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Thermodynamics of 1

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Title: Thermodynamics of 1 2 Sodium Borate Minerals
Physical Description: 1 online resource (58 p.)
Language: english
Creator: Ruhl, Laura
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: borate, borax, kernite, properties, sodium, thermodynamic, tincalconite
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The stability of borate minerals is an important consideration for assessing biogeochemical processes in primordial and extant Earth-surface environments, in addition to many industrial applications. Borate minerals are found in a variety of environments from playas to evaporite deposits and serve as the most important industrial source of boron used in glass production, fire retardation, and cleaning purposes. Despite the geological and industrial importance of borate minerals, their phase stability and formation remains poorly constrained. In the present study, the thermodynamic properties of borax Na2B4O5(OH) 4?8H2O, tincalconite Na2B4O5(OH) 4?2.667H2O, kernite Na2B4O6(OH) 2?3H2O, and the reactions between them were assessed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hydrofluoric (HF) acid solution calorimetry, X-ray powder diffraction (XRPD), and equilibrium observations. Observations of equilibrium between borax and tincalconite were conducted as a function of temperature and relative humidity (RH) using saturated salt solutions. Mass changes in TGA and isotherm measurements confirm the stoichiometry of tincalconite to be Na2B4O5(OH)4?2.667H2O. The minimum relative humidity of borax equilibrium is 64.92% at 298.15, 74.68% at 313.15K, and 89.19% at 328.15K. The maximum relative humidity in equilibrium with tincalconite increases from 52.89% at 298.15K to 71.00% at 313.15K to 80.7% at 328.15K to 79.85% at 338.15K. At 338.15K and 95% humidity the sodium borate deliquesced. The increase in the equilibrium constant for the dehydration reaction from borax to tincalconite is consistent with an enthalpy of dehydration (to water vapor) at 323.15K of -55.3 +/- 2.0 kJ/mol of H2O. This is also consistent with the enthalpy of dehydration determined by HF solution calorimetry of -54.33 +/- 0.24 kJ/mol of H2O and by DSC of -55.6kJ +/- 1.1 kJ/mol of H2O. The enthalpy of dehydration from borax to kernite and tincalconite to kernite determined by HF calorimetry was -56.8 kJ/mol and -1.96 kJ/mol respectively. These results, along with heat capacities for borax, tincalconite, and kernite determined by DSC and molar volumes determined by XRPD, were used to assess the thermodynamic properties of borax dehydration as a function of temperature and pressure. The resulting phase diagram is consistent with geologic observations of Na-borate stability, indicating that in the Na2B4O7?H2O system borax is the primary precipitate and stable at earth's surface conditions. Tincalconite is metastable under most of these conditions and kernite is the result of secondary mineralization. Tincalconite holds a small stability field at 298.15K, therefore making it stable at surface conditions.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Laura Ruhl.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Neuhoff, Philip S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022766:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022766/00001

Material Information

Title: Thermodynamics of 1 2 Sodium Borate Minerals
Physical Description: 1 online resource (58 p.)
Language: english
Creator: Ruhl, Laura
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: borate, borax, kernite, properties, sodium, thermodynamic, tincalconite
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The stability of borate minerals is an important consideration for assessing biogeochemical processes in primordial and extant Earth-surface environments, in addition to many industrial applications. Borate minerals are found in a variety of environments from playas to evaporite deposits and serve as the most important industrial source of boron used in glass production, fire retardation, and cleaning purposes. Despite the geological and industrial importance of borate minerals, their phase stability and formation remains poorly constrained. In the present study, the thermodynamic properties of borax Na2B4O5(OH) 4?8H2O, tincalconite Na2B4O5(OH) 4?2.667H2O, kernite Na2B4O6(OH) 2?3H2O, and the reactions between them were assessed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hydrofluoric (HF) acid solution calorimetry, X-ray powder diffraction (XRPD), and equilibrium observations. Observations of equilibrium between borax and tincalconite were conducted as a function of temperature and relative humidity (RH) using saturated salt solutions. Mass changes in TGA and isotherm measurements confirm the stoichiometry of tincalconite to be Na2B4O5(OH)4?2.667H2O. The minimum relative humidity of borax equilibrium is 64.92% at 298.15, 74.68% at 313.15K, and 89.19% at 328.15K. The maximum relative humidity in equilibrium with tincalconite increases from 52.89% at 298.15K to 71.00% at 313.15K to 80.7% at 328.15K to 79.85% at 338.15K. At 338.15K and 95% humidity the sodium borate deliquesced. The increase in the equilibrium constant for the dehydration reaction from borax to tincalconite is consistent with an enthalpy of dehydration (to water vapor) at 323.15K of -55.3 +/- 2.0 kJ/mol of H2O. This is also consistent with the enthalpy of dehydration determined by HF solution calorimetry of -54.33 +/- 0.24 kJ/mol of H2O and by DSC of -55.6kJ +/- 1.1 kJ/mol of H2O. The enthalpy of dehydration from borax to kernite and tincalconite to kernite determined by HF calorimetry was -56.8 kJ/mol and -1.96 kJ/mol respectively. These results, along with heat capacities for borax, tincalconite, and kernite determined by DSC and molar volumes determined by XRPD, were used to assess the thermodynamic properties of borax dehydration as a function of temperature and pressure. The resulting phase diagram is consistent with geologic observations of Na-borate stability, indicating that in the Na2B4O7?H2O system borax is the primary precipitate and stable at earth's surface conditions. Tincalconite is metastable under most of these conditions and kernite is the result of secondary mineralization. Tincalconite holds a small stability field at 298.15K, therefore making it stable at surface conditions.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Laura Ruhl.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Neuhoff, Philip S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022766:00001


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1 THERMODYNAMICS OF 1:2 SO DIUM BORATE MINERALS By LAURA SUZANNE RUHL 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 2008

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2 2008 Laura Suzanne Ruhl

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3 To my family, who have kept my inquisitive spirit and thirst for knowledge alive, always encouraging me, and giving me strength.

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4 ACKNOWLEDGMENTS First and forem ost, I thank the chair of my co mmittee, Phil Neuhoff, for all of his time and all of the knowledge he has bestowed upon me throughout my time as his student. I would also like to thank my committee members, Ellen Martin and Guerry McClellan, for their time and support in my academic endeavors. I thank Jie Wang and Gkce Atalan for their help in the lab, for many scientific discussions and support. I also thank Derrick Newkirk for his unending encouragement and support. Major thanks go to my parents and si blings for always being there to help in any endeavor and th eir never-ending support and love. Lastly, I a ppreciate all of the never-ending encouragement from my friends and family. The National Science Foundation funded this project.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........6 LIST OF FIGURES.........................................................................................................................7 ABSTRACT.....................................................................................................................................8 CHAP TER 1 INTRODUCTION..................................................................................................................10 Mineralogy and Crystal Ch e mistry of Na-Borates.................................................................11 Geological Occurrence of Na-borate Minerals ....................................................................... 12 Questions Addressed in the Present Study............................................................................. 14 2 METHODS.............................................................................................................................22 Samples and Characterization................................................................................................. 22 Equilibrium E xperim ents........................................................................................................ 22 Calorimetry.................................................................................................................... .........23 3 RESULTS...............................................................................................................................25 Thermal Analysis....................................................................................................................25 Equilibrium Ob servation s....................................................................................................... 26 Heat Capacity.................................................................................................................. ........28 Calorimetric Observations of Heats of Hydration ..................................................................28 4 THERMODYAMIC MODEL AND PROPERTIES.............................................................. 38 5 DISCUSSION.........................................................................................................................43 Comparison with Previous Thermal Analysis Results........................................................... 43 Comparison with Previous Equilibrium Observations and Thermodynamic Results............ 44 Phase Relations between 1:2 Sodium Borates........................................................................46 Other Borate Systems........................................................................................................... ..49 6 CONCLUSIONS.................................................................................................................... 52 LIST OF REFERENCES...............................................................................................................53 BIOGRAPHICAL SKETCH.........................................................................................................58

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6 LIST OF TABLES Table page 1-1 Mineral phases, stoichiometries, and crys tal structures of 1:2 Na:B borate m inerals....... 17 3-1 Equilibrium experiment results showing the range of relative humidity at which the borax to/from tincalconite reaction takes place. ................................................................30 3-2 Heat Capacities of borax, tincalconite, a nd kernite m easured by differential scanning calorimetry.................................................................................................................... .....30 3-3 Measurements made during the HF experiments............................................................... 31 3-4 Thermochemical cycles employed in th e calculation of enthalpy of reaction. ..................31 3-5 HR calculated from HF calorimetric measurements and DSC calorimetric measurements.....................................................................................................................32 4-1 Thermodynamic properties of borax, tinca lconite, and kernite in this study..................... 42 4-2 Maier Kelley Coefficients of borax, tincalconite, and kernite. .......................................... 42

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7 LIST OF FIGURES Figure page 1-1 Borax borate structure..................................................................................................... ...18 1-2 Tincalconite borate structure..............................................................................................19 1-3 Kernite borate structure................................................................................................... ...20 1-4 Paragenetic relationships between borax, tincalconite, and kernite ..................................21 3-1 Thermal analyses of borax, tincalconite, and kernite........................................................ 33 3-2 Experimental observations of borax and tincalconite at 298.15K ..................................... 34 3-3 X-ray powder diffractograms of phase pure borax, tincalconite, and an interm ediate sample......................................................................................................................... .......35 3-4 Experimental observations of borax and tincalconite stability. .........................................36 3-5 Heat Capacities of borax tincalconite, and kernite. ..........................................................37 5-1 Phase diagram of borax, tincalconite, and kernite............................................................. 50 5-2 Phase diagram with geologic observations........................................................................51

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8 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 1:2 SO DIUM BORATE MINERALS By Laura Suzanne Ruhl December 2008 Chair: Phil Neuhoff Major: Geology The stability of borate minerals is an important consideration for assessing biogeochemical processes in primordial and extant Earth-surf ace environments, in addition to many industrial applications. Borate minerals are found in a vari ety of environments from playas to evaporite deposits and serve as the most im portant industrial source of boron used in glass production, fire retardation, and cleaning purposes. Despite the geological and industria l importance of borate minerals, their phase stability and formation remains poorly constrained. In the present study, the thermodynamic properties of borax [Na2B4O5(OH) 4H2O], tincalconite [Na2B4O5(OH) 42.667H2O], kernite [Na2B4O6(OH) 2H2O], and the reactions between them were assessed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), hydrofluoric (HF) acid solution calorimetry, X-ra y powder diffraction (XRPD), and equilibrium observations. Observations of equilibrium between borax a nd tincalconite were conducted as a function of temperature and relative humidity (RH) using sa turated salt solutions. Mass changes in TGA and isotherm measurements confirm the st oichiometry of tincalconite to be Na2B4O5(OH)42.667H2O. The minimum relative humidity of borax equilibrium is 64.92% at 298.15, 74.68% at 313.15K, and 89.19% at 328.15K. The maximum relative humidity in equilibrium with tincalconite increases from 52.89% at 298.15K to 71.00% at 313.15K to 80.7%

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9 at 328.15K to 79.85% at 338.15K. At 338.15K and 95% humidity the sodium borate deliquesced. The increase in the equilibrium constant for th e dehydration reaction from borax to tincalconite is consistent with an enthalpy of dehydration (to water vapor) at 323.15K of -55.3 +/2.0 kJ/mol of H2O. This is also consistent with the enth alpy of dehydration determined by HF solution calorimetry of -54.33 +/0.24 kJ/mol of H2O and by DSC of -55.6kJ +/1.1 kJ/mol of H2O. The enthalpy of dehydration from borax to kernite and tincalconite to kern ite determined by HF calorimetry was -56.8 kJ/mol and -1.96 kJ/mol respectively. These results, along with heat capacities for borax, tincalconite, and kernite de termined by DSC and molar volumes determined by XRPD, were used to assess the thermodynamic properties of borax dehydration as a function of temperature and pressure. The resulting ph ase diagram is consistent with geologic observations of Na-borate stabili ty, indicating that in the Na2B4O7H2O system borax is the primary precipitate and stable at earth's surface conditions. Tincalco nite is metastable under most of these conditions and kernite is the result of secondary mineralizati on. Tincalconite holds a small stability field at 298.15K, therefor e making it stable at surface conditions.

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10 CHAPTER 1 INTRODUCTION Although boron is relatively sc arce in the Earths crust (t wenty-seventh in abundance with an average content of 15 ppm; Anovitz and Grew, 1996), its aqueous and solid phase speciation play a critical role in biogeochemical processes. Seaw ater is enriched in boron relative to the crust, and acid-base equilibria involving boric acid are an important secondary influence on seawater pH (Dickson, 1990; Krauskopf a nd Bird, 1995). Despite its relatively low abundance, boron is an important accessory constituent of many rocks formed through hydrothermal, diagenetic, and metamorphic pr ocesses (Leeman and Sission, 1996) occurring dominantly as borosilicate minerals such as tourmaline (Anovitz and Grew, 1996). At Earth's surface, boron occurs most often in borate anion-based salts formed during evaporation and subsequent diagenesis of playa evaporite de posits (Anovitz and Grew, 1996). Surficial borate deposits serve as the most important industrial source of boron used in glass production, fire retardation, and cleaning purposes. They have also been observed to stabilize ribose, possibly playing a role in the beginning of life on Earth (Ricardo et al., 2004). Despite their wide geologic and industrial importance, the stability of borate minerals remains poorly constrained. The present study ad dresses this issue through an integrated thermodynamic study of the stability of the Naborate minerals, borax, tin calconite, and kernite that employs a combination of equilibrium, thermal analysis, and calorimetric methods. The following sections of this chap ter explore previous observations of the geologic occurrence and stability of these minerals. This discussion raises several critical questions that form the basis of the research described in later chapters.

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11 Mineralogy and Crystal Chemistry of Na-Borates Boron can form threeor four-coor dinated borate groups, denoted as B 3 and B 4, respectively ( : O2or OH-) (Kemp, 1956). It is the most el ectronegative element in its group, resembling a non-metal, like carbon, rather than the Group III metals (Kemp, 1956). Kemp (1956) also reports that crystalline borates structurally resemble the silicates, therefore just as the SiO4 tetrahedra can link together in comp lex silicates, so can the BO3 triangles. Borax, tincalconite, and kernite all consist of a borate-anion based formula with 1:2 Na:B ratios, but differ in their degree of hydration (Table 1-1). Borax [Na2B4O 5(OH) 4H2O] is a primary evaporite phase that often is pseudomor phed by the isostructura l but less hydrous phase tincalconite Na6[B4O5(OH) 4]3H2O (Pabst and Sawyer, 1948;Chri st and Garrels, 1959) when exposed to a dry atmosphere. Borate structures contains corner-sharing B 3 and B 4 polyhedra that polymerize to from sheets, chains, or fram eworks (Burns et al., 1995 ). Table 1-1 lists the number of triangularly coordinated (B 3) or tetrahedrally coordinated (B 4) borate ions in each of the minerals in this inves tigation (Burns et al., 1995). Borax and tincalconite each contain a polyanion with two borate triads a nd two borate tetrahedrons polymer ized to form the structures shown in Figures 1-1 and 1-2, respectively (Bur ns et al., 1995; Levy and Lisensky, 1978; and Luck and Wang, 2002). Their identical polyanion struct ures allow for easy transition from one to the other with relatively low activation en ergy (Christ and Garrels, 1959). Kernite [Na2B4O6(OH) 2H2O] is structurally distinct due to its intr icate chain of borate polyanions. Formation of kernite from either tincalconite or borax re quires higher activation en ergy, and is therefore considerably slower and is not readily reversible (Christ and Garrels, 1959). Figure 1-3 reveals the complexity of the kernite structure compared to borax and tincalconite (Grice et al., 1999). Compositional relations similar to those exhi bited by the 1:2 Naborates are exhibited by both Caand NaCa-borates. The calcium borates follow a sequence very similar to the sodium

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12 borates with myerhofferite [Ca2B6O6 (OH)102H2O], inyoite [Ca2B6O6 (OH)106H2O], and colemanite [Ca2B6O115H2O] differing only in their resp ective degrees of hydration. NaCa borates also mimic the Na borates, alt hough with only two hydration states. Ulexite [NaCaB5O6(OH)65H2O] and probertite [NaCaB5O7(OH)43H2O] are the NaCa high hydrates equivalent to borax and kernite, respectively. Geological Occurrence of Na-borate Minerals Most of the borate deposits m ined today are from fossil deposits formed thousands to millions of years ago. Searles Lake and Clear Lake (in California) are two borate deposits that are currently forming (Bowser, 1965). Searles Lake is currently undergoing mining in which the salt brine is pumped out from beneath a salt cr ust on the lake and left to evaporite, leaving behind the valued minerals (Bixler and Sawyer, 1957). Many deposits have a variety of other minerals accompanying the Na-borates (Bowse r, 1965). Some of the common minerals, in addition to the Na-borates, found at Sear les lake include ha lite (NaCl), trona (Na2CO3.NaHCO3.2H2O), nahcolite (NaHCO3), Aragonite/calcite (CaCO3), and dolomite (CaMg(CO3)2), in addition to many others (Eugster and Smith, 1965). Borax, tincalconite, and kernite are among the most common Na-borates found in economic borate deposits. Borates are common consti tuents of evaporite deposits in regions with a history of volcanism or hydrothermal activity (Woods, 1994). Evaporitic borate deposits occur in many countries throughout the world. The larg est volume producers of borates are the United States, Turkey, Russia, Kazakhsta n, Peru, China, Chile, Bolivia, Argentina (Warren, 2006). The US and Turkey provide 50% of the borates in the world on a product ton basis (Warren, 2006). Some of the most significant Na-borate deposits are found in California (USA), Argentina, and Turkey. Schaller (1936) discusses the Kramer Na-borate deposit as having been the replacement of Ca-borates and Na-Ca borates. Warren (2006) re ported that the mineral zonation seen in many

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13 of the borate deposits around the world were prev iously thought to be a diagenetic overprint (Inan et al.,1973), like that desc ribed by Schaller (1936). Later studi es have shown that some of these deposits are fractionated by hydrologically controlle d separation of evaporite minerals and reflect subsidence and climate (i.e. layering) (Warren, 2006 and Helvaci and Orti, 2004). Geologic and experimental observa tions generally indicate that borax is the first Na-borate mineral to form in evaporite deposits (in the Na2B4O7-H2O system), and later converts into tincalconite and/ or kernite (Chr ist and Garrels, 1959). Tincalc onite appears to be the first dehydration product from borax, forming readily at ambient surface temperatures when borax is exposed to air. When tincalconite is exposed to higher humidity, it returns to the borax phase (Christ and Garrels, 1959). The revers ibility of the borax to tincalc onite transitions appear to be dependent on humidity and temperature. At low temperatures, borax dehydrates either to an amorphous phase (Christ and Garrels, 1959) or converts to tincalconite The less hydrous and denser kernite becomes stable at elevated temperatures (Smith and Medrano, 1996). Kernite occurs at greater depths in ev aporite deposits, and appears to fo rm through thermal diagenesis of borax (Christ and Garrels, 1959). Field relations suggest that direct conversion of borax to kernite occurs at 331 5 K and at a depth of 760 150 m in the Boron deposit, California, without formation of tincalconite (Christ and Garre ls, 1959). Formation of kernite in the lab from borax or aqueous solutions has been successful only above 365 K (Christ and Garrels, 1959; Kemp, 1956). Kernite crystals grow slowly in laboratory conditions, indicating that kinetic factors may in part explain the discrepancy betw een geologic and laborator y observations of this paragenesis (Smith and Medrano, 1996). Tincalconite has been found as a primary mine ral at Searles Lake, California (Pabst and Sawyer, 1957, and Bowser, 1965). They reported that tincalconite crystals formed directly from

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14 solution, instead of psuedomorphing borax, as seen at the Kramer Deposit (in the pure Na2B4O7H2O system) (Christ and Garrels, 1959). Pabs t and Sawyer (1957) attribute the formation of primary tincalconite to th e presence of other ions, such as Na+, Cl-, SO4 -. The relative stabilities of borax, tincalc onite, and kernite can be described through chemical equations relating their stability in the presence of liqui d or gaseous water: OHOHOHOBNaOHOHOBNa2 2 4 542 2 4 542333.56.2)( 8 (1-1) borax tincalconite OHOHOHOBNaOHOHOBNa2 2 2 642 2 4 54263 8 (1-2) borax kernite and OHOHOHOBNaOHOHOBNa2 2 2 642 2 4 54223 6.2)( (1-3) tincalconite kernite Note that the stoichiometry of equations (1-1 ) and (1-3) are dependent on the stoichiometry of tincalconite, which is re-assessed in this study. Phase stability of these sequences is intriguing because borax converts rapidly to tincalco nite despite the apparent metastablity of the latter (Bowser, 1965). Figure 1-4 summarizes reported phase relations of this suite of sodium borate minerals. Questions Addressed in the Present Study The stability, formation, and phase relations of borax, tincalconite, and kernite are poorly understood. Borax is known to precipitate as a pr imary phase, but the occu rrence of tincalconite as a primary phase is controversial. Most author s consider tincalconite to be a purely metastable phase among 1:2 sodium borates in contrast to reports of its a pparent formation as a primary phase. Thus, does tincalconite have a true stabil ity field, and if so, under what conditions (e.g.,

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15 temperature, pressure, relative humidity)? Tincalconite and kern ite are often found as pseudomorphs of borax, but what conditions allo w mineral transformation to borax? In addition to the questions about tincalconite's stability, it s stoichiometry has been debated throughout the literature. Several researchers ha ve attempted to refine the stoi chiometry, but which is correct? A complete set of thermodynamic properties ha ve been measured for borax under standard conditions, which facilitates retrieval of the proper ties of the other substances from equilibrium observations. However, no experimental observations are available for the heat capacity (Cp) as a function of temperature of borax (or the other two minerals) ne cessary for evaluating properties of reactions at elevated temper ature. In addition, there are cons iderable discrepancies between reported values of the enthalpy a nd Gibbs energies of formation ( Hf and Gf, respectively). Therefore what are the thermodynamic propert ies of formation and reaction of borax, tincalconite, and kernite? Once the thermodynamic properties of the minerals are determined, they can be utilized to answer some of the questions addresse s above, although there may not be enough detail to answer all of them. The present study addresses the stability and paragene sis of Na-borates through experimental observations and modeling of the stability and thermodynamic properties of borax, tincalconite, and kernite. The thermodynamic propert ies were determined using a combination of differential scanning calorime try, hydrofluoric acid calorim etry, X-ray diffraction, and equilibrium observations. The stability of the mi nerals was addressing using new experimental techniques, such as the dual TGA-DSC (Neuhoff and Wang, 2007b) and equilibrium experiments. Experimental observations were synthesized through th ermodynamic modeling to assess the stability of each phase as a function of temperature, pressure, and system composition in order to constrain geochemical processes in B-rich systems at Earth's surface. Due to the

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16 similarities in the structure and behavior, these findings can be applied to other borate systems, like the Ca, Na-Ca, and Mgborates.

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17 Table 1-1. The mineral phases, stoichiometries, and crystal structures of 1:2 Na:B borate minerals. (Burns et al. 1995) Mineral Phase Stoichiometry Crystal Structure Borax Na2B4O 5(OH) 4H2O 2 B 3 triangular complexes and 2 B 4 tetrahedrons Tincalconite Na2B4O5(OH) 4.667H2O 2 B 3 triangular complexes and 2 B 4 tetrahedrons Kernite Na2B4O6(OH) 2H2O 3 B 3 triangular complexes and 4 B 4 tetrahedrons; 3 of the above 2 with one borate polyhedral connection

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18 Figure 1-1. Borax borate structure looking down the C axis. The green spheres represent sodium, the blue polyhedra are boron, and the red spheres are oxygen. Borax is made up of two borate triangles and two borate tetrahedrals. (Figure dr afted in XtalDraw (Downs, 2005) using atomic coordinates repo rted by Levy and Lisensky, 1978)

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19 Figure 1-2. Tincalconite borate structure looking down the C axis The green spheres represent sodium, the blue polyhedra are boron, and th e red spheres are oxyge n. Tincalconite is made up of two borate triangles and two borate tetrahedrals. (Figure drafted in XtalDraw (Downs, 2005) us ing atomic coordinates reported by Luck and Wang, 2002)

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20 Figure 1-3. Kernite borate stru cture looking down the C axis. The green spheres represent sodium, the blue polyhedra are boron, a nd the red spheres are oxygen. Kernite is made up of three borate tria ngles and four borate tetrah edrals. (Figure drafted in XtalDraw (Downs, 2005) us ing atomic coordinates re ported by Cooper et al., 1973)

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21 NaBO(OH)8HO Borax24542NaBO(OH)2.667HO Tincalconite2454 2NaBO(OH)3HO Kernite24622 Not Observed+/5.333 HO2 (in air)R e h y d r a t i o n D e h y d r a t i o nD i a g e n e s i s W e a t h e r i n g+/6HO2(Primary) (Primary or Secondary) (Secondary) Figure 1-4. Paragenetic relationshi ps between borax, tincalconite, and kernite. The reaction from borax to tincalconite involves a loss of 5.333 water molecules (per Na2) and is common during surficial diagenes is under relatively dry c onditions. Tincalconite can reconvert to borax by gaining 5.333 water mo lecules to become borax. The reaction from borax to kernite involves 6 water molecules, with borax undergoing thermal diagenesis to the less-hydrous phase ke rnite. Kernite can reconvert under humid conditions to borax. Direct reaction between tincalconite and kernite has not been observed.

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22 CHAPTER 2 METHODS Samples and Characterization The experiments described below were conducted on phase pure samples of borax, tincalconite, and kernite. Synthetic ultrapure (>99%) borax (sodium tetraborate decahydrate; Fisher Chemical) was used in the experiments a nd as a starting material for the preparation of tincalconite. Tincalconite was synthesized by dehydrating synthetic borax in a desiccator containing a saturated solution of LiCl (11.30% RH) at room temper ature (298.15 K) for one month. Natural kernite from Boron, CA (USA) was used in the experiments. Phase pure kernite was obtained by lightly crushing a large crystal fragment and hand-picked under a binocular microscope. Although the chemical composition of th e kernite sample used in this study was not determined directly due to difficu lties in quantifying B contents, it is assumed that this sample is stoichiometric as are all previously reported analyses from Boron, CA and other localities (Hurlbut et al., 1973). All samples were hand ground in an agate mortar. Sample purity was assessed by X-ray powder diffr action (XRPD) using CuKat 45 kV, and 30 mA, with typical scans covering 5-50 2 Water contents were measured by thermogravimetric analysis (TGA) by scanning heating in dry N2 at heating rates of 15 K/min from 298 to 973K on a Netzsch STA 449C Jupiter simultaneous differential scanning calorimeter-thermogravimetric analysis (DSCTGA) apparatus. Equilibrium Experiments Phase relations between borax and tincalconite were determined as a function of temperature and relative humidity using a modified version of the salt buffer methods described by Chou et al. (2002). Samples of borax and tin calconite (~0.20.4g) were placed in preweighed 0.5 ml Eppendorf tubes. Th e open Eppendorf tubes were then set in shell vials within

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23 scintillation vials containing various buffer salt solutions of known relative humidity (RH) ranging from ~ 5 to ~97% (saturated solutions of K2SO4, KCl, NaCl, NaNO3, NaI, LiI, LiBr, LiCl, KI, CoCl2, MgNO3, and MgCl2; Greenspan, 1977; or NaCl solutions of fixed molality; Cherife and Resnik, 1984). The borax and tincalc onite samples were reacted at 298.15 K, 313.15 K, 328.15 K, and 338.15 K for ~ 4 days, except for those at 298.15 K which were allowed to equilibrate for 4 weeks. Temperature was contro lled by submerging the sc intillation vials in a constant temperature water bath except those at 298.15 K which were placed in a climate controlled laboratory. Reaction pr ogress was assessed by monitoring mass changes of individual samples and phase identities of experimental products determined by XRPD. Calorimetry Heats of solution in HF of borax, tincalcon ite, and kernite were measured at Lafayette College using a calorimetric system described by Hovis and Roux (1993) and Hovis et al. (1998). Samples (~200 mg) were dissolved in 9 10.1 g (~ 1 L) of 20.1 wt % hydrofluoric acid (HF) at 323.15 K under isoperibolic conditions utilizing an internal sample container (Waldbaum and Robie 1970). Experiments were run in duplicate. Heat capacities of borax, tinca lconite, and kernite and the heat of dehydration of borax to tincalconite were measured by DSC on a Netz sch STA 449C Jupiter simultaneous DSC-TGA apparatus. Gas flow (ultrapure N2) was maintained at ~30 mL/min using mass-flow controllers. A multipoint temperature calibration curve wa s developed using the melting points of H2O, Ga, In, Sn, Bi, Zn, and Al along with the solid-solid transition points of ces ium chloride and quartz (Cammenga et al. 1993, Gmelin and Sarge 2000; Hhne et al. 1990; Sabbah et al. 1999, Price 1995). Due to the incompatibility of many of these materials with the Pt-Rh crucible used in the experiments, temperature calibration was conducte d in identical crucibles lined with a sub-mm think insert of alumina. Caloric calibrati on was accomplished using the DSC response of

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24 synthetic sapphire (Gmelin and Sarge 2000; Sabbah et al. 1999; Sarge et al. 1994; Stlen et al. 1996). All experiments were conducted in Pt-Rh cr ucibles with unsealed, pe rforated lids using sample masses between 20 and 30 mg. Heat capacity measurements were conducted from 273 to 315 K. Samples were run in duplicate with each experiment consisting of f our separate runs: (1) a background correction with an empty crucible; (2) a si ngle-crystal sapphire caloric ba ckground; (3) the mineral sample; and (4) repeat of the background correction. Standard response was compared between experiments to test for reproducibility. Co mpanion experiments on quartz yielded Cp measurements within 1% of pr eviously reported values. The enthalpy of dehydration of borax to tincalconite was measured at 348 K using a modified version of the technique describe d by Neuhoff and Wang (2007b) Measurements were made at a heating rate of 15 K per min by sca nning DSC measurements made on a Netzsch STA 449C Jupiter simultaneous DSC-TGA apparatus. Ultrapure N2 was used for the experiments. Each sample of borax was kept over a saturated K2SO4 solution until transfer to the crucible in order to prevent dehydration. For each run, 20-30 mg of borax was heated rapidly (15 K.min) from room temperature to 348 K under ultrapure N2. Temperature was held constant for a period of 2 hours to allow the sample to dehydrate. Reaction progress and heat effects were monitored by continuous application of th e DSC and TGA signals. Calculati ons of the heats of dehydration were performed based on correlations between the DSC and TGA signals following the methods of Neuhoff and Wang (2007b).

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25 CHAPTER 3 RESULTS Thermal Analysis Thermogravimetric analysis and differentia l scanning calorimetric observations of borax, tincalconite, and kernite are shown in Figure 31. Borax exhibits thr ee distinct dehydration events, as evidenced by the three endothermic p eaks in the DSC curve. This initial mass loss corresponds to a dehydration even t evidenced by the endothermic peak centered at 365 K. A second mass loss and dehydration event occurs at 391 K corresponding to a 25% mass loss. The third dehydration event occurred at 434 K with a mass loss of 22% Comparison with the results for tincalconite discussed below indicates that th e first two dehydration ev ents correspond to the transition from borax to tincalconite. The third dehydration event results in complete loss of water from borax to form anhydrous Na2B4O7 phase. The total mass loss to 975 K was 47%, in good agreement with the expected 47.24% loss e xpected from the stoichiometric amounts of molecular water and hydroxyl groups in the formula. Tincalconite exhibited one distinct dehydrat ion event. The dehydration event occurred at 434 K with a mass loss of ~29%. This matched the expected mass loss from release of its water molecules (Luck and Wang, 2002, and Pabst and Sawy er, 1998). At 918 K an exothermic peak in the DSC curve is present that does not correlate with any mass change. This peak may represent amorphization of the sample. Outside of this ex othermic peak, the tincalconite TGA and DSC curves replicate the latter portion of the borax curves. After the borax loses the initial 25% mass (its first two dehydration events), the TGA cu rve mimics the gradual mass loss shown in the tincalconite curve. The tincalconite DSC curve also matches the third dehydration event in borax, consistent with initial transformation of borax into tincalconite followed by complete dehydration of a tincalco nite phase in the borax experiments.

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26 Although the stoichiometry of tincalconite has traditionally been given as Na2B4O 5(OH) 4H2O (Muessig and Allen, 1957), recent X-ray diffraction results suggest that the stoichiometric tincalconite is somewhat less hydrous. Based on water site occupancies determined through single crystal diffracti on, Luck and Wang (2002) suggested that the composition of tincalcon ite corresponds to Na2B4O 5(OH) 4.667H2O. In the experiments shown in Figure 3-1, the initial two dehydration events (which are not observed in tincalconite) correspond to a mass loss of ~25.5%. Given molecu lar weights for borax and tincalconite of 381.36g/mol and 285.28g/mol, respectively (based on the formula unit proposed by Luck and Wang, 2002), the corresponding mass loss during the transition between these two phases should be 25.2%. This is consistent with the observations shown in Figure 3-1. In contrast, if the more hydrous stoichiometry of tincalconite were correct, the resulting mass loss would amount to only 23.5%. The discrepancy between this prediction and the experiment al results shown in Figure 31 is larger than the likely error in the expe rimental results (~0.1% of the mass change). Kernite exhibits two distin ct dehydration events, followed by a gradual loss of water resulting in a total mass loss of 26.4%. The initial mass loss corresponds to ~10% of the initial mass related to an endothermic reaction at 438 K. The second dehydration event occurred at 469 K with an additional 7% mass loss. The mass con tinues to decrease with temperature, finally leveling off at a 26.4% loss. The expected tota l mass loss from the dehydration of kernite is 26.1% (Hurlbut, 1973). Equilibrium Observations Figure 3-2 shows the results of an experime nt run at 298.15 K left to equilibrate over 4 weeks. Borax (open circles) did not lose any mass above an RH of 53%, below which mass loss signaled the dehydration to tincalconite. The ma ss of borax decreased systematically with decreasing humidity down to 18% RH, at whic h point the mass loss was 25.5% corresponding to

PAGE 27

27 complete transformation to tincalconite. The tinc alconite samples (dark circles) exhibited no mass change at RH values below 65%; above this humidity, tincalconite began to absorb water and gained mass as it converted to borax. The mass of tincalconite systematically increased with increasing relative humidity until 84% RH, at which point the mass gain was ~33% signaling the complete transition to borax. (The percentage in crease in hydration is greater than the percent decrease because of the smaller mass at comm encement in hydration reaction). As discussed below, the progressive decrease in the mass of borax with decreasing humidity and increase in the mass of tincalconite with increasing humidity reflects incomplete conversion due to kinetic limitations that depend on the driving force of the reaction (humidity) and do not reflect solid solution between these phases. Thus at 298.15 K equilibrium between borax and tincalconite occurred at 59 +/6% relative humidity. Figure 3-3 shows the results of XRPD character ization of borax, tinca lconite, and a sample of intermediate composition labeled "sample X" (borax sample that lost ~12.12% of mass) formed by partially dehydrating borax at 313.15 K and 48.42% relative humidity for 4 days. Sample X is analogous to the samples in Figure 32 that exhibited an intermediate mass loss (i.e. not entirely transformed to borax or tincalconite ). Phase pure borax and tincalconite are clearly distinguished with unique peaks for borax, 14.88, 15.54, and 18.32 2 while tincalconite peaks occur at 10.06, 18.84, and 20.24 2 The borax and tincalconite peaks are all present in the pattern of sample X. No additional peaks ar e observed in the pattern of sample X. This suggests that sample X is a mechanical mixture of borax and tincalconite rather than being a solid solution of these phases (which would be evidenced by shifting peak positions and/or identities).

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28 The results of the experiments as a functi on of temperature and relative humidity are summarized in Figure 3-4 and Ta ble 3-1. The black line in Figur e 3-4 represents equilibrium between borax and tincalconite calculated from the thermodynamic model described in Chapter 4. Also shown for comparison are previous observations of equilibrium between these two phases. It can be seen in Figure 3-4 and Table 31 that the RH at equilibrium between borax and tincalconite increases steadily from 59 6% RH at 298.15 K to 84.5 4.5% RH at 328.15 K. Borax was not stable at 338.15 K with tinca lconite stable up to 80% RH and borax transformation to tincalconite over this range of RH as we ll. At 338.15 K and 97% RH both samples deliquesced as evidenced by the formati on of an aqueous solution on the sample and an increase in mass above that of borax. Delique sence under these conditions indicates that a sodium tetraborate-saturated solution has an equilibrium vapor pressure lower than 97% at 338.15 K. Each phase reported was confirmed by XRPD. Heat Capacity The measured heat capacities (Cp) of borax, tincalconite, and ke rnite are listed in Table 32 and plotted as a function of te mperature in Figure 3-5. It can be seen in Figure 3-5 that Cp increases monotonically for each mineral over the temperature range shown, indicating that no phase transitions or dehydration events occu rred during the heat capacity measurements. Calorimetric Observations of Heats of Hydration Table 3-3 shows the measurements from the HF calorimetric experiments. The average enthalpy of solution ( H(sol)) for borax, tincalconite, and kernite, as well as errors taken to be twice the standard deviat ion of the individual experimental re sults, are shown as well. The error associated with borax, tincalconite, and kernit e is 0.1%, 0.7%, and 0.5% of the total heat of solution, respectively.

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29 The results given in Table 3-3, along with the previously determined Hsol for liquid water (G. Hovis, pers. communication), were used to calculate the enthalpies of dehydration reactions 1-1 through 1-3 using the thermochemical cy cles shown in Table 3-4. The resulting HR for reactions 1-1 through 1-3 at 323.15 K are shown in Table 3-5. Using the heat capacity measurements described above, the HR at 298.15 K was calculated from the relation dTC H Hcesubsp KR KR 15.323 15.298 tan, 15.298, 15.323, (3-1) and is also listed in Table 3-5. Evaluation of HR for reaction (8) was determined directly from simultaneous DSC-TGA measurements following the methods of Neuhoff and Wang (2007b). Due to lack of a suitable background, HR was calculated from a linear regressi on of the DSC signal and the first derivative of the TGA signal (dTGA; cf Neuhoff and Wang, 2007b) via the equation HR,T,P = kA ( m )-1 (MWH2O) (3-2) where A is the area of the DSC peak (in Vs), k is the calibration factor (in W/ V), m is the mass change measured by TGA, and MWH2O is the molecular weight of water. Over the range of mass change expected for the borax to tincalc onite transition, the DSC and dTGA signals were linearly correlated indicating that the nature of the reaction was c onstant over the whole range of reaction progress. This observation helped conf irm findings between individual experiments as well, despite visible differences in the shape of the DSC and dTGA peaks between experiments. The enthalpy of reaction at 323.15 K calculated from these experiments (taking into account the enthalpy of vaporization of water from Wagner andPru 2002) is given in Table 3-5 along with that evaluated via equation 8 at 298.15 K. It can be seen in Table 3-5 that HR determined through HF calorimetry and by DSC agree within error.

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30 Table 3-1. Equilibrium experiment results showing the range of relative humidity at which the borax to/from tincalconite reaction takes place. Temperature (K) Maximum RH for Tincalconite Stability (%) Minimum RH for Borax Stability (%) 298.15 52.98 64.92 313.15 71.00 74.68 328.15 80.70 89.19 338.15 79.85 NA Table 3-2. Heat Capacities of borax, tincalconite and kernite were measured by differential scanning calorimetry. Cp (J/molK) Temperature (K) Borax Tincalconite Kernite 253.15 308.8 255.15 311.0 257.15 313.3 259.15 315.7 261.15 318.1 263.15 320.6 265.15 323.0 267.15 325.5 269.15 327.8 271.15 330.3 273.15 535.2 332.8 313.4 275.15 539.6 335.4 315.4 277.15 544.3 337.2 317.5 279.15 548.3 339.8 319.3 281.15 552.5 342.3 321.3 283.15 556.7 344.9 323.4 285.15 560.6 347.3 325.2 287.15 564.5 349.7 327.0 289.15 568.6 352.2 329.0 291.15 572.6 354.6 330.8 293.15 576.5 357.1 332.7 295.15 580.5 359.8 334.6 297.15 584.5 362.4 336.4 299.15 588.6 365.2 338.3 301.15 592.5 367.8 340.0 303.15 597.0 370.7 342.0 305.15 601.4 373.6 343.9 307.15 606.2 376.6 345.9 309.15 611.0 379.7 347.9 311.15 616.4 383.1 350.2 313.15 622.4 386.7 352.6 315.15 355.4

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31Table 3-3. Measurements made during the HF experiments. Dissolved in acid of the preceding experiment. Table 3-4. Thermochemical cycles employed in the calculation of enthalpy of reaction. Equation number Reaction H R (kJ/mol) 1 Borax + HF = Solution 1 = H Sol(Borax) 2 Tincalconite + HF = Solution 2 = H Sol(Tincalconite) 3 H2O + HF = Solution 3 = H Sol(Water) 4 (Reaction 1-1) Borax = Tincalconite + 5.333H2O = H Sol(Borax) H Sol(Tincalconite) -5.333 H Sol(Water) 5 Kernite + HF = Solution 4 = H Sol(Kernite) 6 (Reaction 1-2) Borax = Kernite + 6H2O = H Sol(Borax) H Sol(Kernite) -6 H Sol(Water) 7 (Reaction 1-3) Tincalconite= 3Kernite + 2H2O = H Sol(Tincalconite) H Sol(Kernite) -5.333 H Sol(Water) Sample Assumed composition Formula Weight (g/mol) Sample Weight (g) Temperature change during dissolution (C) Mean Solution temperature (C) Calorimeter Cp before dissolution (J/deg) Calorimeter Cp after dissolution (J/deg) Enthalpy of solution from Cp before dissolution (kJ/mol) Enthalpy of solution from Cp after dissolution (kJ/mol) Average Enthalpy of Solution Standard Deviation Borax Na2B4O7.10 H2O 381.37 0.26 0.046 49.876 3866.3 3866.1 -256.83 -256.82 Borax* Na2B4O7.10 H2O 381.37 0.25 0.044 49.892 3874.5 3872.4 -256.70 -256.56 -256.73 0.255 Tincalco nite Na2B4O5(O H)4.2.6667H2O 285.29 0.25 0.071 49.933 3871.0 3871.1 -311.10 -311.10 Tincalco nite* Na2B4O5(O H)4.2.6667H2O 285.29 0.25 0.071 49.935 3870.3 3869.6 -312.16 -312.10 -311.61 1.191 Kernite Na2B4O6(O H)2-3H2O 273.28 0.20 0.059 49.920 3866.4 3865.3 -313.27 -313.18 Kernite* Na2B4O6(O H)2-3H2O 273.28 0.20 0.060 49.932 3864.2 3864.3 -313.94 -313.95 -313.59 0.832

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32 Table 3-5. HR calculated from HF calorimetric measurements and DSC calorimetric measurements. Reactions (Table 3-4) HR (kJ/mol) 298.15K (HF Method) 323.15K (HF Method) 298.15K (DSC Method) 323.15K (DSC Method) 1-1 -50.62 1.22 -54.83 .22 -51.4 .1 -55.6 1.1 1-2 -52.10 1.45 -56.80 1.45 1-3 2.49 0.87 -1.96 0.87

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33 KerniteTemperature ( K) 300400500600700800900 DSC (mw/mg) -6 -4 -2 0 TGA (Mass %) 70 75 80 85 90 95 100 Temperature ( K) 300400500600700800900 DSC (mw/mg) -5 -4 -3 -2 -1 0 1 Tincalconite Temperature ( K) 300400500600700800900 DSC (mw/mg) -6 -4 -2 0 TGA (Mass %) 50 60 70 80 90 100 Borax TGA (Mass %) 65 70 75 80 85 90 95 100 Figure 3-1. Thermal analyses of borax, tincalconi te, and kernite (heated from 298K to 973K at 1bar at 15K/min). TGA is shown with the red dotted line and DSC with the blue solid line.

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34 T i n c a l c o n i t e t o B o r a xB o r a x t o T i n c a l c o n i t eEquilibrium Relative Humidity (%) 02 04 06 08 01 0 0 Mass Change (%) -30 -20 -10 0 10 20 30 Borax Tincalconite Figure 3-2. Experimental observations of borax and tincalconite reaction at 298.15K. The mass of borax remains constant down to a relativ e humidity of 53%, then systematically decreases with the RH, eventually completing the transition to tincalconite (25% mass loss) at the RH 18%. The mass of tincalcon ite remains constant up to a RH of 65% and increases with the relative humidity until it becomes borax at RH 84%.

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35 2 510152025 tincalconite borax "sample X" Figure 3-3. X-ray powder diffractograms of phase pure borax, tincalconite, and an intermediate sample formed by partially dehydrati ng borax at 313.15K and 48.42% relative humidity for 4 days. The dashed lines indicate the positions of characteristic reflections for borax and tincalconite; the di ffraction peaks for sample X occur at the same positions. All of the reflections exhi bited by sample X correspond to peaks for either borax or tincalconite.

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36 Relative Humidity (%) 02 04 06 08 01 0 0 Temperature (K) 280 300 320 340 Tincalconite (Present Study) Borax (Present Study) Bowser, 1964 Menzel, 1935 Menzel and Schulz, 1939 DeliquescenceTincalconite Stable Borax Stable Figure 3-4. Experimental observations of borax and tincalconite stability as a function of temperature and RH. Shown for comparison are observations of RH in equilibrium with borax and tincalconite from Menzel (1935), Menzel and Shulz (1940), and Bowser (1965). The gray symbol denotes conditions under which sodium tetraborate deliquesces. The black curve shows the calcu lated equilibrium RH as a function of temperature derived in this study.

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37 Temperature (K) 250260270280290300310320 Cp (J/molK) 250 300 350 400 450 500 550 600 650 Borax Tincalconite Kernite Figure 3-5. Heat capacities of borax, tincalconite, and kernite meas ured by differential scanning calorimetry.

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38 CHAPTER 4 THERMODYAMIC MODEL AND PROPERTIES The equilibrium constants ( K ) for reactions 1-1, 1-2, and 1-3, respectively, are given by 3 333.5 112borax OHtetincalconiK (4-1) borax OHniteK 6 ker 212 (4-2) tetincalconi OH niteK 2 3 ker 312 (4-3) where X is activity of phase X The standard Gibbs energy of reaction (PTrG,,) is related to the K at T P by PT PTrKRT G, ,,ln (4-4) where R is the gas constant. The standard enthalpy and entropy of reaction (PTrH,,and PTrS,,, respectively) are related to PTrG,, by PTr PTr PTrSTHG,, ,, ,, (4-5) The change in a thermodynamic prope rty across a reaction at T and P, PTr ,, is given by PT j j PTr ,, (4-6) where the summation is over all species j, j is the stochiometric reaction coefficient for species j, and PT, is the corresponding property of the substa nce. The Gibbs energy of reaction at T, P is related to the standard mo lal Gibbs energy of formation ( Gf) at 298.15 K, 1 bar (Tref, Pref) via

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39 equation (4-6) by noting that th e apparent (cf. Benson, 1968; He lgeson et al., 1978) molal Gibbs energy of formation at T P ( Gf,T,P) is given by P P PT T T P T T P ref PT PTf PTfref refref ref ref refref refrefdPVTdTdT TTCC SGGln )(\, ,, ,, (4-7) where STref,Pref and VTref,Pref are the standard molal entropy and volume at 298.15 K and 1 bar, respectively, and Cp is the standard molal heat capacity at T, 1 bar. The thermodynamic properties adopted by Wagm an et al. (1982) for borax (except Cp, for which Wagman et al., 1982 did not evaluate the temperature dependence) were adopted as the basis for subsequent evaluation of the ther modynamic properties of tin calconite and kernite from the results of this study (Table 4-1). Previously reported values of VTref,Pref for tincalconite and kernite were adopted, though were not directly used in de rivation of other thermodynamic properties as all relevant calcula tions are based on observations at 1 bar. The heat capacity values listed in Table 3-2 were used to regre ss Maier-Kelley (1932) polynomial expressions of the temperature dependence of this property 2 cTbTaCp (4-8) for evaluation of the Cp integrals in equation (4-7 ). In converting between STref,Pref and entropies of formation in these calculations, STref,Pref of the elements reported by Robie and Hemingway (1995) were adopted. The STref,Pref value was calculated from equation (4-9) elements PTi T T p PTPTfrefref ref refrefSTdTC SS ,,ln (4-9) which was then used to determine the STref,Pref using equation (4-10). elements PTi PTPTfrefref refref refrefS SS ,, (4-10)

PAGE 40

40 The other thermodynamic properties were then retrieved from equation (4-11) refref refref refrefPTf PTf PTfSTH G,, ,, ,, (4-11) by utilizing the value for SfTref,Pref from equation (4-10) and Gf,Tref,Pref from equation (4-7). The thermodynamic properties of the formation of tincalconite were calculated from the measurements from the equilibrium observations and DSC measurements. The equilibrium constant of the borax/tincalconite reaction was determined using equation (4-1) and utilizing the activities from the equilibrium observations. Once the equilibrium constant was known it was used to determinePTrG,,. Then PTrH,, was determined from the DSC measurements, therefore both PTrG,, and PTrH,, were plugged into equation (4-12) to get the PTfG,, andPTfH,,, respectively. fBorax OfH R fTinc 2333. 5 (4-12) with representing each of the properties (G and H) of tincalconite. Then Sr,T,P and Sf,T,P were calculated using equations (4-1 0) and (4-9) respectively. Finally, Gf,Tref,Pref and Hf,Tref,Pref were calculated using equations (4-7) and (4-11) respectively. The error associated with the DSC measurements and equilibrium observations were compounded by summing the squared errors for each variable, then taking the s quare root, therefore giving the total error of the thermodynamic properties of formation at 298.15K. The properties of reaction for kernite were cal culated in the same method as tincalconite explained above, although the HR came from the HF measuremen ts. Properties of kernite were calculated from the calorimetric observations noted above and the values fr om Christ and Garrels (1959) of kernite-borax-solution equilibrium at 58.5 C, 1 bar, with 96.6% equilibrium RH over the saturated solution. The equilibrium constant of the borax/kernnite reaction was determined

PAGE 41

41 using equation (4-2) and the activity from the Blasdale and Slanksy (1939) and Menzel and Schulz (1940) value of 96.6% RH at 331.65K. On ce the equilibrium constant was known it was used to determine PTrG,,(T= 331.65K). Then PTrH,, was determined from the HF measurements, therefore both PTrG,, and PTrH,, were plugged into equa tion (4-13) to get the PTfG,, andPTfH,,, respectively, Borax OH R Kernite 26 (4-13) with representing each of the propertie s (G and H) of kernite. Then Sr,T,P and Sf,T,P were calculated using equations (4-10) and (4-9) respectively. Finally, Gf,Tref,Pref and Hf,Tref,Pref were calculated using equations (4 -7) and (4-11) respectively. The error associated with the HF measurements and calculations were com pounded by summing the squared errors for each variable, then taking the square root. Errors on the thermodynami c properties were taken to be twice the standard devi ation of the individual experimental results.

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42 Table 4-1. Thermodynamic properties of borax, tin calconite, and kernite derived in this study. At 298.15K Volume (cm3) G298.15 (kJ/mol) H298.15 (kJ/mol) S298.15 (J/molK) Borax 222.7 0.2a -5516.02a-6288.59a 586.01a Tincalconite 2279.3b -4244.48 8.58d-4713.60 8.58d 360.66 2.83dKernite 143.54.02c -4086.30 8.57d-4580.19 8.57d 337.01 2.67d a-Wagman et al. 1982; bLuck and Wang, 2002; cCooper et al. 1973; d-Present study. Table 4-2. Maier Kelley Coefficients of borax, tincalconite, and kernite. Minerals a b c Borax -9.2440.50207 0.0 Tincalconite -3.2520.30318 0.0 Kernite 11.810.23099 0.0

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43 CHAPTER 5 DISCUSSION Comparison with Previous Thermal Analysis Results The thermal analysis results of this study generally agree well with those of prior observations of the thermal behavior of borax, tin calconite, and kernite. In Figure 3-1, it can be seen that the first endothermic reaction of bor ax occurred at 93C, compared to 90C as compared observed by Giese and Kerr (1959) a nd 74C as observed by Waclawska (1998). The second endothermic reaction was observed at 117C in this study, as oppos ed to 130C (Giese and Kerr, 1959) and 102C (Waclawska, 1998). The third reaction was observed in this study at 161C, as opposed to 155C (Giese and Kerr, 1959) or 133C (Waclawska, 1998). In all three studies, the first two reactions signaled the tran sition of borax to tincalconite via dehydration. The transition temperatures observed in the pres ent study are systematically higher than observed by Giese and Kerr (1959) and systematically lower than observed by Waclawska (1998). The differences in temperature could be related to th e heating rates of the experiments because faster heating rates cause the reaction to appear at higher temperatures. Waclawska (1998) used a much slower heating rate at 2.5K/min compared to the 15K/min rate us ed in this study. Giese and Kerr (1959) did not report their heating rate. Tincalconite's first reaction (endothermic) o ccurs at the same temperature as the third borax reaction (161 C), compared to Waclawska (1998) that obser ved this transi tion at 137C. The second peak (exothermic) occurs at 646 C, suggesting the mineral becoming amorphous. Waclawska (1998) did not heat the experimental samples to that high of a temperature. Kernite has two endothermic reactions; one at 165C and the other at 196C. Waclawska has the two endothermic reactions referenced prev iously at 155C and 159 C, but has two earlier reactions at 84C and 10 0C in which the OH groups and water are split from the structure. The

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44 first two reactions of kernite s een by Waclawska are not found in our TGA curves. This could be due to differences in the heating rate utilized by each scientist. There have been reports of another phase formed from the dehydration of kernite. Metakernite, Na2B4O6(OH)2.5H2O, has been formed from the heating of kernite by Muessig (1959) and Sennova, et al. (2005). Sennova et al.(2005) determined that this new phase consisted of the same chains (minus water molecules) that kernite did, which allow for a simple transition from kernite to meta-kernite. The two dehydration events, seen in th e kernite TGA curve in the presen t study (Figure 3-1), result in the formation of meta-kernite (Sennova, 2005). Comparison with Previous Equilibrium Observations and Thermodynamic Results The results from experimental observations of this study for the dehydration of borax to tincalconite (Figure 3-4) are in good agreement with previous obser vations of this reaction. The curve in Figure 3-4 calculated from the thermod ynamic properties derived in this study similarly agrees with previous observations. Our resu lts confirm and extend the observations of Menzel(1935), Menzel and Schulz (1940), and Bowser (1965) on the T-RH conditions of reaction between borax and tincalc onite (Figure 3-4). As temperature increases, tincalconite becomes increasingly stable with respect to borax at pure-H2O-undersaturated conditions until it is the only phase stable in the presence of an aqueous solution. The present study reports a se ries of new observations of the temperature dependence of Cp for 1:2 sodium borates. The heat capacity of borax at 298.15K was determined to be ~586.5 J/molK; lower than evaluated by Wagman et al.(1982; 615 J/molK). The cause of this discrepancy is unclear. To my knowledge th ere are no previous determinations of Cp for tincalconite or kernite. At 298.15K tincalconite's heat capacity is 367.45 J/molK and kernite is 337.32 J/molK (Table 1).

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45 The results of the present study perm it assessment of the properties of Cp for the waters of hydration in borax that are lost during the conversion to tincalconite. Barrer (1978) reasoned that as water molecules adsorb into the structure of a mineral, such as a zeolite or other mineral hydrate, the loss of rotational and translational degrees of freedom in the crystal structure over those in an ideal gas should be manifest by an increase in Cp. His statistical mechanical model also predicts that for each of the six degrees of motional freedom (three translational and three rotational) lost to vibrational modes in the cr ystal during sorption, Cp should increase by 0.5 R (R is the gas constant; 8.314 J/molK) to a maxi mum of 3R. At 298.15K the average Cp of the water molecules in borax relative to tincalconite is 37.98J/molK per water molecule. This is an increase of ~0.5R over Cp of H2O vapor, corresponding to a loss of one degree of motional freedom during incorporation of these water molecules into the borax structure. The thermodynamic properties of borax, tincal conite, and kernite were calculated from the thermodynamic model and calorimetric experi mental results. Previously, borax's properties had not been calculated from direct measurements, and very few thermodynamic properties for tincalconite and kernite had b een published (Anovitz and Hemi ngway, 1996). Navrotsky et al. (unpublished; cited in by Anovitz and Hemingway, 1996) assessed Hf of tincalconite calorimetrically through two different thermochem ical cycles to be -4770.5kJ/mol (relative to boric acid) and -4785.1kJ/mol (relati ve to caliborite). Our value for Hf was significantly lower at -4713.6kJ/mol. Their kernite Hf is -4507.4 kJ/mol, while ours was -4580.19 kJ/mol. The discrepancies could be due to the difference in methods of measurement. Using the group contribution method (adding up thermodynamic propertie s of the individual ions in a mineral), Li et al. (2000) calculated the H0 f of borax, tincalconite, and kernite. The numbers they calculated were -6268.5 kJ/mol, -4816.4 kJ/mol, and -4506.21 kJ/mol respectively, compared to our values

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46 of tincalconite and kernite at -4713.6 and -4580.19 kJ/mol, respectively. The Go f of borax was reported by Bassett (1976) as -5516.60kJ/mol based on its solubility in water, compared to Wagman's value of -5516.02 kJ/mol. The gr oup contribution method resulted in a borax G0 f of -5518.01 kJ/mol. Phase Relations between 1:2 Sodium Borates Figure 5-1 shows the calculated phase rela tions among 1:2 sodium borates calculated from the thermodynamic properties in Table 4-1. Th e solid line is representative of the transition between borax and kernite, where kernite is stab le phase below the solid line while borax is stable above. The dotted line represents the tran sition between borax (sta ble above the line) and tincalconite (stable below the line). The three do ts with the error bars are the equilibrium conditions of borax-tincalconite from the equilibri um observations. The error bar on the point at 298.15K intersects the borax-kernite curve repres enting the conditions where tincalconite could be a stable phase. The dashed line represents the RH of a saturated sodium borate solution (Blasdale and Slansky, 1939, and Menzel a nd Schulz, 1964), which is near the point of deliquesence. Blasdale and Slanksy (1939) a nd Menzel and Schulz ( 1940) report the boraxkernite transition occurs at 331.65K at 96.6% RH (star in Figure 5-1), while the transition of borax to tincalconite occurs at 333.95K (inverted triangle in Figure 5-1) (Christ and Garrels, 1959). In this purely Na2B4O7-H2O system, borax and kernite are the stable phases at other temperatures and RH, therefore tincalconite is metastable (Christ and Ga rrels, 1959, and Bowser, 1965). Figure 5-2 reveals the calculat ed phase relations seen in Figure 5-1, but now includes geologic observations from the Kramer Deposit in California of borax and tincalconite. Kernite is seen in the Kramer deposit at much greater depths (~ 1 km) (Christ and Garrels, 1959). When geothermal gradient is taken into account the temperature increases by ~ 33 km, therefore

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47 pushing the tincalconite point seen in Figure 5-2 further into the ke rnite stability field. The NaCl saturated solution line is shown for reference because borate deposits of ten contain other salts such as NaCl. With the addition of NaCl, the re lative humidity of the syst em would be decrease to the NaCl saturated solution line resulting in ti ncalconite or kernite becoming the stable phase. With the addition of other ions in solution tinca lconite's solubility is decreased due to the common ion effect, therefore allowing primary pr ecipitation of tincalconite (Bowser, 1965). For instance, Pabst and Swayer (1948) report that if sulfate ions ar e introduced into solution, the tincalconite will form down to temperatures of 322.45K, but with the addition of sulfate and ammonium ions, the transition temperature lowers even further to 314.85K. They also predicted that in the presence of other additional ions tincalconite may precipitate at even lower temperatures. Bowser (1965) and Pabst and Sawyer (1948) have reported that other non-marine evaporite lakes, such as Searles Lake, have prec ipitated tincalconite out of solution (as a primary precipitate not a psuedomorph). Th is indicates that the tincalconite transition line is lowered and tincalconite becomes a stable phase at more than just 298.15K and 65% RH. The Kramer deposit, the world's largest and most important s ource of Na-borates in the world is free of the other salty sediments found el sewhere (Bowser, 1965; Muessi g and Allen, 1957). Without significant amounts of other ions the Kramer borates are chem ically similar to the phase diagram (Figure 5-1). As was stated by Christ and Garrels (1959), the reaction between the minerals is a function of temperature, pressure and vapor pressure of water (relative humidity). The reaction between borax and kernite can be dependent on the energy associated with the reaction and chemical arrangement. The reversible reaction from borax to tincalconite (a nd vice versa) occurs rapidly at surface conditions, while the borax to kernite reaction takes much more time with

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48 increased temperature and pressure (Christ and Garrels, 1959; Bowser, 1965). Borax and tincalconite both cont ain the polyanion [B4O5(OH)4]-2 and therefore can convert rapidly to each other with little energy associated with the reaction (loss/gain of water)(Christ and Garrels, 1959;Morimoto, 1956). Christ and Garrels (1959) calculated the pressu re (depth) based on geothermal gradient and temperature associated with the borax to kernite reaction to be 2500 feet. Kernite differs in that it contains chains of [B4O6(OH)2]n -2n polyanions, resulting in the polymerization reaction of n[B4O5(OH)4]-2 = [B4O6(OH)2]n -2n + nH2O requiring appreciable activation energy (Christ and Garr els, 1959). Blasdale and Slans ky (1939), as well as Menzel and Schulz (1940), produced a solubility temperature curve for the system Na2B4O7-H2O in which the transition point for borax to kernite occurs at 58.5 C, while the borax to tincalconite occurs at 60.8 C. From the experimental observa tions we observe tincalconite as the secondary mineral that forms at both of these temperatures Because of the struct ure and activation energy needed in the transformation from borax to ke rnite, tincalconite forms instead (meatastably). Borax is described as a primary evaporite becau se it is most often the first of the Na-suite to form at surface conditions (Muessig, 1959). Muessig (1959) describes primary borate evaporites as minerals that form at the lowest temperatures under surf ace conditions of playas, also having the lowest specific gravity and hence the highest water content. Because tincalconite psuedomorphs borax where it has been exposed to dry desert air and then transforms back to borax when re-exposed to moisture (Christ and Garrels, 1959), tincalconite was formally described as secondary and metast able, but now can be described as primary and stable (Figure 5-1). Kernite is regarded as a secondary mineral in the Na2B4O7-H2O system; a product of thermal diagenesis of borax. Kernite is seen deep in the Kramer deposit in irregularly shaped masses, indicative of secondary mineralization (Christ and Garrels, 1959). The transformation

PAGE 49

49 from kernite back to borax occurs when ke rnite is exposed to weathering (Warren, 1999), confirming that borax is a stable phase at surface conditions. This was also seen in our laboratory experiments where kernite was left in a 97% RH environment at 25C for 3 months resulting in borax. Other Borate Systems The findings from this investig ation of Na-borates can be ap plied to other borate systems (Ca, Mg, NaCa). The calcium borate suite mirrors the sodium suite with a high, mid, and low hydrate. Inyoite [Ca2B6O1113H2O] and myerhoffite [Ca2B5O107H2O] contain the polyanion [B3O3(OH)5]-2. Colemanite [Ca2B5O115H2O], like kernite, contains ch ains of polyanions, but of the composition [B3O4(OH)3]n -2n. The polymerization reaction is n[B3O3(OH)5]-2 = [B3O4(OH)3]n 2n + n H2O (Christ and Garrels, 1959). The similarities in the two reactions of borax to kernite and inyoite to colemanite make the study of the Na-system important to the Ca-system.

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50 Temperature (K) 280285290295300305310315320325330335340 Relative Humidity (%) 0 20 40 60 80 100 Borax KerniteB o r a x T i n c a l c o n i t e Figure 5-1. Phase diagram of borax, tincalconite, and kernite. The solid line represents equilibrium between borax and kernite, while the dashed line represents equilibrium between borax and tincalconite. The star represents the borax-kernite transition recognized by Menzel and Schulz (1940) a nd Blasdale and Slansky (1939). The inverted triangle is the transition of borax to tincalconite reco gnized by Christ and Garrels (1959). The open circle is the point of deliques cence and the dashed line is the RH of a saturated sodium borate solution. The dots with error bars are the equilibrium between borax and tincalconite realized in the equilibrium observations.

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51 Temperature (K) 280290300310320330340 Relative Humidity (%) 40 50 60 70 80 90 100 SolutionBorax KerniteB o r a x Ti n c a l c o n i t eKramer Deposit (Borax) Kramer Deposit (Tincalconite) NaCl saturated solution Figure 5-2. Phase diagram with geologic observations of borax, tincalconite, and kernite. The solid line represents equilibrium between borax and kernite, while the dashed line represents equilibrium between borax and tin calconite. The thick dashed line is the RH of a saturated sodium borate solution. Geologic observations from the Kramer Deposit, California match the stability fields of borax and tincalconite shown in the phase diagram. The NaCl saturated solution line is shown for reference, revealing the RH of a system saturated with NaCl as seen in some borate deposits.

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52 CHAPTER 6 CONCLUSIONS This suite of sodium borate minerals is very important economically, therefore understanding their stability and formation is of the utmost importance. Through experimental observations, the stability of borax and tincalconite were established in the Na2B4O7-H2O system. Borax seems to be the primary precipitate in the Na2B4O7-H2O system, but tincalconite does have a small stability field at 298.15K and 65%. Kernite is a thermal diagenesis product of borax. With the addition of other ions, the solu bility of tincalconite and kernite are lowered allowing them to become a stable phase and precip itate directly from solu tion as seen at Searles Lake. The differences in structure between borax tincalconite, and kernite were discussed to help explain the difficulty in transformation to kern ite. Due to the similarities in the structure and behavior, these findings can be applied to other borate systems, like the Ca, Na-Ca, and Mgborates. The TGA curves from each mineral confir m the structure and water content of the minerals. TGA measurements and equilibrium observations confirm the best stoichiometry of tincalconite as Na2B4O5(OH) 4.667H2O. Through HF calorimetry, DSC-TGA analysis, X-ray diffraction, and equili brium experiments the thermodynamic properties of these borax, tincalconite, and kernite (Cp, Hf, S, Gf at 298.15 K) were measured and calculated using the thermodynamic model. The heat capacity of the hydration reaction is equal to 0.5R which reveals that one degree of motional freedom among the water molecules was lost.

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Waclawska, I. (1998) Controlled rate thermal analysis of hydrated borates. Journal of Thermal Analysis, 53, 519-532. Waldbaum, D.R. and Robie, R.A. (1970) An internal sample c ontainer for hydrofluoric acid solution calorimetry, Journal of Geology, 78, 736-741. Wagman D.D., Evans, W.H., Parker, V.B., Schu mm, R.H., Halow, I, Bailey, S.M., Churney, K.L., and Nuttall, R.L. (1982) The NBS tables of chemical thermodynamic properties: Selected values for inorganic and C1 and C2 organic substances in SI units Journal of Physical Chemistry Reference, 11, 2. Wagner, W. and Pru, A. (2002) The IAPWS formulati on 1995 for the thermodynamic properties of ordinary water substance for genera l and scientific use. Journal of Physical and Chemical Reference Data, 31, 387-535. Warren, J.K. (1999) Evaporites: Their Evolut ion and Economics. Blackwell Science, Malden, MA. Warren, J.K. (2006) Evaporites: Sediments, Resources, and Hydrocarbons. Springer, Berlin, NewYork. Woods, W. (1994) An introduction to boron: hi story, sources, uses, and chemistry. Environmental Health Perspectives, 102, 7.

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BIOGRAPHICAL SKETCH Laura was born in Monroe, Louisiana, and grew up there and in Ft. Myers, FL. She attended Ft. Myers High School and complete d the International Baccalaureate program. Afterward, she attended the University of Florida, just as her grandfather, father, aunt, and uncle did. She earned a Bachelor of Science degree in geological sciences in 2006. Laura decided to continue her education at the University of Fl orida and worked on her Master of Science in geology under the direction of Philip Neuhoff. Wh ile at the University of Florida Laura has maintained many great friendships and taken part in many extracurricular activities, thoroughly enjoying her time as a gator.