Carbonates: Mineralogy and Chemistry 9781501508134, 9780939950157

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REVIEWS in MINERALOGY Volume 11 CARBONATES:

MINERALOGY and CHEMISTRY RICHARD J. REEDER, Editor The Authors:

F R E D T. M A C K E N Z I E

D A V I D J. BARBER Dept. of Physics University of Essex Colchester, England C 0 4 3SQ W I L L I A M D. BISCHOFF

JANESCHOONMAKER Dept. of Oceanography University of Hawaii Honolulu, Hawaii 96822

F I N L E Y C. BISHOP

JOHN W. MORSE

Dept. of Geological Sciences Northwestern University Evanston, Illinois 60201

Dept. of Oceanography Texas A & M University C o l l e g e Station, Texas 77843

W I L L I A M D. C A R L S O N

R I C H A R D J. REEDER

Dept. of Geological Sciences University of Texas at Austin Austin, Texas 78712

Dept. of Earth & Space Sciences State University of New York Stony Brook, New York 11794

ERIC J. ESSENE

J. A L E X A N D E R SPEER

Dept. of Geological Sciences University of Michigan Ann Arbor, Michigan 48109

Dept. of Geological Sciences Virginia Polytechnic Institute & State University Blacksburg, Virginia 24061

J U L I A N R. GOLDSMITH Dept. of Geophysical Sciences University of Chicago Chicago, Illinois 60637

J A N VEIZER Dept. of Geology University of Ottawa Ottawa, Ontario, Canada K I N 6N5

MICHELE LOIJENS H A N S - R U D O L F WENK

R O L A N D WOLLAST Laboratoire d'Oceanographie Université Libre de Bruxelles 1050 Brussels, Belgium

Series Editor:

Dept. of Geology ic Geophysics University of California Berkeley, California 94720

P A U L H. RIBBE

Dept. of Geological Sciences Virginia Polytechnic Institute & State University Blacksburg, Virginia 24061

MINERALOGICAL SOCIETY OF AMERICA

COPYRIGHT

1983

MINERALOGICAL SOCIETY of AMERICA PRINTED BY BookCrafters, Inc. Chelsea, Michigan 48118

REVIEWS in MINERALOGY (Formerly:

SHORT COURSE NOTES) ISSN 0275-0279

Volume 11:

CARBONATES: MINERALOGY and CHEMISTRY ISBN 0-939950-15-4

ADDITIONAL COPIES of this volume as well as those listed below may be obtained at moderate cost from:

Mineralogical Society of America 2000 FLORIDA AVENUE, N.W. WASHINGTON, D. C. 20009 Volume 1

SULFIDE MINERALOGY, P.H. Ribbe, editor

(1974)

2

FELDSPAR MINERALOGY, P.H. Ribbe, editor

3

OXIDE MINERALS, Douglas Rumble III, editor

4

MINERALOGY and GEOLOGY of NATURAL ZEOLITES, F.A. Mumpton, editor

(1975; revised 1983) (1976) (1977)

362 p. 502 p. 232 p.

5

ORTHOSILICATES, P.H. Ribbe, editor

6

MARINE MINERALS, R.G. Burns, editor

7

PYROXENES, C.T. Prewitt, editor

8

KINETICS of GEOCHEMICAL PROCESSES, A. C. Lasaga and R.J. Kirkpatrick, editors (1981)

391 p.

9A

AMPHIBOLES and OTHER HYDROUS PYRIBOLES - MINERALOGY, D.R. Veblen, editor (1981)

372 p.

9B

AMPHIBOLES: PETROLOGY and EXPERIMENTAL PHASE RELATIONS, D.R. Veblen and P.H. Ribbe, editors (1982)

390 p.

10

CHARACTERIZATION of METAMORPHISM through MINERAL EQUILIBRIA, J.M. Ferry, editor (1982)

397 p.

11

CARBONATES,

(1980; revised 1982)

284 p.

(1979)

(1980)

R.J. Reeder, editor

ii

(1983)

450 p. 380 p. 525 p.

CARBONATES:

MINERALOGY and CHEMISTRY FOREWORD This is Volume 11 of REVIEWS in MINERALOGY, a series begun as "Short Course Notes" for the Mineralogical Society of America in 1974.

This review

of the mineralogy and chemistry of carbonates appears just four years after the M.S.A. Short Course on Marine Minerals which covered marine manganese oxides and iron oxides, silica polymorphs, and the zeolite, clay, phosphorite, barite, evaporite, and placer minerals. Volume 6, Marine

Minerals,

Carbonates were not included in

because, as editor Roger Burns put it, ". . .

coverage of this important mineral group warrants a separate monograph." Several years ago, Rich Reeder, who was and still is very excited about new vistas of research centered around application of the transmission electron microscope to carbonates, volunteered to organize a short course on carbonates for the Mineralogical Society of America.

His success is commemorated

by this volume with its fifteen authors, nine of whom lectured October 28-30, 1983, just prior to the annual meetings of M.S.A. and the Geological Society of America at Indianapolis, Indiana. Paul H. Ribbe Blacksburg, VA S e p t e m b e r 1, 1983

PREFACE and ACKNOWLEDGMENTS This volume of Reviews

in Mineralogy

attempts to synthesize our present

understanding of certain aspects of the mineralogy and chemistry of the rockforming carbonates. of research.

Hopefully, it reflects the presently more active areas

This review follows, by ten years, a major assessment of (sedi-

mentary) carbonate minerals by Lippmann (1973).

There is only minor overlap

of subject material, and I hope that this difference reflects fairly how this field has developed. In some respects carbonates are unique, for they are one of the few mineral groups providing an abundant record of biological, physical, and chemical processes throughout much of geologic time.

Because of their re-

lative importance in sedimentary rocks, low-temperature examples are given more emphasis here.

Moreover the obvious correlation with energy resources iii

has been a significant factor contributing to the current resurgence of interest in this area.

However, the broader interest in carbonates is also a

reflection of their widespread occurrence in vastly different geologic environments, including metamorphic and igneous settings, as well as an appreciation of their role in both atmospheric and oceanic chemistry, both past and present. In this volume, some of the papers are general (i.e., those addressing crystal chemistry and phase relations), and they provide overviews of a fundamental nature and are of interest to many.

Others are more specialized in

coverage and generally reflect the different approaches used in carbonate geochemistry.

The final chapter introduces transmission electron microscopy,

a relatively new and powerful technique for mineralogical research that has great potential in carbonate research. Owing to the short time interval between the completion of manuscripts and publication, much of the newer material in this volume is still "fresh." The various reviewers, all gratefully acknowledged, were expeditious in their efforts.

A hurried schedule, however, allows for unnoticed errors to persist;

these should be brought to my attention. This volume would not have been possible without the cooperation and help of the authors, all of whom I thank.

Many others contributed in various

ways; in particular I thank my friends and colleagues at the State University of New York at Stony Brook.

Support in various but essential forms was pro-

vided both by SUNY and by Virginia Polytechnic Institute and State University. Special thanks go to Paul H. Ribbe for advice, reviews, copy editing, and for producing the final product.

Typing of the final manuscript was done by

Ada Simmons, Sarah Vaughan, Theresa Beddoe and Margie Strickler, with editorial assistance by Julie Anne Ribbe. Richard J . Reeder Stony Brook, NY September 1, 1983

SPECIAL ACKNOWLEDGMENT The Mineralogical Society of America is g r a t e f u l to AMOCO Production Company, A R C O Oil and Gas Company, and SHELL Development Corporation for providing scholarships for graduate students to attend the Short Course on carbonates.

iv

CARBONATES:

MINERALOGY and CHEMISTRY TABLE of CONTENTS COPYRIGHT; LIST OF PUBLICATIONS

Page ii

FOREWORD

iii

PREFACE; ACKNOWLEDGMENTS

Hi

1

Chapter 1

CRYSTAL CHEMISTRY of the RHOMBOHEDRAL CARBONATES Richard J. Reeder

INTRODUCTION

1

RHOMBOHEDRAL VERSUS HEXAGONAL AXIAL SYSTEMS

2

THE CO 3 GROUP AS A STRUCTURAL UNIT

4

THE J?3c CARBONATES The calcite structure Calcite isotypes Atomic thermal vibrations Structural variation Solid solutions of R3o carbonates solid solution The magnesian calcite The CaC03-CdC03 solid solution The Ni-Mg carbonate solid solution parameters in other solid solutions THE R3Lattice CARBONATES The dolomite structure Interatomic distances Octahedral distortion Thermal parameters Dolomite isotypes Ankerite and ferroan dolomite Kutnahorite Other transition metal dolomites Calcian dolomites and ankerites Cation order in dolomite-structure carbonates Thermal disordering Structural changes Disorder in lew-temperature dolomites Lattice parameters of dolomite-type carbonates

6 6 8 10 14 17 17 20 20 21 22 22 23 26 28 28 28 31 31 32 32 32 35 35 37

OTHER DOUBLE CARBONATES

40

CRYSTAL CHEMISTRY AT HIGH TEMPERATURES AND PRESSURES

43

Thermal expansion High-temperature transformations of calcite CaC0 3 (II) structure

43 45 45

SUGGESTIONS FOR FUTURE WORK

46

ACKNOWLEDGMENTS

47 v

Chapter 2 2

PHASE RELATIONS of RHOMBOHEDRAL CARBONATES Julian R. Goldsmith Page

INTRODUCTION AND EXPERIMENTAL RESULTS

49

THE END-MEMBER CARBONATES

51

DOLOMITE-TYPE COMPOUNDS

51

BINARY PHASE RELATIONS

51

CaC0 3 -MgC0 3 Relations at moderate pressures Relations at higher pressures and temperatures Additional considerations in subsolidus relations CdC0 3 -MgC0 3 CaC0 3 -MnC0 3 CaC0 3 -FeC0 3 Additional binary joins with a single solvus CaC03-NiC03; CaC03-CoC03; MgC03-NiC03 Binary joins with extensive solid solubility The join CaC0 3 -ZnC0 3 and Zn-dolomite A note on the asymmetry of the solvi TERNARY PHASE RELATIONS

51 51 55 58 60 60 62 63 63 64 64 65 66

CaC0 3 -MgC0 3 -FeC0 3 CaC0 3 -MgC0 3 -MnC0 3 Order-disorder relations in Fe- and Mh-containing dolomites The systems CaC0 3 -MgC0 3 -CoC0 3 and CaC0 3 -MgC0 3 -NiC0 3

66 70 70 72

SUGGESTIONS FOR FUTURE WORK

75

ACKNOWLEDGMENTS

76 Chapter 3

3

SOLID SOLUTIONS and SOLVI among METAMORPHIC CARBONATES with APPLICATIONS to GEOLOGIC THERMOBAROMETRY Eric J. Essene

INTRODUCTION

77

EXPERIMENTAL DATA BASE

77

DETERMINATION OF CARBONATE COMPOSITIONS

78

COMPOSITIONS OF METAMORPHIC CARBONATES Rhombohedral carbonates Orthorhombic carbonates

80 81 84

SOLVUS LIMITS IN METAMORPHIC CARBONATES Solvi in the system CaC0 3 -MgC0 3 -FeC0 3 Solvi in the system CaC0 3 -MgC0 3 -MnC0 3 APPLICATIONS OF CALCITE-D0L0MITE THERMOMETRY Regional metamorphic rocks Contact metamorphic rocks

85 85 86 88 89 91

THE ARAGONITE-CALCITE TRANSITION AS A THERM0BAR0METER . . . . . . . . . vi

93

CHAPTER 3, continued

Page

COEXISTING ORTHORHOMBIC AND RHOMBOHEDRAL CARBONATES

94

SUGGESTIONS FOR FURTHER WORK

95

SUMMARY

95

ACKNOWLEDGMENTS

96 Chapter 4

4

MAGNESIAN CALCITES: LOW-TEMPERATURE OCCURRENCE, SOLUBILITY and SOLID-SOLUTION BEHAVIOR Fred T. Mackenzie, William D. B i s c h o f f , Finley C. Bishop, Michele Loijens, Jane Schoonmaker & Roland Wollast

INTRODUCTION

97

LCW-TEMPERATURE OCCURRENCE

98

Skeletal magnesian calcites Magneslan calcite cement UNIT CELL PARAMETERS:

98 98

BIOGENIC AND SYNTHETIC INORGANIC PHASES

High-temperature synthetic materials Synthesis techniques Unit cell parameters Low-temperature synthetic materials Biogenic materials SOLUBILITIES AND SOLID SOLUTION BEHAVIOR Solubilities of magnesian calcites Dissolution experiments Precipitation experiments Interpretation of experimental data Theoretical considerations Thermodynamic equilibrium Stoichiometric saturation Experimental tests of stoichiometric saturation Expression of solubility Evaluation of thermodynamic properties from dissolution experiments Other approaches to estimation of magnesian calcite properties Solid solution properties Estimation of free energy of mixing from high-temperature data Estimation of excess lattice energy Evaluation of the heat of mixing by calorimetric measurements Hypothesis of an hydrated magnesian calcite SOME CONCLUDING REMARKS

INTRODUCTION

104 104 105 107 108 112 114 114 118 118 119 120 124 126 127 129 132 132 135 136 138 140 142

ACKNOWLEDGMENTS

5

104

144 Chapter 5 CRYSTAL CHEMISTRY and PHASE RELATIONS of ORTHORHOMBIC J. AlexanderCARBONATES Speer

CRYSTALLOGRAPHY

145 145

vii

CHAPTER 5, continued

Page

Crystal structures Charge distribution /4C03 polymorphism Twinning Morphology The CaBa(C03)2 phases Carbocernaite

145 150 151 152 152 154 157

CHEMISTRY

158

Inorganic orthorhombic carbonates Biogenic orthorhombic carbonates Reoent shells Fossil shells ISOTOPIC COMPOSITION

169

Dependence on crystal chemistry Dependence on physical conditions Dependence on source PHASE RELATIONS

170 170 171 171

Unary systems Binary systems CaC03-SrC03 CaC03-BaC0 3 CaC03-PbC03 SrC03-BaC03 SrC03-PbC03 BaC03-PbC0 3 Ternary systems Phase relations in aqueous systems Phase relations involving noncarbonate minerals OCCURRENCE OF ORTHORHOMBIC CARBONATES AND DISCUSSION OF THEIR COMPOSITIONS AND MINERAL ASSEMBLAGES PHYSICAL PROPERTIES

171 173 173 174 174 174 175 175 176 177 178 178 182

Density Lattice parameters Optical properties Luminescence Magnetic properties Infrared and Raman spectra

182 184 185 188 188 188

SUGGESTIONS FOR FUTURE WORK

6

158 161 161 167

189 Chapter 6

The POLYMORPHS of C a C 0 3 and the ARAGONITE—CALCITE TRANSFORMATION William D. Carlson

INTRODUCTION

191

THE POLYMORPHS OF CALCIUM CARBONATE The calcite-aragonite equilibrium CaC03(II) and CaC03(III) CaC03(IV) and CaC03(V) The role of rotational disorder of C03 groups Evidence for disordering

191 193 195 196 198 198

viil*

CHAPTER 6, continued

Page

Possible disordering schemes Speculations on disorder in CaCO3 as a function of pressure and temperature ARAGONITE-CALCITE

TRANSFORMATIONS

The calcite-to-aragonite

IN THE SOLID STATE

201 202

transformation

202

The transformation under conditions of aragonite stability The transformation outside the aragonite stability field The aragonite-to-calcite

200

transformation

202 203 203

204 Pétrographie observations Experiments measuring bulk transformation rates 204 207 Independent determination of growth rates Extraction of nucleation rates from bulk transformation rates 207 Summary 209 THE ARAGONITE-CALCITE Observational

TRANSFORMATION

evidence

IN THE PRESENCE

for the nature

OF WATER

of the aqueous

211

transformation

Marine environments Fresh water environments Mechanisms

212

212 212

of the transformation

213

Dissolution at surface of parent crystal Nucleation of product crystal Transport of complexes in solution Precipitation at surface of product crystal

214 214 214 214

Interpretation of natural occurrences in terms of reaction mechanisms

215

Reasons for extremely limited extent of transformation in sea water 215 Reasons for apparent differences in transformation type 215 Reasons for oriented overgrowths and topotaxial replacement 217 218 Reasons for selective transformation of skeletal materials Reasons for long-term preservation in ancient limestone 218 Experimental

attempts

to quantify

reaction

kinetics

219

The significance of transport processes The time dependence of the overall transformation Inconsistency of a time-squared volume dependence with other observations A possible explanation for the time-squared volume dependence CONCLUSIONS

AND OPPORTUNITIES

FOR FURTHER

RESEARCH

222 225

Chapter KINETICS

220

224

ACKNOWLEDGMENTS

^The

219 219

of CALCIUM

CARBONATE John

W.

7 DISSOLUTION

and

PRECIPITATION

Morse

INTRODUCTION

227

BASIC PRINCIPLES

228

REACTION

KINETICS

IN SIMPLE SOLUTIONS

231

Dissolution

231

231 234

General observations Models and mechanisms ix

CHAPTER 7, continued

Page

Precipitation General observations and models Secondary nucleation DISSOLUTION AND PRECIPITATION REACTIONS OF NONBIOGENIC CARBONATES IN COMPLEX SOLUTIONS General considerations Major influences Models for reacting surfaces and inhibitors Individual seawater-component influences Magnesium Sulphate Reaction kinetics in seawater and related solutions Dissolution Precipitation Other specific influences Phosphates Heavy metals Organics SPECIFIC TOPICS

238 238 239 241 241 241 242 244 244 246 246 246 247 249 249 252 253 254

Biogenic carbonate dissolution kinetics General considerations Influence of grain size Prediction of solubility from kinetics Kinetic influence on coprecipitation reactions Experimental methods Cation coprecipitation with calcite ACKNOWLEDGMENTS

254 254 256 258 261 261 262 264

Chapter 8 O

TRACE ELEMENTS and ISOTOPES in SEDIMENTARY CARBONATES Jân Veizer

INTRODUCTION

265

INCORPORATION OF TRACE ELEMENTS INTO CARBONATE MINERALS

267

INCORPORATION OF STABLE ISOTOPES OF OXYGEN AND CARBON INTO CARBONATE MINERALS Theoretically predicted composition of carbonate minerals Isotopic variations in natural waters Isotopic composition of natural carbonates INCORPORATION OF RADIOGENIC ISOTOPES INTO CARBONATE MINERALS Radiocarbon U-series disequilibrium nuclei Isotopes of strontium

271 272 276 278 279 279 282 284

DIAGENETIC REPARTITIONING OF TRACERS

285

RECORD OF TERRESTRIAL EVOLUTION IN ANCIENT CARBONATES Oxygen isotope paleothermometry Post-Triassic paleoceanography Carbon isotopes of ancient oceans: the story of life Strontium isotopes and buffering of the oceans x

288 289 289 292 294

CHAPTER 8, continued

Page

Oxygen isotopic composition of sedimentary carbonates Secular variations in chemical composition of carbonates CONCLUDING REMARKS

296 298 299

ACKNOWLEDGMENTS

300 Chapter 9

^

MICROSTRUCTURES in CARBONATES Hans-Rudolf Wenk, David 3. Barber & Richard 3. Reeder

INTRODUCTION

301

ORIGINS OF MICROSTRUCTURES

303

Deformation microstructures Transformation microstructures Growth microstructures dislocations Growth bands Faults Stacking disorder Twins

303 306 311 311 311 313 313 313

METHODS OF ANALYSIS

313

Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) Scanning transmission electron microscopy (STEM) High resolution electron microscopy (HREM) POLYMORPHIC TRANSFORMATIONS

314 315 317 318 319

Cation ordering in dolomite Aragonite calcite transformation Observations in sedimentary rocks Observations of the solid state transformation in the TEM VARIATIONS IN STACKING ORDER

319 321 322 326 328

Periodic basal superstructure in dolomite Stacking disorder and polytypes in rare earth carbonates MODULATED STRUCTURES PARALLEL TO r = {1014}

328 329 332

Calcian dolomites Saddle dolomites Calcite Carbonatites Interpretation

332 338 343 344 348

OTHER SEDIMENTARY CARBONATES

352

Recent dolomites Magnesian calcites

352 352

DEFORMATION

354

Deformation mechanisms Calcite Dolomite The effects of twinning Biaxiality

354 355 356 357 359

xi

CHAPTER 9, continued

Page

Other defects in deformed carbonates Dislocations Stacking faults Stylolites - "Pressure solution" ACKNOWLEDGMENTS

359 359 360 364 367

REFERENCES

xii

369

1

C R Y S T A L CHEMISTRY of the R H O M B O H E D R A L

CARBONATES

Richard J. Reeder INTRODUCTION Crystal chemistry plays an important role in understanding the behavior of minerals, both in terms of reactivity and stability.

Crystal growth processes,

whether in the solid state or in aqueous systems, are influenced by the arrange ment of atoms in the crystal and the constraints imposed by coordination and bonding.

Stability differences among natural minerals, so important for inter-

preting the potential for chemical change, reflect detailed differences in structure and bonding, as do physical properties. is available

A multitude of techniques

to study these, but in all cases the essential element is an

understanding of the crystal structures involved. The history of crystal structure investigations of the common rhombohedral carbonates has been long and somewhat sporadic in nature.

The general structur

of calcite was firmly established early in the century by Bragg (1914), and that of dolomite just ten years later by Wasastjerna (1924) and Wyckoff and Merwin (1924).

Accurate structural refinements awaited the development of more

sophisticated data collection methods, and by 1965, studies had yielded fairly precise determinations of positional and thermal parameters for calcite (Sass et al., 1957; Inkinen and Lahti, 1964; Chessin et al., 1965) and dolomite (Steinfink and Sans, 1959).

Lippmann (1973) thoroughly reviewed data available

at that time, giving a more complete historical development.

Since then, con-

tinued interest in carbonate minerals has fostered many new crystal structure studies, and today modern refinements have been undertaken for most of the common rhombohedral carbonates.

The present review is primarily based on the more

recent studies; for references to much of the older work, see Lippmann (1973). The preferred technique for structural studies has been x-ray diffraction, although recently transmission electron microscopy and electron diffraction have been used (see Chapter 9).

Studies of the vibrational spectra (IR and

Raman) of carbonates have been reviewed by White (1974) and Scheetz and White (1977). Of the rock-forming carbonates, calcite and dolomite are the most abundant accounting for more than 90% of natural carbonates.

The structure of calcite

is descriptively fairly simple, and it is the same as that taken by several other anhydrous, single carbonates including magnesite (MgCO^), siderite (FeCO^ rhodochrosite (MnCO^), otavite (CdCO^), smithsonite (ZnCO^), sphaerocobaltite (CoCO,) , and gaspeite (NiC0„) . Single carbonates with cations larger than Ca 1

2+

form with the orthorhombic

aragonite structure. 2+ to the size of Ca

The explanation given for this preference usually relates

, which is near the limit for 6-fold coordination.

aragonite structure, the cation is 9-coordinated.

In the

It is interesting to note,

however, that at high temperatures the orthorhombic carbonates BaCO^ and SrCO^ invert to a rhombohedral, calcite-like structure (Chang, 1965). RHOMBOHEDRAL VERSUS HEXAGONAL AXIAL SYSTEMS When dealing with the rhombohedral carbonates, one has the choice of using either a rhombohedral or hexagonal axial system.

It should be noted that the

smallest unit cells for both calcite and dolomite are acute primitive rhombohedrons (Fig. 1) with cell contents of 2 CaCO^ and 1 CaMgiCO^^, respectively. Rhombohedral cells are described entirely by two parameters —

the edge

length, and a^, the angle between any two of the three equal length edges forming the apex of the rhomb.

Since the line connecting the apices of the rhombo-

hedron is a three-fold axis, all lattice points of the rhombohedron can be equally well described using a hexagonal axial system.

The rhombohedron may

have two possible orientations with respect to the hexagonal axes called the obverse and the reverse settings (International Tables for X-ray Crystallography, Vol. I).

The obverse setting shown in Figure 2 is now standard.

In

both calcite and dolomite, the heights of the hexagonal cells are the same as for the acute rhombohedral cells, although they contain 6 CaCO^ and 3 CaMg(CO^). respectively, and are triply primitive. The primary advantage of the hexagonal axial system lies in the disposition of the axes relative to one another.

The three a axes, of equal length,

intersect at angles of 120° with respect to one another, and all lie perpendicular to the a axis.

This arrangement makes it relatively easy to visualize

geometric aspects of the crystal structure, especially planar features at distinct levels along the a axis.

This becomes rather awkward using the rhombo-

hedral axes, particularly so when comparing different minerals because the angle between axes changes from one mineral to the next. Modern descriptions of rhombohedral carbonates are given almost exclusively in terms of the hexagonal cell.

Much of the older literature, however,

uses a rhombohedral description, which may be based on either the true unit cell or the morphologic cell (also called a cleavage cell).

The latter (shown

in Fig. 3) should be avoided since it is not a correct unit cell.

A proper

morphologic unit cell does exist, but its a cell edge is twice that of the morphologic cell shown in Figure 3, and its cell contents are 16 times that of the acute rhombohedral cell. Owing to the presence of the fourth axis (a%) used in the hexagonal axial

2

Figure 1 (to the left). Schematic illustration of the true rhombohedral unit cell for calcite containing 2 CaCOß. The apical angle a is ^ 46° for this acute rhombohedron. Closed circles represent Ca positions; two CO3 groups are shown. After Lippmann (1973).

Figure 2 (below). Illustrations showing the obverse setting of the rhombohedron in relation to the hexagonal unit cell: (a) side view, (b) plan view. From Int'l Tables for X-ray Crystallography, Vol. I.

•0

»0

Figure 3. Schematic illustration showing the relationships between the true rhombohedral unit cell (the acute rhombohedron) and the morphologic (or cleavage) cell in calcite. Notice that the true cell is doubled in height w i t h respect to the morphologic cell. The hexagonal unit cell is also shown. From Hurlbut and Klein (1977).

3

h k i SL, where

system, four-symbol Miller-Bravais indices should be used — h + k + i = 0.

Although some authors prefer to drop the superfluous i when

writing indices, its presence leaves no doubt that hexagonal indices are in fact being used. The following expressions are useful w h e n converting between hexagonal and rhombohedral indices:

K

h

= h

k. = k h

i,

h

r r

- k

- i

= h + k r

h

r r r

r

r

2/3 h,+ 1/3 k, + 1/3 n n

k

= -1/3 h,+ 1/3 K + 1/3 n n *h

I

= - 1 / 3 hh-

r

+ a

=

r

2/3 kh + 1/3

£

h

Transformations for fractional atomic coordinates between the two are as follows: x h = 2/3 x r - 1/3 y r - 1/3 z r

xr =

y h = 1/3 x r + 1/3 y r - 2/3 z f

yr =

+ y h + zh

z, = 1/3 x + 1/3 y + 1/3 z h r r r

z

- y, + z, •'h h

r

xh

=

+ zh

Cell parameters can be related by the following conversions :

a, = 2a h

a

h

a

r

sin

a

2

1= (9a 2 - 3al) 2 r h

r

= 1/3 (3a} + a}) h

h

V2

"r sin -r- = a,/2a L n r

A l l descriptions in the present chapter will refer to the hexagonal unit cell. THE C 0 3 GROUP AS A STRUCTURAL UNIT The CO^ group is the fundamental chemical unit from which the carbonate minerals derive their identity.

This anion is also present in some minerals

that might not be best described as carbonates, scapolite being a good example. There in fact exists a large number of minerals where the CO^ group is one of several anions (or polyanions) in the structure.

Hydroxyl is perhaps the most

common "other" anion in these mixed-anion carbonates. Despite the wide variety of carbonate mineral structures, the basic configuration of the COj group is found to be remarkably uniform in all but a few cases.

In its general form the CO^ group resembles an equilateral triangle with

oxygen atoms at the corners and a carbon atom in the center.

This exact con-

figuration is attained in calcite and its isotypes, where the local point symmetry of the group is 32.

Zemann (1981) notes that a higher point symmetry

(6m2) characterizes one of the two distinct CO^ groups in fairchildite 4

(K^Ca

(CO^^)» although the structure of the group is essentially identical with that in calcite.

Slight deviations from this form reduce the point symmetry below

that in calcite, and reflect differences in coordination or electronic environment . Zemann (1981) has compiled recent structural data for CO^ groups in 30 dif ferent carbonate minerals.

He found the mean value for the O-C-O angle to be

120° in agreement with the ideal value.

Deviations from this are generally

rather small, and the great majority of carbonates have CO^ groups where the O-C-O angle approximates the ideal value rather closely.

In a few minerals,

large deviations may be found (up to 11° in one case), but seem to occur only where the edge of a CO^ group is shared with an edge of a coordination polyhedron of an adjacent cation.

In acid carbonates, such as NaHCO^ and KHCO^,

the O-C-O angles (now for an HCO^ radical) are found to deviate from 120° by several degrees. Zemann found the mean C-0 bond length to be 1.284 A with a standard deviation of only 0.004 A for average values in a CO^ group.

Tabulated values range

between 1.25 and 1.31 A in all but a few extreme cases.

A very useful compari-

son is provided by the x-ray structural data of Effenberger et al. (1981) for calcite, magnesite, siderite, rhodochrosite and smithsonite.

Within this iso-

structural series the maximum variation of the C-0 bond length is only 0.005 A , a clearly measurable but quite small variation.

Thus the effect of different

cations on the C-0 bond length within this structure type is minimal.

The

result of the short C-0 bond length is a rather close approach for oxygen atoms within the CO^ group —

2.22 A .

Slightly shorter nonbonding 0...0 distances

are known for NaNO^ and stishovite (cf. Table 5 of Shannon and Prewitt, 1969). In contrast to the CO^ group, the C-0 bond lengths in the acid carbonates vary considerably within individual HCO^ ions.

Sass and Scheurman (1962) repor

bond lengths of 1.346, 1.264, and 1.263 A for NaHC03< In those carbonates where layers of CO^ groups separate layers of differen; cation types, the carbon atom is displaced from the plane formed by the 3 oxygei atoms.

Dolomite, with its two distinct cation layers, serves as a good example

Recent x-ray structural refinements of ordered dolomites (see Table 8 below) show displacements of 0.018, 0.020, 0.022 and 0.027 A .

The latter value (from

Althoff, 1977) appears to be somewhat large, and the other values are perhaps more typical for dolomite.

In buetschliite l^Ca(CO^^, a displacement of 0.038

A is reported by Knobloch et al. (1980).

The direction of the displacement is

always toward the layer of cations with the smaller radius.

Since the juxta-

position of unlike cation layers forms an ordered superstructure, the average displacement is sensitive to the degree of cation ordering. 5

For dolomite,

Reeder and Wenk (1983) have shown that the magnitude of the average displacement decreases as cation disorder increases.

However in most carbonates where

the deviation occurs it is quite small. Based on the uniformity of bond lengths and angles, it is reasonable to consider the CO^ group as a fairly rigid structural unit.

Such a viewpoint is

reinforced by consideration of the C-0 bond, which is generally regarded as being strongly covalent in character.

Recent studies of electron density dis-

tribution in calcite (Peterson et al., 1979) and dolomite (Effenberger et al., 1983; Reeder, unpublished data) show a buildup of charge density between C and 0, supporting this view.

In general, the C-0 bond will be considerably stronge

than any other M-0 bond typically found in carbonates.

In calcite, for example

the C-0 bond strength calculated using the empirical Brown-Shannon method (Brov and Shannon, 1973) is approximately four times greater than for the Ca-0 bond. When we consider atomic positions in the calcite structure in the following section, it will become clear that the excellent cleavage can, in part, be attributed to this substantial difference in bond strengths.

The cleavage

plane, {10l4}, is one that breaks the least number of Ca-0 bonds and no C-0 bonds. Several authors have considered the charge distribution in the C0^ group (Effenberger et al., 1981; Yuen et al., 1978; and references therein).

While

there is much disagreement in earlier studies, Yuen et al. and Effenberger et al. find point charges of approximately +1 and -1 for C and 0, respectively, for single rhombohedral carbonates. The i?3c CARBONATES The calcite structure Traditionally the calcite structure has been described using the NaCl structure as a starting point.

The primitive unit cell of the face-centered

cubic NaCl structure is a rhombohedron whose 3-fold axis is coincident with the 3-fold axis of the cubic unit cell (i.e., the body diagonal), but whose volume is one quarter of the latter.

If Na atoms are replaced by Ca atoms

and CI atoms by C0^ groups, a structure somewhat resembling calcite results. By necessity, the rhombohedron must be compressed in a relative sense along the 3-fold axis to accommodate the "flat" C0^ groups now lying perpendicular to this axis. At once we see the limitations of this description.

The CI atoms have a

spherical symmetry, being centers of symmetry in the NaCl structure, while the CO^ groups are triangular.

The different orientations of the C0^ groups in

successive carbonate layers are nowhere foreshadowed in the NaCl structure. 6

Figure 4 (to the left). Schematic illustration of the hexagonal unit cell of calcite, a = 4.99 A, a = 17.06 Â. Ca atoms are open circles; CO3 groups are triangles; z. coordinates for Ca layers are shown on the right. From Lippmann (1973).

Figure 5 (above). The CO3 layer in calcite as seen down the c axis. The CO3 groups have identical orientations throughout the layer. Dashed circles show the positions of Ca atoms projected from the cation layer above the CO3 plane. Relative sizes of the Ca and 0 circles correspond to crystal radii as assigned by Shannon and Prewitt (1969).

In fact, we will see that the rhombohedral cell derived from the NaCl structure is not a proper unit cell for calcite. Figure 4 shows a schematic model for the calcite structure.

It can be seen

that layers of Ca atoms alternate with carbonate layers along the a axis.

The

COj groups have like orientations within each layer, but reversed orientations in successive layers.

Thus the translation along a to similarly oriented CO^

layers is doubled in comparison to the analogous translation in the NaCl structure.

This requires a doubling of the unit cell height as shown in Figure 3.

Perhaps a better description of the structure is provided if the positions of the oxygen atoms are deferred to a pattern of hexagonal close packing (cf. Megaw, 1970, 1973).

While only approximate, this analogy does show the orien-

tational relationships among CO^ groups in successive layers.

Within a given

layer of hexagonally-packed oxygens, carbon atoms are introduced such that each oxygen coordinates with only one carbon resulting in a hexagonal distribution of C in that layer.

Calcium atoms then fill those octahedral intersticies

(between oxygen layers) that avoid coordination with two oxygens of the same CO^ group, since that 0-0 distance would be very short (Fig. 5).

C atoms in

successive oxygen (or CO^) layers are distributed so as to avoid superposition over either Ca or C atoms in adjacent cation and C0^ layers.

This ensures that

Ca is then coordinated to six oxygens, each belonging to different C0^ groups. The unit translation along the direction of stacking (i.e., along the a axis) 7

, i 1., Formal ,description , ... Table T

Space group:

R3c (No. 167)

Unit cell: Cell contents:

»e*aS°nal6 CaCOj

„ ,, . Cell parameters*:

. , ..

is equivalent to the height of six CO. ^ 3 layers.

of calcite structure

In

rhombohedrally-centered

c =17.0610

°

a n d

i n

f a c t

t h e

i d e a l

hexagonal pattern is not attained owingo o r

X

Ca in 6 (b) : 0, 0.

in

,

k e d r

.OOQ- o a = 4.9896 A

Atom positions":

reality, the oxygens are not

c l Q s e s t

tQ t h e

o

foreshortening of the 0-0 distanc
os rt e o •O h rt o

UJ c

ai OI

« +J 4> in rt h V CTI Í-» B0 -H «> h V C •O h -H C 4> T3 a) M O C h IH •H O tu u, aa a> o o CM (N o o o o o o rH r i ' ^

W K3 cr> w CM »O rH