Upon completion of master's study, he will keep

on working with Dr. Neuhoff for his Ph.D. degree.