Sulfate Minerals: Crystallography, Geochemistry, and Environmental Significance 9781501508660, 9780939950522

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I&ŒVliïEWS M MUMŒIRA mmû (Q®(D€IE[IËMII^M¥

— SULFATE MINERALS

—

Crystallography, Geochemistry, and Environmental Significance Editors:

Charles N. Alpers John L. Jambor D. Kirk Nordstrom

U.S. Geological Survey, Sacramento,

California

Leslie Research and Consulting and Department of Earth and Ocean Sciences, University of British Columbia, Vancouver U.S. Geological Survey, Boulder,

Colorado

F R O N T - C O V E R P H O T O G R A P H : Melanterite [(Fe 2 , ,Zn,Cu)S0 4 -7H 2 0] stalagmites (blue and blue-green) from the Mattie deposit, Richmond tunnel, Iron Mountain, California. Yellow and brown minerals are products of melanterite oxidation and dehydration. Field of view is 1 0 x 7 cm. BACK-COVER PHOTOGRAPHS: a . Melanterite stalactite (blue-green, with orange, Fe 3 + -rich water inside) from the Mattie deposit, Richmond tunnel, Iron Mountain, California. Photo copyright: Bud Eagle 1992. Field of view: 5 x 8 cm. b. Copiapite stalagmite (yellow) with halotrichite needles (white) from the Richmond mine, Iron Mountain, California. Field of view: 25 X 40 cm. c. Voltaite from the Richmond Mine, Iron Mountain, California. Canadian Museum of Nature specimen CMNOC 3525. Photo courtesy of George Robinson. Width of specimen: 8 mm. d. Coquimbite (pale purple) with copiapite, and voltaite from the Richmond mine, Iron Mountain, California. Canadian Museum of Nature specimen CMNOC 3526. Photo courtesy of George Robinson. Width of specimen as viewed is 2 cm.

Series Editor: Paul H. Ribbe Virginia Polytechnic Institute and State Blacksburg, Virginia

University

SOCIIBTY ®lf AMÏB3UCA Wasihaaigtoin, L> C

COPYRIGHT 2 0 0 0

MINERALOGICAL SOCIETY OF AMERICA The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright o w n e r ' s consent that copies of the article can be made for personal use or internal use or for the personal use or internal use of specific clients, provided the original publication is cited. The consent is given on the condition, however, that the copier pay the stated per-copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to other types of copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. For permission to reprint entire articles in these cases and the like, consult the Administrator of the Mineralogical Society of Amcrica as to the royalty due to the Society.

REVIEWS IN MINERALOGY AND GEOCHEMISTRY ( Formerly: REVIEWS IN MINERALOGY)

ISSN 1529-6466

Volume 40

Sulfate Minerals:

Crystallography,

Geochemistry, and Environmental

Significance

ISBN 0-939950-52-9 ** This volume is the second of a series of review volumes published jointly under the banner of the Mineralogical Society of America and the Geochemical Society. The newly titled Reviews in Mineralogy and Geochemistry has been numbered contiguously with the previous series, Reviews in Mineralogy. Additional copies of this volume as well as others in this series may be obtained at moderate cost from: T H E M I N E R A L O G I C A L S O C I E T Y OF A M E R I C A 1 0 1 5 EIGHTEENTH STREET, N W , SUITE 6 0 1 WASHINGTON, D C 2 0 0 3 6 U . S . A .

- SULFATE MINERALS Crystallography, Geochemistry, and Environmental Significance 40

Reviews in Mineralogy and Geochemistry

40

FOREWORD The review chapters in this volume were the basis for a short course on sulfate minerals sponsored by the Mineralogical Society of America (MSA) November 11-12, 2000 in Tahoe City, California, prior to the Annual Meeting of MSA, the Geological Society of America, and other associated societies in nearby Reno, Nevada. The conveners of the course (and editors of this volume of Reviews in Mineralogy and Geochemistry), Charles Alpers, John Jambor, and Kirk Nordstrom, also organized related topical sessions at the GSA meeting on sulfate minerals in both hydrothermal and low-temperature environments. A special issue of a journal yet to be identified is being planned for the publication of research articles based on several of these presentations. Taken together, the MSA short course and the related GSA sessions represent the most comprehensive grouping of technical meetings ever devoted to sulfate minerals. ERRATA (if any) may be found at the MSA website together with access to the black-and-white and some color representations of many STRUCTURE DRAWINGS in Chapter 1: http ://www.minsocam.org (click on Rev Mineral Geochem entry)

Paul H. Ribbe, Series Editor Virginia Tech, Blacksburg December 9, 2000 PREFACE

Sulfate is an abundant and ubiquitous component of Earth's lithosphere and hydrosphere. Sulfate minerals represent an important component of our mineral economy, the pollution problems in our air and water, the technology for alleviating pollution, and the natural processes that affect the land we utilize. Vast quantities of gypsum are consumed in the manufacture of wallboard, and calcium sulfates are also used in sculpture in the forms of alabaster (gypsum) and papier-máché (bassanite). For centuries, Al-sulfate minerals, or "alums," have been used in the tanning and dyeing industries, and these sulfate minerals have also been a minor source of aluminum metal. Barite is used extensively in the petroleum industry as a weighting agent during drilling, and celestine (also known as "celestite") is a primary source of strontium for the ceramics, metallurgical, glass, and television face-plate industries. Jarosite is a major waste product of the hydrometallurgical processing of zinc ores and is used in agriculture to reduce alkalinity in soils. At many mining sites, the extraction and processing of coal or metal-sulfide ores (largely for gold, silver, copper, lead, and zinc) produce waste materials that generate acid-sulfate waters rich in heavy metals, commonly leading to contamination of water and sediment. Concentrated waters associated with mine wastes may precipitate a variety of metal-sulfate minerals upon evaporation, oxidation, or neutralization. Some of these sulfate minerals are soluble and store metals and acidity only temporarily, whereas others are insoluble and improve water quality by removing metals from the water column. 1529-6466/00/0040-0000S05.00

DOI: 10.213 8/rmg. 2000.40.0

There is considerable scientific interest in the mineralogy and geochemistry of sulfate minerals in both high-temperature (igneous and hydrothermal) and low-temperature (weathering and evaporite) environments. The physical scale of processes affected by aqueous sulfate and associated minerals spans from submicroscopic reactions at mineral-water interfaces to global issues of oceanic cycling and mass balance, and even to extraterrestrial applications in the exploration of other planets and their satellites. In mineral exploration, minerals of the alunite-jarosite supergroup are recognized as key components of the advanced argillic (acid-sulfate) hydrothermal alteration assemblage, and supergene sulfate minerals can be useful guides to primary sulfide deposits. The role of soluble sulfate minerals formed from acid mine drainage (and its natural equivalent, acid rock drainage) in the storage and release of potentially toxic metals associated with wet-dry climatic cycles (on annual or other time scales) is increasingly appreciated in environmental studies of mineral deposits and of waste materials from mining and mineral processing. This volume compiles and synthesizes current information on sulfate minerals from a variety of perspectives, including crystallography, geochemical properties, geological environments of formation, thermodynamic stability relations, kinetics of formation and dissolution, and environmental aspects. The first two chapters cover crystallography (Chapter 1) and spectroscopy (Chapter 2). Environments with alkali and alkaline earth sulfates are described in the next three chapters, on evaporites (Chapter 3). barite-celestine deposits (Chapter 4), and the kinetics of precipitation and dissolution of gypsum, barite, and celestine (Chapter 5). Acidic environments are the theme for the next four chapters, which cover soluble metal salts from sulfide oxidation (Chapter 6), iron and aluminum hydroxysulfates (Chapter 7), jarosites in hydrometallugy (Chapter 8), and alunite-jarosite crystallography, thermodynamics, and geochronology (Chapter 9). The next two chapters discuss thermodynamic modeling of sulfate systems from the perspectives of predicting sulfate-mineral solubilities in waters covering a wide range in composition and concentration (Chapter 10) and predicting interactions between sulfate solid solutions and aqueous solutions (Chapter 11). The concluding chapter on stable-isotope systematics (Chapter 12) discusses the utility of sulfate minerals in understanding the geological and geochemical processes in both high-and low-temperature environments, and in unraveling the past evolution of natural systems through paleoclimate studies. We thank the authors for their comprehensive and timely efforts, and for their cooperation with our various requests regarding consistency of format and nomenclature. Special thanks are due to the numerous scientists who provided peer reviews, which substantially improved the content of the chapters. This volume would not have been possible without the usual magic touch and extreme patience of Paul H. Ribbe, Series Editor for Reviews in Mineralogy and Geochemistry. Finally, we thank our families for their support and understanding during the past several months. Charles N. Alpers U.S. Geological Survey, Sacramento John L. Jambor Leslie Research and Consulting & Department of Earth and Ocean Sciences, University of British Columbia, Vancouver D. Kirk Nordstrom U.S. Geological Survey, Boulder October 6, 2000 1529-6466/00/0040-0000S05.00

DOI: 10.213 8/rmg. 2000.40.0

RiMG Volume 40. SULFATE MINERALS: Crystallography, Geochemistry, and Environmental Significance Table of Contents U

The Crystal Chemistry of Sulfate Minerals Frank C. Hawthorne, Sergey V. Krivovichev, Peter C. Burns

INTRODUCTION CHEMICAL BONDING STEREOCHEMISTRY O F SULFATE T E T R A H E D R A IN MINERALS Variation in ( S - 0 ) distances Variation in S - O distances General polyhedral distortion in sulfate minerals S6* T"+ substitution in minerals and its influence on ( S , 7 ) - 0 distances Hydrogen bonding in sulfate minerals STEREOCHEMISTRY O F THIOSULFATE T E T R A H E D R A Variation in ( S - 0 ) distances Variation in S - O distances The formal valences of S in the thiosulfate group STEREOCHEMISTRY O F F L U O R O S U L F A T E TETRAHEDRA Variation in S - 0 distances M O L E C U L A R - O R B I T A L STUDIES O F S 0 4 POLYHEDRA The role of 3c/-orbitals in bonding in sulfates Stability of the (S0 4 ) 2 " tetrahedron (H 2 S0 4 ) and (H 2 S 2 0 7 ) clusters: prediction of equilibrium geometry Bond angles in (S0 4 ) 2 ~tetrahedra Alkali metal-sulfate clusters Experimental studies of electron density Theoretical studies of electron densities Models of chemical bonding H I E R A R C H I C A L O R G A N I Z A T I O N O F C R Y S T A L STRUCTURES POLYMERIZATION O F S 0 4 A N D OTHER 7 0 4 TETRAHEDRA A S T R U C T U R A L H I E R A R C H Y FOR SULFATE MINERALS STRUCTURES B A S E D ON SULFATE T E T R A H E D R A A N D D I V A L E N T AND/OR TRIVALENT CATION OCTAHEDRA Graphical representation of octahedral-tetrahedral structures Structures with unconnected S 0 4 groups Structures with finite clusters of polyhedra Structures with infinite chains Structures with infinite sheets Structures with infinite frameworks STRUCTURES WITH NON-OCTAHEDRAL CATION-COORDINATION P O L Y H E D R A Calcium-sulfate minerals Alkali-metal- and NH 4 -sulfate minerals

v

1 1 1 2 3 3 3 5 6 6 8 8 8 9 10 10 10 10 11 12 13 14 14 14 16 17 17 17 18 24 30 40 53 61 61 66

Apatite-like structures Sulfates with the barite structure Pb 4 (S0 4 )(C0 3 ) 2 (0H) 2 polymorphs Uranyl sulfates Sulfates with non-sulfate tetrahedral sheets or frameworks Basic sulfates of Sb3+ and Bi3+ Miscellaneous sulfates STRUCTURES WITH ANION-CENTERED TETRAHEDRA THIOSULFATE MINERALS SULFITE MINERALS FLUOROSULFATE MINERALS ACKNOWLEDGMENTS APPENDIX: Index of mineral names and the table numbers in which they appear REFERENCES

%

80 80 80 80 82 84 89 90 91 94 98 98 99 101

X-ray and Vibrational Spectroscopy of Sulfate in Earth Materials Satish C. B. Myneni

INTRODUCTION X-RAY SPECTROSCOPY X-ray absorption spectroscopy (XAS) X-ray absorption spectroscopy at the sulfur L-edge X-ray photoelectron spectroscopy X-ray imaging and spectromicroscopy VIBRATIONAL SPECTROSCOPY Symmetry and vibrational modes of sulfate and its complexes Data collection and analysis Vibrational spectra of sulfate in solids Vibrational spectra of sulfate in aqueous solutions Vibrational spectra of sulfate at the interfaces Spectromicroscopy of sulfates COMPLEMENTARY SPECTROSCOPIC METHODS Scattering methods Infrared emission spectroscopy Optical spectroscopy SUMMARY AND FUTURE DIRECTIONS ACKNOWLEDGMENTS REFERENCES

3

113 115 116 137 139 140 140 143 145 146 147 158 164 165 165 165 165 166 166 167

Sulfate Minerals in Evaporite Deposits Ronald J. Spencer

SOLUBILITY CONTROLS ON MINERAL PRECIPITATION AND PATHS OF EVAPORATION Solubility of Na-K-Ca-Mg-bearing sulfate minerals Chemical divides in the system Ca2+-S042"-HC03" Precipitation sequences of Na-K-Mg-bearing sulfates vi

173 174 175 176

Evaporation paths and mineralogy of marine evaporites Evaporation paths and mineralogy of non-marine evaporites MINERAL TEXTURES AND FABRICS Criteria for syndepositional features Criteria for burial alteration features Ambiguous features SUMMARY REFERENCES

4

Barite-Celestine Geochemistry and Environments of Formation

179 182 185 185 187 188 189 189

Jeffrey S. Hanor

INTRODUCTION Geological significance of barite and celestine Economic importance Some conventions and terms used in this chapter PHYSICAL CHEMISTRY Crystal chemistry and solid-phase relations Solubility of barite and celestine in aqueous solutions Stability ranges in multicomponent systems Ba, Sr, AND S IN CRUSTAL ROCKS AND NATURAL WATERS Crustal abundance and controls on the distribution of Ba and Sr Sulfate geochemistry Waters in sedimentary basins Meteoric groundwaters Seawater River and estuarine waters Waters in crystalline rocks Continental rifts Seafloor hydrothermal vents Review of controls on Sr/Ba ratios in natural waters ENVIRONMENTS OF FORMATION OF BARITE AND CELESTINE: AN OVERVIEW Mechanisms for the precipitation of barite and celestine Relation of barite and celestine occurrences to global tectonics and regional hydrogeology Relation of barite and celestine occurrences to the secular evolution of sedimentary rock types Marine versus continental barite CHEMICAL AND ISOTOPIC COMPOSITION OF BARITE AND CELESTINE: AN OVERVIEW Frequency distribution of compositions in the barite-celestine series Ba-Sr zoning in barite-celestine Other cations Strontium isotopic composition Sulfur isotopic composition Oxygen isotopic composition vii

193 193 193 194 195 195 196 202 205 205 206 206 208 208 210 210 210 211 211 211 212 214 214 214 215 215 217 218 218 220 222

Radium Fluid inclusions BARITE IN SUBMARINE VOLCANIC HYDROTHERMAL SYSTEMS Sulfide-poor barite deposits of Archean age Volcanic-hosted massive sulfide (VHMS) deposits Modern submarine hydrothermal barite SEDIMENTARY EXHALATIVE (SEDEX) DEPOSITS OF BARITE Proterozoic barite Phanerozoic convergent continental margins Ordovician-Devonian Roberts Mountain allochthon, Nevada Cenozoic strike-slip margins Sedex barite deposits in active marine evaporite settings The question of metal-bearing and metal-free bedded barite deposits CENOZOIC PELAGIC BARITE AND DISPERSED BARITE IN DEEP SEA SEDIMENTS Barite in seawater Mode(s) of precipitation of pelagic barite Fate of barite in the water column Barite in deep-sea sediments EPIGENETIC BARITE DEPOSITS AND EVAPORITES Carbonate-hosted barite deposits Continental rifts Barite in late-stage thrust belts Dispersed cements and nodules of barite BARITE OF CONTINENTAL IGNEOUS AND IGNEOUS-HYDROTHERMAL ORIGIN Carbonatites Other magmatic examples Hydrothermal barite in the Cordilleran of the western United States Barite from outer space BEHAVIOR OF BARITE DURING WEATHERING, DIAGENESIS, AND METAMORPHISM Formation of barite in soil environments Subaerial weathering of barite Solution and reprecipitation of barite in diagenetic redox fronts Behavior of barite during metamorphism CELESTINE IN SEDIMENTARY ENVIRONMENTS Pelagic celestine Carbonate sediments Celestine in coastal carbonate-evaporite sequences ENVIRONMENTAL ASPECTS Barium in potable water supplies Barite and the uranium industry Problems related to oil and gas production An example of a "bad rap" for barite CONCLUDING REMARKS ACKNOWLEDGMENTS REFERENCES viii

222 222 223 224 227 227 228 229 230 232 237 238 238 239 239 239 241 241 245 245 250 251 252 253 253 254 254 255 255 255 255 255 256 257 257 258 259 260 260 261 261 262 263 263 263

S

Precipitation and Dissolution of Alkaline Earth Sulfates: Kinetics and Surface Energy A. Hina and G. H. Nancollas

INTRODUCTION DRIVING FORCES FOR GROWTH AND DISSOLUTION Supersaturation DEFINITION AND DETERMINATION OF GROWTH RATE Rate determination CRYSTALLIZATION AND DISSOLUTION KINETICS Homogeneous nucleation Heterogeneous nucleation Determination of interfacial free energy Interfacial energies between minerals and aqueous solutions CONTACT-ANGLE METHOD Surface-tension component theory Contact-angle measurement and thin-layer wicking CONCLUSIONS NOMENCLATURE Symbols Subscripts and superscripts REFERENCES

(5

277 277 277 278 279 282 282 285 286 290 292 292 293 295 296 296 297 297

Metal-sulfate Salts from Sulfide Mineral Oxidation John L. Jambor, D. Kirk Nordstrom, Charles N. Alpers

COMPOSITIONS AND CRYSTAL CHEMISTRY OF HYDRATED METAL SALTS Divalent cations Trivalent cations Mixed divalent-tri valent salts Other minerals PROCESSES OF FORMATION, TRANSFORMATION AND DISSOLUTION Pyrite oxidation Field studies Dissolution during rainfall events Laboratory studies Solubilities and stability relationships PARAGENESIS ACKNOWLEDGMENTS REFERENCES

ix

305 305 315 317 319 319 319 321 325 327 327 339 340 340

H

Iron and Aluminum Hydroxysulfates from Acid Sulfate Waters J. M. Bigham, D. Kirk Nordstrom

INTRODUCTION TO ACID SULFATE ENVIRONMENTS Mine drainage Residues from mineral extraction and ore processing Rock weathering Acid sulfate soils (cat clays) FORMATION OF ACID SULFATE WATERS AND ASSOCIATED WEATHERING PRODUCTS OF Fe AND A1 The Fe system The A1 system Fe AND A1 HYDROXYSULFATES OF LOW CRYSTALLINITY Schwertmannite [Fe 8 0 8 (0H) 6 S0 4 nH 2 0] Hydroxysulfates of A1 FORMATION AND DECOMPOSITION OF Fe- AND Al-HYDROXYSULFATES OF LOW CRYSTALLINITY Biological influences on mineral formation GEOCHEMICAL CONTROLS ON MINERAL FORMATION The Fe System The A1 system ENVIRONMENTAL IMPLICATIONS OF TRACE ELEMENT SORPTION Sorption of metal cations Sorption of oxyanions Mineral instability and possible affects on sorbed species SUMMARY REFERENCES :

'S

351 O« 1

J

353 353 354 356 356 360 362 362 369 376 376 378 379 384 385 387 389 391 391 393

Jarosites and Their Application in Hydrometallurgy John E. Dutrizac, John L. Jambor

INTRODUCTION SYNTHESIZED MEMBERS OF THE JAROSITE SUBGROUP OCCURRENCES OF THE JAROSITE SUBGROUP Oxidized sulfide deposits and pyritiferous rocks Nodules and disseminations in clays Acid soils Hypogene jarosite Alteration of minerals of the jarosite subgroup CONDITIONS AFFECTING THE SYNTHESES OF THE JAROSITE SUBGROUP Sodium, potassium, and ammonium jarosites JAROSITE PRECIPITATION IN THE ZINC INDUSTRY Outline of the jarosite process Metallurgical problems and environmental concerns KINETICS OF JAROSITE PRECIPITATION Flowsheets x

405 408 408 408 411 411 412 412 416 416 421 421 422 423 427

IMPURITY INCORPORATION IN SYNTHETIC JAROSITES Monovalent substitutions Divalent substitutions Trivalent substitutions Higher valency substitutions CELL DIMENSIONS OF THE SUBGROUP CONCLUSIONS AND FUTURE TRENDS REFERENCES

430 431 433 435 436 438 440 443

Alunite-Jarosite Crystallography, Thermodynamics, and Geochronology R. E. Stoffregen, C. N. Alpers, J. L. Jambor CRYSTALLOGRAPHIC DATA Unit-cell parameters THERMODYNAMIC DATA Alunite and natroalunite Jarosite and natrojarosite Other minerals in the alunite-jarosite supergroup Mixing relations GEOCHEMISTRY AND OCCURRENCES Alunite and natroalunite Jarosite and natrojarosite GEOCHRONOLOGY USING ALUNITE AND JAROSITE REFERENCES

Id)

454 456 457 457 460 462 462 468 468 472 473 475

Solid-Solution Solubilities and Thermodynamics: Sulfates, Carbonates and Halides Pierre Glynn

INTRODUCTION DEFINITIONS AND REPRESENTATION OF THERMODYNAMIC STATES Thermodynamic equilibrium states Primary saturation states Stoichiometric saturation states COMPARISON OF SOLID-SOLUTION AND PURE-PHASE SOLUBILITIES Stoichiometric saturation solubilities Primary saturation and thermodynamic equilibrium solubilities ESTIMATION OF THERMODYNAMIC MIXING PARAMETERS Miscibility-gap data Spinodal-gap data Critical mixing-point data Distribution coefficient measurements Stoichiometric saturation solubilities xi

481 482 482 483 485 486 487 489 491 493 494 500 501 501

Thermodynamic equilibrium solubilities 502 Excess-free-energy data for selected sulfate, carbonate and halide solid solutions . 504 Multicomponent solid-solution systems 505 SUMMARY AND CONCLUSIONS 507 ACKNOWLEDGMENTS 508 REFERENCES 509

o

Predicting Sulfate-Mineral Solubility in Concentrated Waters Carol Ptacek, David Blowes

INTRODUCTION BACKGROUND Compositions of concentrated waters Modeling approaches MODEL DESCRIPTION Summary of model formulation Internal consistency of model data-sets MODEL DEVELOPMENT TO PREDICT SULFATE-MINERAL SOLUBILITY Sul fate-mineral formation in the system N a - C a - M g - K - C l - S 0 4 - H 2 0 Formation of Ba, Sr, and Rb sulfates Formation of metal sulfates Formation of sulfates in acid systems Formation of sulfates in basic systems APPLICATION OF THE ION-INTERACTION APPROACH TO FIELD SETTINGS Results of the applications CONCLUSIONS REFERENCES

H

513 513 513 514 516 516 518 520 520 522 523 526 529 529 530 535 536

Stable Isotope Systematics of Sulfate Minerals Robert R. Seal, II, Charles N. Alpers, Robert O. Rye

INTRODUCTION FUNDAMENTAL ASPECTS OF STABLE ISOTOPE GEOCHEMISTRY ANALYTICAL METHODS REFERENCE RESERVOIRS FACTORS THAT CONTROL STABLE ISOTOPE FRACTIONATION EQUILIBRIUM FRACTIONATION FACTORS Sulfur Oxygen Hydrogen Geothermometry PROCESSES THAT CAUSE STABLE ISOTOPIC VARIATIONS Kinetics of isotope exchange reactions Sulfate reduction, sulfide oxidation, and associated processes xii

541 542 544 546 549 550 550 551 554 555 557 559 561

Mechanisms of precipitation and dissolution of sulfate minerals GEOCHEMICAL ENVIRONMENTS Secular variations in seawater sulfate isotopic compositions Seafloor hydrothermal systems Magmatic systems Continental hydrothermal systems Metamoiphism of sulfate minerals Surficial environments SUMMARY ACKNOWLEDGMENTS REFERENCES

INDEX

by D. Kirk Nordstrom

xiii

page 603

566 566 566 571 575 576 585 587 592 593 593

1

The Crystal Chemistry of Sulfate Minerals Frank C. Hawthorne Department of Geological Sciences University of Manitoba Winnipeg, Manitoba, Canada R3T2N2

Sergey V. Krivovichev and Peter C. Burns Department of Civil Engineering and Geological Sciences 15 6 Fitzpatrick Hall, University of Notre Dame Notre Dame, Indiana 46556

INTRODUCTION Sulfur is the fifteenth most abundant element in the continental crust of the Earth (260 ppm), and the sixth most abundant element in seawater (885 ppm). Sulfur (atomic number 16) has the ground-state electronic structure [ N e ] 3 / V , and is the first of the group VIB elements in the periodic table (S, Se, Te, Po). In minerals, sulfur can occur in the formal valence states S S°, S4+, and S6+, corresponding to the sulfide minerals, native sulfur, the sulfite minerals, and the sulfate minerals. In the sulfide minerals, S2~ functions as a simple anion (e.g. CuFeS2, chalcopyrite) and as a compound S2 anion (e.g. FeS2, pyrite). In the sulfosalts, S2~ functions as a component of a complex anion (e.g. ASS3 in tennantite, CU12AS4S13). In the sulfite minerals, S4+ has four valence electrons available for chemical bonding, and occurs in triangular pyramidal coordination with O. In the sulfate minerals, S 6+ has six valence electrons available for bonding, and occurs in tetrahedral coordination with O. In addition, there are the thiosulfate minerals, in which S is in the hexavalent state, but is coordinated by three 0 2 ~ anions and one S2~ anion. Chemists frequently write the thiosulfate group as S2O3; however, we write it as SO3S to emphasize that one of the S atoms is an anion and is involved in a tetrahedral group. Although the focus of this chapter is the sulfate minerals, we will deal also with the sulfite and thiosulfate minerals, as they occur in the same types of geochemical environments. CHEMICAL BONDING We adopt a pragmatic approach to matters involving chemical bonding. We use bondvalence theory (Brown 1981) and its developments (Hawthorne 1985a, 1994, 1997) to consider structure topology and hierarchical classification of structures, and we use molecular-orbital theory to consider aspects of structural energetics, stereochemistry and spectroscopy of sulfate minerals. These approaches are compatible, as bond-valence theory can be considered as a simple form of molecular-orbital theory (Burdett and Hawthorne 1993; Hawthorne 1994). STEREOCHEMISTRY OF SULFATE TETRAHEDRA IN MINERALS The variation of S O ( 0 : unspecified anion) distances and 0 - S - 0 angles is of great interest for several reasons: (1) mean bond-length and empirical cation and anion radii play a very important role in systematizing chemical and physical properties of crystals; (2) variations in individual bond-lengths give insight into the stereochemical behavior of structures, particularly with regard to the factors affecting structure stability;

1529-6466/00/0040-0001 $ 10.00

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Hawthorne, Krivovichev & Burns

2

(3) there is a range of stereochemical variation beyond which a specific oxyanion or cation-coordination polyhedron is not stable; it is of use to know this range, both for assessing the stability of hypothetical structures (calculated by DLS [Distance LeastSquares] refinement) and for assessing the accuracy of experimentally detennined structures. Here, we examine the variation in S - 0 distances in minerals and review previous work on polyhedral distortions in SO4 tetrahedra. Data for 206 (SO4) tetrahedra were taken from 112 refined crystal structures with R < 6.5% (for a definition of R, see Ladd and Palmer 1994) and standard deviations of î

1

Elerman et al. (1978)

Cd

SA

(H-OJj

1

Baggio et al. (1997)

Ba

SA

H.O

1

Akaet ai. (1980)

(H,0> 3

1

Gjerrestad and Maroy (1973)

Ba 1

Te2*

(SA)2

contains O s S 5 "-S ! - S ' A group

STEREOCHEMISTRY OF THIOSULFATE TETRAHEDRA The thiosulfate group consists of a central S6+ cation surrounded by four anions, three 02~ and one S" , arranged at the vertices of a tetrahedron (Fig. 4); this group is conventionally written as S2O3, but it is much more informative to write it as (S +C>3S2~). As sidpietersite, Pb2%(S6+03S2~)02(0H)2 (Cooper and Hawthorne 1999), is the only tliiosulfate mineral for which structural data are available, here we examine stereochemical variations in some synthetic thiosultate compounds. These are listed in Table 2; all are refined to R indices between 2 and 9%, and provide us with 23 distinct thiosultate groups.

Figure 4. The thiosultate group in sidpietersite shown as atoms (left) and as a tetrahedron (right) in which the S 2 anion is identified as a large black circle. S 6 + is the random-dot-shaded circle, and ( )2 are the unshaded circles.

Variation in ( S - 0 ) distances The variation in (S 6 + -0) distances in thiosulfate structures is shown in Figure 5a. The grand (S 6 + -0) distance in thiosultate compounds is 1.459 Â, the minimum and maximum distances are 1.429 and 1.476 Â, respectively, and the range of variation is 0.047 Â. The grand distance is 2.038 Â, the minimum and maximum S -S" distances are 1.965 and 2.123 Â, respectively (Fig. 5b), and the range of variation is 0.158 Â. The grand distance of 1.459 Â_ is fairly close to the sum of the empirical radii of Shannon (1976) for [4]S6+ and P^O 2 ": 0.12 + 1.360 = 1.480 Â ([3.25] is an average value for the coordination of O in sulfate minerals). However, there is no reason that the distance should be equal to the sum of the constituent radii as there is another ligand in the

Crystal

Chemistry

of Sulfate

Minerals

J.lll.

Lli

í Thioaulfate Averaga Bond Langtha (A)

I Thloftulfat»

3 3 i

1 S

- 5(2-) B o n d Lengths (A)

c

ï

l

l

I Í i

l

l

l

1 i

i

ï

i

l

i

l

i

l

i

Figure 5. (a) Variation iir(S-C^ distances in structures containing (S^0 3 S z ~) tetrahedra (n = 23); (b) variation in S ^ - S 2 - distances in structures containing (S^0 3 S z ~) tetrahedra (n = 23); (c) variation in S-O distances in structures containing (S° 0 3 S'") tetrahedra (n = 69).

Thloaulfata Individual S(6+) • 0 B o n d Langtha {£)

1.49 t.4fl

1.47 S

1.46

O

£

1.45 1.44

1.43

142 1.90

1.95

2.00

2.05

2.10

2 15

S®*- S3' (Aj Figure 6. The (S 6 + -0) distance as a fonction of S6+-S2~ distance in thiosulfate groups from the synthetic compounds listed in Table 1 and sidpietersite (black triangle). The curve shows the relation for ideal bondvalence satisfaction at the central S 6+ cation.

8

Hawthorne, Krivovichev & Burns

coordination polyhedron: S2~. According to the valence-sum rule (Brown 1981; Hawthorne 1994, 1997), there should be an inverse relation between (S 6 + -0) and S6+-S2~ in thiosulfate groups; Figure 6 shows this to be the case. The data for sidpietersite lie on the general trend, albeit near the lower end of the range of S6+-S2~ distances. The curve in Figure 6 shows ideal agreement with the valence-sum rule calculated using the observed (S-O) distances. There is reasonable agreement between the slope of the curve and that of the data, but the curve is displaced approximately 0.02 A below the trend of the data. The bond-valence curve for S + - 0 can be considered as reliable. The value reported by Brese and O'Keeffe (1991) is the same as that reported by Brown (1981), and gives close-to-ideal bond-valence sums at S6+ in (SO4) groups. The problem, therefore, is likely to be with the g6+_g2- b o n d _ v a i e n c e c u r ve of Brese and O'Keeffe (1991). Variation in S-O distances The variation in S 6 + -0 distances is shown in Figure 5c; the grand (S-O) distance is 1.459 A. The minimum and maximum observed S-O distances are 1.408 and 1.497 A, respectively, and the range of variation is 0.089 A. According to the bond-valence curve for S 6 + -0 (the universal curve for second-row elements) from Brown (1981), the range of variation in S-O bond-valence is 1.41 to 1.83 vu. According to the bond-valence curve for S6+-S2~ (from Brese and O'Keeffe 1991), the variation in S6+-S2~ bond-valence is 0.87 to 1.33 vu. The formal valences of S in the thiosulfate group On the basis of XANES spectroscopy, Vairavamurthy et al. (1993) proposed that the valences of S in thiosulfate are 5+ and 1~ instead of the conventionally assigned 6+ and 2~. This proposal may be tested in a very simple manner using structural data for sidpietersite and bond-valence theory. The bond-valence incident at the O anions of the thiosulfate group, from the rest of the structure, is calculated independent of the formal charges of S in the structure. The valence-sum rule (Brown 1981) states that the sum of the bond valences at an atom is equal to the magnitude of the formal valence of that atom. Hence the difference between the formal valence of oxygen (02~) and the sum of the bond valence incident at that oxygen (exclusive of the S-O bond) gives the bond valence of the S-O bond. Summing the bond valences thus calculated for the S-O bonds gives a value of 4.83 vu. If the formal charge on S6+ were actually 5+, the bond-valence for the S^-S 2 ~ bond would be 0.17 vu. This result seems unlikely from several perspectives. First, the four long Pb-S2~ bonds would be required to supply 0.83 vu, at a mean Pb2+-S2~ bond-valence of 0.21 vu. This requirement is not in accord with the incident bond-valence sums around the Pb sites, as it would require a bond-valence sum around one specific Pb site of -2.36 vu. Second, it is unlikely that the thiosulfate group would be a prominent complex in aqueous solutions at a range of pH (and Eh) values if it were defined by such a weak S6+-S2~ bond. Third, it seems intuitively unlikely that a group involving such a weak bond would occur with such reproducible stereochemistry in a range of structures. Thus it does not seem possible that the formal valences of S in the thiosulfate group are 5+ and 1~. STEREOCHEMISTRY OF FLUOROSULFATE TETRAHEDRA The fluorosulfate group consists of a central S6+ cation surrounded by four anions, three 0 2 ~ and one F~, arranged at the vertices of a tetrahedron: (SOsF)-. Reederite-(Y), ideally Nai5Y2(C03)9(S03F)Cl (Grice et al. 1995), is the only known fluorosulfate mineral, and fortunately, structural data are available. Here, we examine stereochemical variations in 16 synthetic fluorosulfate compounds. These are listed in Table 3; all are refined to R indices between 2 and 9%, and provide us with 27 distinct fluorosulfate groups.

Crystal Chemistry of Sulfate

Minerals

Table 3. Fluorosulfate compounds, Sb

(SO.F), Zhang eta!. (1996)

(H,0)

(S0 3 F)

Mootz & Bartmaon (199!)

CSj Pt

(SO,F)„ Zhang etal. (1996)

H

(S0 3 F)

Bartmann & Mootz (1990)

Cs

Au

(SO,F) 4 Zhang etal. (1996)

I2

(S0 3 F) 2 Birchall e t a l . (1990)

Cs

H

(SO,F) 2 Zhang etal. (1996)

Se,„

(S0 3 F) 2 Collins et al. (1986)

(SO,F)

Zhang etal. (1996)

(S 4 N.)

(S0 3 F), Gillespie etal. (1981a)

Ir

(CO), (SO,F) s Wang e t a l .