NATIONAL BUREAU OF STANDARDS REPORT

9985

Progress Report

on THE CRYSTAL STRUCTURE OF CaC03 -6H2 0 at -120°C

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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:

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

31 1.05-1 1-3 1 10 56 1 December 31, 1968

NBS REPORT

9985

Progress Report

on THE CRYSTAL STRUCTURE OF CaC0

3

*6H

2 Q at -120°

C

by

Brian Dickens* and Walter E. Brown+

* Research Chemist, Dental Research Section, National

Bureau of Standards, Washington, D. C. 20 234

+ Director, Research Associate Program of the American

Dentarl Association, Dental Research Section, National

Bureau of Standards, Washington, D. C. 20234

This investigation is part of the dental research program

conducted by the National Bureau of Standards, in cooperation with the Council on Dental Research of the American

Dental Association; the National Institute for Dental Research;

the Dental Research Division of the U. S. Army Medical Research and Development Command; the Dental Sciences Division

of the School of Aerospace Medicine, USAF; and the Veterans

Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF ST/

for use within the Government,

and review. For this reason, the whole or in part, is not authori Bureau of Standards, Washingto

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in

Office of the Director, National

i the Government agency for which

pies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THE CRYSTAL STRUCTURE OF CaC03 *6H2 0 at -120°C

by

Brian Dickens and Walter E. Brown*

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

ABSTRACT

The crystal structure of CaC03 »6H2 0 has been determined

from 1420 x-ray diffraction data collected photographically by

the oscillation technique from a single crystal held at about

-120°C. The unit cell is a = 8.87 ± .02 A, b = 8.23 ± .01 A,

_c = 11.02 ± .02 A, 3 = 110.2 ± .2° and the space group is 02/c.

or Cc with z = 4. Refinement is satisfactory in 02/a. R w = / (s(w||f0 Hfc II) 2 /(Zw|

0 I)

2

) =0.12. R = e||f0 I — I

c ||A|f0 I =

0.10. The calculated density at about -120°C is 1.80 g/ml,

the observed density at about 0°C is 1.82 g/ml. The structure

contains discrete CaC03 ion pairs, each surrounded by an

envelope of 18 water molecules. Thus Ca is preferentially

coordinated to C03 .

*Director, Research Associate Program of the American Dental

Association at the National Bureau of Standards
CaC03 *6H2 0 was found to be more stable than CaC03 »H2 0

in water near 0°C. The formation of CaC03 »6H2 0 from an equiva

lent amount of calcite (CaC03 ) and water is accompanied by

a 20% decrease in volume. This may be important in explaining

the scarcity of calcareous material in life in the ocean

depths

.
-3-

INTRODUCTION

In calcium carbonate mineralization, nearly all of the

attention has been given to the anhydrous CaC03 forms (calcite,

aragonite and vaterite) , which are known to occur both simultaneously and individually in the shells of molluscs 1 and in

other biological minerals, 2 including gallstones. 3 The two

known hydrated calcium carbonates, CaC03 *6H2 0

4,5 and CaC03 *H3 0

4,6

have been neglected in this respect, possibly because of their

metastability with respect to the anhydrous salts. This neglect

may not be warranted because in aqueous environments (a) very

small particles of hydrated salts frequently may have greater

stability than similar particles of anhydrous salts because

of their probably lower surface energies, (b) hydrated crystals

tend to grow more easily than anhydrous crystals, (c) polyphosphate4 and magnesium6 ions repress the growth of anhydrous

calcium carbonates more than that of the hydrated salts, and

(d) low temperatures and high pressures, such as those in the

oceanic deeps, should favor the formation of the hydrated

compounds
-4-

The report of the structure of CaCQ3 ®6H2 0 given here

is part of a larger study of hydrated salts which have potential importance in biological mineralization. The structures

of CaNa2 (C03

) 2

• 5H2 0 (gaylussite) 7 and CaNa2 (C03 ) 2

• 2HS 0

(pirssonite) 7 9

8

have been reported. One of the major objectives

of our studies is to learn more about the interaction between

water molecules and the calcium and carbonate ions in hydrated

salts *

Early work on CaC03 *6H2 0 and CaC03

s H2 0, including some

chemical properties and preparative details using crystal

growth poisons , is summarized by Brooks , Clark and Thurstont

Powder x-ray diffraction data have been reported. 4

EXPERIMENTAL METHODS

Crystals of CaC03 ®6H2 Q up to 2 mm in diameter were grown

from calcium carbonate gels left for several months at 3-5 °C

in a solution containing 400 ppm sodium polyphosphate. The

calcium carbonate gels were prepared following a procedure

similar to that given by Brooks , Clark and Thurston. 4 Thus,

2.4 g CaCl2 dissolved in 100 ml water were added to 150 ml

water containing 4.6 g Na2 C03 and 0.5 g sodium polyphosphate.

After about six days the gel, which formed initially, had

disappeared and spherulites had formed. These were identified
-5-

as CaC03 *Hs 0 from an x-ray powder pattern. On standing

for several months, the spherulites dissolved and crystals

of CaC03 »6H2 0 were formed.

Two distinct views of the CaC03 »6H2 0 crystals were

common. (a) Plates with low birefringence and parallel extinction. These, when modified by (110) -type faces, exhibited

2/m symmetry with symmetrical extinction and with the fast

direction, Ng, parallel to b; the trace of the (110) (110)

,

93.4°, yields an axial ratio a/b = 1.05. (b) Short, highly

birefringent {010} rods or plates with the slow direction, N ,

extinguishing at 17° from c: in acute J3; these give £

= 109.4°

and c/a = 1.28. Difficulties in working with unstable crystals

prevented precise optical and morphological measurements, but

these values are in reasonable agreement with those derived

from x-ray data and they serve to orient the optical properties

with respect to the unit cell.
-6 -

Single crystals of CaC03 «6H2 0 were sealed in borate

glass capillaries at about 0°C and transferred to a precession

during which a stream of air at about 2°C was blwon over the crystal

camera for preliminary studies „/ A crystal with maximum diameter

about 0.3 mm ( u

jyio ” 8.8 cm”1

) was mounted on a Weissenberg

camera modified9 to allow the routine collection of inclined

oscillation data at low temperatures and was cooled down to

•~120 °C in a stream of cold gaseous nitrogen. The unit cell*

at -120°C was determined from Weissenberg and oscillation

photographs to be a^ = 8.87 ± .02 A, b = 8.23 ± .01 &,

_c = 11.02 ± .02 &, 6 = 110.2 ± .2° (assuming Xjy[0 = 0.71069 A)

with space group C2/c or Cc (reciprocal lattice extinctions

h + k = 2n + 1 for hki , i = 2n + 1 for hoi.) . A superimposed

aluminum powder pattern (_a = 4.0493 K) taken at room temperature

was used as a standard. With four formula weightsper unit

cell, the calculated density at about -120°C is 1.80 g/ml;

*The uncertainties in the cell dimensions are estimates

derived from the measurements. In the authors' opinion these

may be treated as standard deviations.
— 1 —

the density observed at about 0°C is 1,82 g/ml. 10 Data from

one hemisphere of the reciprocal lattice were collected in

18 oscillations about the _c axis with an inclination angle

of 30°, Molybdenum radiation filtered through 0.001 inch Nb

and eight films interleaved with 0.001 inch brass sheets were

used. The oscillation photographs were indexed using a program

written in FORTRAN V from a flow sheet of a similar program

written in MAD by Schilling and Nordman. 11 Oscillation photographs, rather than Weissenberg photographs, were taken to

decrease the time of data collection, to enable us to place

all the data on a common scale from one crystal setting, and to

collect data with higher l than would be possible from c:

axis equi-inclination photographs alone.

About 5,000 reflections were estimated by visual comparison with an intensity scale prepared from timed exposures of

a single reflection of the crystal. They were corrected for

the Lorentz-polar ization and Cox-Shaw1

effects and a common

scale was calculated from the scale factors

Kij =d8 Ki^Ki:j )/N

j£=i

between the _i and j. photographs where N is the number of Kj_ eKe j

pairs which gave non-zero products. Rejection tests were used

to discard individual inter-reflection ratios which differed

from the average by more than three times the mean deviations.
*** 8-

After two cycles of rejections, the scale

factors relating all other films to the first film remained relatively « All the reflections were then merged into a unique

set. No corrections for absorption were made and the steady

formation of a thin layer of ice on the capillary was ignored

because it did not appear to affect the relative intensities®

If absorption is attributed entirely to the crystal, an estimated

maximum absorption error of about 7% was thus introduced into

the observed diffracted intensities.

STRUCTURE DETERMINATION

The subsequent calculations were performed using the

crystallographic computing system (X-ray 63) assembled under

the editorship of J. M, Stewart at the University of Maryland.

Statistics on the quasi-normalized structure factors, 13

(< j E

2

j > made equal to 1) indicate that the space group is

probably centrosymmetr ic, i.e. C2/c. The structure refined

to a satisfactory residual in this space group and refinement

in Cc was therefore not attempted. The atomic scattering

factors used were taken from reference 14, except for those

of hydrogen, which were taken from reference 15. The quantity

E(w||f 0 |-|F C II) 3 was minimized in the full matrix least squares

refinements. The intensities were given weights of

for reflections less intense than , 1.0 for reflections
-9-

between I I and I 2 , and I 2 /l for reflections more intense

than I 2

. I x and I 2 were chosen as the lower and upper ends

of the easily readable range of the intensity scale used in the

visual estimation, and is the intensity after scaling

in the multiple film pack, but uncorrected for Lorentzpolarization and Cox-Shaw effects.

The structure was solved from the sharpened Patterson

function, calculated from the (E 2 -l) coefficients, and from

subsequent F0 Fourier syntheses. It was refined isotropically

to R = Z||F0 | — | F c ||/e|f0 | =0.14.

Because of the many high-angle reflections to which Ca

makes the primary contribution, a high correlation was found

between the scale factor and the Ca isotropic temperature

factor. The Ca temperature factor was therefore fixed at

BCa = 0.1 A

2

. Several classes of reflections were considered

unreliable and were not used in subsequent calculations.

These were (a) the reflections with sin0/\ < 0.35 because of

high background on the films at low angles; (b) the reflections within 1 cm of the center line of the films because

misalignment greatly affects their Lorentz-polar ization
- 10-

corrections; (c) the hkO reflections because of streaking

by white radiation, and (d) reflections with sin§/X > 1.05

because of difficulties with indexing. The number of observed reflections was thus reduced to 1420. Rejection of

class (a) means that the positions of the hydrogen cannot

be determined unequivocally from the x-ray data.

The structure was refined isotropically to R = 0.10 with

the Ca temperature factor and the hydrogen parameters kept

constant. This agreement is considered to be the limit of

the experimental data. Two cycles of anisotropic refinement

did not decrease R w significantly. The observed and calculated structure factors are given in Table 1, and the atomic

parameters are given in Table 2. A difference electron density

synthesis in which the coefficients were weighted by the

least squares weights was calculated. None of the ten highest

peaks in this difference synthesis was in a plausible position

for a hydrogen atom or any other unassigned atom. As expected,

the hydrogen atoms could not be found because the reflections

at low sinQ had been discarded. Probable hydrogen positions.

Table 3, were calculated from stereo-chemical considerations.

as described in the section on water environments
- 11- Table 1

Observed and Calculated Structure Factors

Columns are JL

,

!OF0 , 10F C for CaC03

• 6H2 O.

-I 12a 130 • -1 SOU 371 -

156 -250

)95 -628

61) 6)0

398 -625

6.0.L

8 I ) •- 1 l 65

6 <5 2'»» 8 3)6 282

13 631 -6)3

12 6)5 66T

-15 152 — 1 2*» -16 17) -165

-l) 180 168

-11 217 -205

-4 165 165 -H 22) 216

-6 )27 -360

-6 1 68 136 -3 2*1 -286

-2 119 87

-l 151 161

322

11 207 -202

1.) 187 176

-8 200 -292

-6 267 2)6

-5 158 1)6 -6 133 -1)8

-16 288 - )21 -l) -12 327 356

16? -8 20* -176

-5 163 16R -3 268 -?B?

-2 137 156

-1 2)2 285

18,2,1

-15 156 -177

-12 Do -166

-ll 181 -205

207 202 -5

i 016 -6 115 96 -4 -6)3 -2 156 -176 -3

181 -188 -15

261 -260 669 696 676 -521 5J3 501 386 -605 336 3)6 668* -661

518 559

10,0,1.

269 -226

6 16* -822

256 27)

265 270

236 -260

3)4 626

27) -335

-16 339 -356 -12 139 160

- 1 U 259 -263

-8 636 668

-6 337 -317

-6 261 299

-2 315 -351

10 309 3)8

12 3C-. -353

16 220 256

12 28) -306

1) 282 -36)

16 115 107

8 628 -622

9 369 -615

11 286 337

13 151 -179

16 262 258

260 -218

611 -6)1

262 266

299 -296

306 361 237 -272

251 218

161 131

635 -566

10 308 295

13 115 -106

15 3)6 322

-o 218 195

-5 6I0 628

1 1 5 j -126

2 312 365

3 75 78

6 31s -333

5 177 -162

6 235 260

7 186 181

8 20o -261

11 102 162

12 203 -190

6 199 193

7 250 260

10 330 -366

15 289 26)

611 -42)

233 -196

338 257 -

-8 326 -378

-7 347 -437

-5 6? 1 • 897

-3 548 • -743

-2 480* 647

-1 158 160

1 123 -112

2 168 -140

3 402 434

4 344 -3)1

5 470 -474

6 319 27)

11 286 226

12 198 -152

13 178 -151 8,2,1 -15 291 -321 -14 234 258 -13 212 2)1 -12 203 -196

-11 207 -237 -10 109 -90 -9 302 3 2

J

-8 200 -201

-7 34? -351

-5 307 299

-3 362 -413

-l 397» 507

t 327 -277

2 194 145

3 200 174

« 346 -284

•15 170 -155

•14 220 23?

•1) lit 93 12 206 -229

•ll 194 -173

•10 262 2)9

-5 ?lo 135 -4 203 -191

-3 312 -309

-2 183 137

-1 269 259

1 149 -127

2 367 -.02 3 197 -164

6 210 180

7 4)h 409

9 335 -27)

10 31 v 258

11 199 -136

12 411 -357

-12 190 - 1

*

15 245 -234

13 383 416

l l 369 -364

10 2?8 217

-9 l«J 149 -7 14) -142

3 )2o 260 -

127 -15 24, -203

308 -14 169 178 -245 -10 268 246

278 221 349« 463

87 -101 -3 187 179

-14 15) 164

17,3,1

•15 116 -114 14 155 173 12 195 -202 10 125 143 -7 177 157

-6 208 220

-2 120 178

5 160 -110

7 89 -74

8 421 -457

9 86 67

10 50). 653

11 279 -295

12 2)4 -26)

1) 202 186

14 105 131

-12 26 2 -257

-ll 126 128 -10 171 160

- M 165 -1*3

-4 2*8 -300

-3 182 195 -2 119 118

-3 217 244 -

-15 145 2*5 -251

-8 »2T -52>

-4 3*1 347

-3 261 280

6 205 -1)9

7 264 -201

8 383 311

9 496 440

10 343 -292

10, 6,

IS 21) 188 229 -222

13 257 -263

10 318 -310

-9 265 -272

-8 209 217 -4 320 316

-3 480

•IS 142 131

•14 129 -114 13 192 -134

-9 206 168

8 373 359

-2 232 246 -13 153 15 290 -310

-2

2)2 178

-1*4 -ll 23?

-12 15?

-7 178

-5 1 04

292 256 -1<

246 2)3

485

134

-9 297 -291

-7 265 237

10 165 152

11 237 -219

14 I?) 121

-13 24| 044

-ll J 29 305 -9 3)4 -302

-7 284 279

-5 251 -221

-) 298 26

6

-1 35* -V

-7 162 -11?

-% 34) 294

-13 218 -220

-ll 159 185 -9 223 -219

-7 380 398 -1

120 142 -10 159 148

-8 2*9 -24}

-7 106 189

-6 280 302

-4 190 -226

-1 121 -155

18, ‘

-14 263 307 -

-ll 193 173 -

4 450 -531 -l'

o 410 4)2 -1 7 402 -405

8 200 -182

9 l 69 155

10 299 290

12 320 -327

1) 194 189

14 211 ?«0

15 205 -221

18 107 92

•14 249 268 13 15) -153

-6 475» 647

1 270 335

2 526* 7?5

3 491 -605

6 212 185

7 126 102

9 241 244

10 269 278

11 224 -226

12 420 -494

14 261 26C

ll 119 114 10 497 580 -9 407 -4)6

2 392* 593

3 82 -74

4 131 -101

10 269 254

11 179 -141

12 26) -24)

15 I'.e 105

16 127 157

13 262 -271

12 207 -286

-15 125 -l IT -13 3)0 354 -1C 271 -276

-8 219 194

-6 191 -181

-5 356 404

-4 110 « 94

-3 157 -142

2 369 -)32

4 200 188

5 191 -170

ll.l.L -19 l?8 -121

-11 228 -19V

-10 ?86 -225

-9 107 137 -8 2*2 214

-9 191 152

-13 186 186 -10 161 141 -9 31 o 326

-8 14J -104

-7 250 -249

-4 177 -156

-3 175 -188

-2 178 170

-ll 225 -194 -10 272 247

-8 312 -326

-5 242 238

-5 144 126 -4 2w7 -218

-3 118 -9?

-2 165 174

-l 174 198

-12 15) -1)4

-8 2 44 -261

-7 )?? 326

-6 2*0 221

-5 2)1 -2 25

-4 406 -410

-l lil -142

1 2 »2 274

4 38) -30)

6 260 213

7 120 -124

8 213 -169

14 312 29)

-4 172 -16}

-) 167 140

-2 132 135

2 38) 38b

12.4,1

•15 157 157

•12 162 -146 10 299 258

-9 2)0 -18*

-8 470 -426

-7 219 171 -6 297 281 -2 ITS 197

-11 160 171 -7 167 174 -5 236 -266

-8 147 214 -5 225 -190 -3 187 -171

-5 144 111 29 2 298

-15 296 321 -2 128

-11 193 229 -12 2)7

8 -228 -ll 126

15 25d 263

13 251 -259

12 126 106

120 106

76 -9 23s - 1 8

-8 118 -116

-7 318 301

-6 221 -203

-5 329 -332

-4 2)7 195

3 331 -319

4 236 -200

0 , 6.1 -12 263 2*9

1 222 -219

2 605 -*42

) 572* -660

4 573* 703

5 593» 740

5 351 -3)6

7 203 -156

8 247 221

9 180 151

10 300 -295

11 226 -2C7

12 263 2-9

1) 2)7 222

14 248 -204

15 255 -301

267 -246 -14

203 185 -12 -224 -5 007

246 198

197

'2 -330

-15 187 211 -13 259 -246

-ll 196 179 -10 117 83

-9 173 -150

-8 261 -233

-7 241 209

-6 23) -187

-5 308 -305

-4 5C 1 • 637

-3 462 • 553

3 262 -209

5 555 657

6 109 -174

7 2C2 -230

6 63 62

10 121 116

11 2 62 -2)9

13 256 265

14 U7 -114

5,5,1.

-15 392 409

-14 200 -3w5

-10 1*1 -119

-10 224 -211

-•> 382 -401

-6 477 525

-7 561 679

-5 359 -385

-5 210 -176

-4 273 2*9

-3 261 2)3

-2 441 -518

-l 449» -576

2 274 -237

) 266 -246

5 352 375

7 366 -)}1

0 327 295

* 297 256

10 278 -264

12 223 225

14 186 -2„1

15 180 -179

267 -227 277 -231

1 4 171 -155

•13 219 -221

•12 268 271 10 209 -218

-5 225 -216

-4 204 229

-3 167 16)

202 -10 170 -190

0 282 287

1 368 -447

2 229 -203

3 *29 449

5 20V -177

6 136 105

7 279 234

9 472 -487

10 291 -274

11 3*8 324

12 264 205

1) 184 -161

-l 146 174 -1

-5 229 271 -3 162 -187

-14 226 214 -12 237 -215

-1C 214 176

-8 250 -218 -1*8

-l) 202 -177

-ll 37) 322 -1) 238 2(-b -0 205 -loo

-8 1*2 -118

-5 169 -143

-4 216 -2,3

-3 2)7 224

-2 225 231

152 -ll

67 -1 157 244 200

-7 224 -188

-6 237 -210

-5 173 140 -4 104 162 -2 222 - 22

1 12e -6 ITS -167

-4 197 181 -3 14? 133

•1 124 110

-1)

26 6 221

-1 176 -207 15 26u 25)

16,6,1.

•15 119 118 14 127 -143 13 154 -150

•ll 265 296 10 22) -244

-9 208 -229

-6 159 178

15 227 -234

•I) 32 d 340 ll 335 -29? 10 228 -19)

234 250 -5 2l» 1*7 -3 396 -467

-2 282 -2*4

-10 250 205

-V 260 224

-8 180 -145

-7 274 -268

-6 1*0 -145

-5 275 282

-4 208 -193

-) 2o

9

-287

-2 4C4 452

-l 255 277

1 238 -261

2 270 -251

3 2-3 230

4 251 -242

6 323 306

7 309 2 98

9 279 -254

11 227 183

I ) 191

412 358

-5 23) 194 -4 384 -375

-2 2)4 201

1 207 -179

2 35 1 32-

4 414 -372

6 254 201

7 217 17)

8 219 -189

9 145 -104

10 346 5C3

12 2*6 -226

4,10,

-12 209 -18)

-ll 1*7 -109 -13 296 256

-8 294 -274

-7 151 106

-6 224 188

-5 150 132

10 193 164

11 168 -149

13 24? 228 192 -160 -6

260 241 .y

202 172 .4 15 171

•14 207 187

-9 267 -243

-8 265 -251

-7 325 309

-6 171 15C -5 165 -1)1

l 303 311

3 28* -269

l) 13) 68

3, 13.1

14 202 -171

-7 152 132 -5 207 -158

-4 380 344

l 2)0 207

3 1*2 -132

4 232 20*

18) 152 225 218

-2 211 - 21

V

ll 155 -172

-7 264 -240

-5 267 241

1 337 -370

3 267 247 4 17) 135

9 169 -1*2

-12 226 -2v/6 -ll 225 208 -9 268 -270

-7 189 loC -4 165 -120

-1 140 -116

11,11,1

-12 132 -04

-11 167 1 7C -9 216 -193

-4 157 - 1 i

V

-159 -ll )38 -)|

ll 157 144 -6 115 -123

-4 174 144 -3 245 218

-3 170 -185

-1 202 235

1 267 -279

-9 232 236

-4 1,9 U4

-3 177 -169

15 211

-10 184 -126

22o

-l) 26) -2‘

-2 110 -106

-1 30) -)06

t 360 -365

15 181 -180

-3 398 495

-2 325 -3 j9

334 36?

107 -Id)

172 -161

165 132

156 -141 247 235

155 -157

15 204 107 11 147 147 -9 144 -120

-5 206 -22C

-14 276 -287 -13 416 -437 -12 353 380 -ll 292 294 -10 186 -186

-8 2*3 22

-7 298 291 -6 6*0 -455

-5 5u5 -547

-3 381 443

-2 3)7 -)62

-l 340 -413

1 206 199

2 267 -297

3 105 -69

10 388 -383

11 220 187

12 185 167

• l) 157 -136

179 157

9 28) -250

11 426 381

1 ) 22 ) - 2)6

14 237 -206

15 170 186

-ll 262 -249

-15 209 -226 -1) 265 230 -11 184 -215 -10 339 -205

-9 219 201 -7 261 -264

-6 375 395

-5 359 324

-4 14) -101

429 -396

?S) -2*7

9 311 282

10 266 -251

11 182 -15)

12 215 205

15 162 -141

-1) 28) -287 -12 291 279

-11 276 -257 -10 271 -275

-8 212 207

-7 190 170

-6 36- -346

-5 51) -577

-4 3oT 445

-3 412 505 -2 316 -357

-l 220 -212

1 1)9 117

-15 114 101 -14 150 -11)

-12 222 199 -10 265 -260

-9 183 17? -8 307 30)

-7 137 -100

-5 313 -3 J

-3 306 316 -2 366 -442

-1 234 250

1 318 -346

3 129 -98

4 291 264

5 310 2o2

6 307 -277

10 216 -186

12 393 326

367

1 597 -481

2 39 J -4)5

3 27} 27)

4 247 225

5 37- -351

7 29) 281

8 9/ -85

9 16) -1*4

10 28o -217

12 232 207

15 156 154

-13 205 221

-12 169 -171 -10 271 277

-6 274 -22V

-2 3 * 3 332

-1 l)v 1)5

6 ?)« 2)t

1*1 -124

212 -107

256 240

281 -278

219 217 264 262

189 -189

227 -243 219 256

261 -264

257 239

231 -220

11 310 -2o9

1) 265 265

15 19? -193

-I) 3 12 -320

-ll 520 4o8

-v 247 -2v9

-8 178 136

-5 478 -415

-3 395 374

-1 264 -2)3

1 376 360

5 3P5 2)6

7 >3) -259

9 34 2 2v?

II 291 -2TC

13 271 276

-12 235 -240 -10 175 162

-8 205 -186

279 258

-ll 307 259

208 210

-l 330 -314

1 360 340

2 107 92

-14 182 -173

-12 187 161 -9 167 -1)1 -7 194 191 -4 145 1*3

-2 266 -28)

-1 200 -146

9.13.1

-14 2)5 -216

-12 189 1*9 -11 165 1)5 -6 206 -191

’ -5 191 -176

-4 102 207

11.13.

-11 119 48 -10 146 -12)

-9 16) -137

-l 166 -ll) 13.13.1 •10 152 -170

-ll 259 -22)

1 24o -208

2 207 -158

4 32o 252

6 427 -351

11 257 22)

12 183 169

14 2)9 -23)

1 165 -18?

1)7 128

2)7 -193

222 195

155 -151

161 1)0 221 -161

319 2o5 -12 17s 154 139 -116 -u 25 1 -18?

264 -200 . •» 105 152 317 305 -12 134 319 303 -11 263 -2 159 196

-232

3 19? -163

4 251 274

5 378 398

6 390 - )08

7 417 -445

8 2 79 207

9 128 122

11 ll) -55

12 245 203

13 287 276

14 279 -258

-6 358 -354

2 223 -241 6 244 -265

14 264 -2)9

-3 266 25

9

1 174 -195

2 317 -39)

1 9t -181

ias -i6i

205 -210

414* -542

31b -300

201 -214

-5 2*5 243

198 -20) -15

9 292 2*7

11 279 -2t 1 13 249 292

2 44 -25?

-4 36* -306 -2

1 2)3 -200 2

2 357 -2H6

?8« 217

265 2)4

139 1)3

>1 17,1

2 02 203

201 161 -

15 219 -226

14 224 230

13 2)1 2)4

12 163 -149

ll 219 -190

-255

145 -3 187 -2 117 102

-1 212 -20*

3 279 -266

• 212 175

8 1*0 -117

-246

-179

13

386 -13 19i 199 -268 -) 210 -208

267

2C0 179

409 3)6 .12 is? ll* -73 -10 28') - 238 -200 —6 iso

198 152 .s 217 2*0 219 -7 16) - 347 -320 -6 174 - )4t 374 -5 243 422 -34? -4 354

261 209 .) |48 . 131 -ll* .? JJ 166 -138 1 2*! 314 -321 2 is:

3 36' 0.12.1 4 23' 6 30! 15 149 147 8 )5< (a) F0 and Fc are on an absolutely scale. Reflections marked

* are considered to contain extinction and/or film scaling

errors
- 12 -

Table 2

Atomic Parameters in CaC03 «6Hs 0

Atom X _z B (A2

)

Ca* .5000 .6472(1) .2500 .10

C .5000 .3067 (6) .2500 .37(5)

0(1) .5000 .1408 (6) .2500 .77(5)

0(2) .5263 (4) .3849(3) .1582(3) .49(3)

0(3) .6163(4) .7229(4) .0916(4) .70(4)

0(4) .7883(4) .5576(4) .3825(3) .59(4)

0(5) .6703(4) .8842(3) .3593 (3) o 53 ( 3)

The quantities in parenthesis are standard errors in the last

significant figures, as computed in the full matrix least squares

refinements

.

*temperature factor of Ca held fixed at 0.1 A

2

Average shift/error for last cycle, neglecting scale factor, =0.09.

Maximum shift/error for last cycle, neglecting scale factor, =0.29.

Average shift/error for last cycle, including scale factor, =0.29.
-13-

Table 2

The Proposed Hydrogen Positions

Atom X

H ( 1) .568

H ( 2) .644

H (3) .872

H (4) .822

H (5) o 774

H ( 6) .616

X z

696 .002

835 .092

598 .354

588 .472

891 .350

981 .319
- 14-

Besides the correlation coefficient of 0.89 between the

scale factor and the Ca isotropic temperature factor, the

largest correlation factors are 0.45 between the x and 2

parameters of atoms in general positions, 0.22 to 0.35

between the scale factor and the temperature factors of

atoms other than Ca, and 0.15 between the temperature factors

themselves. Most correlation coefficients are less than 0.02.

Description of the Structure

The Ca and C03 ions lie on the two-fold axes and the

water molecules are all in general positions. The two ions

form an ion pair in which the Ca is bonded to an edge of the

C03 group. Each ion pair is isolated from the other pairs

by an envelope of 18 water molecules. Six of these 18 waters

are bonded to the Ca and eight are hydrogen bonded to oxygens

of the C03 groups. Four are not bonded to this CaC03 ion

pair, but are bonded to adjacent ion pairs and to other water

molecules in the envelope. No water molecule is bonded to

both ions in a given ion pair.
“15-

When viewed along b, the diads on which the ion pairs

lie form a pseudohexagonal array of Ca»COa columns. Adjacent

columns lying in plane parallel to (100) point in opposite

directions; those lying in a planes parallel to (001) point

in the same direction but are staggered by b/2. This arrange-”

ment appears to maximize the attractions between nearestneighbor ion pairs and at the same time permits water molecules

coordinated to calciums in one column to hydrogen bond to

C03 groups in adjacent columns.

The calcium environment. —The environment of Ca is shown

is given in

in Figures 1 and 2 and/Table 4. In the ion pair, 0(2) and 0(2 8 ) are

of the C03 group /coordinated to Ca (2.429 A) . The shortest

is

Ca...O distance (2.397 A) /to water oxygens 0(3, 3') and is

slightly shorter than the distance (2.429 K) to the carbonate

oxygens 0(2, 2'). The coordination of Ca is completed by the

pairs of water oxygens 0(4, 4') and 0(5, 5') thus giving Ca

a coordination number of eight. The eight oxygens form a distorted

tr istetrahedron. All the Ca-to-0 distances are in the normal

range.
-16-

Table 4

The Calcium Environment

Atoms distance, ,

Ca, 0(2) 2.429(3)

0(3) 2. 393(4)

0(4) 2.576(3)

0(5) 2.506(3)

*In all tables of distances and angles the figures in parentheses are standard errors in the last significant figures

and are calculated from the standard errors in the atomic

parameters and the cell dimensions
-17-

The carbonate environment, —The C03 is required by

symmetry to be planar, and its dimensions are in the normal

range (Table 5) . Coordination of the C03 to Ca does not

distort the trigonal symmetry of the C03 significantly. The

environment is summarized in Figures 1 and 3 and Table 5,

Besides being coordinated to Ca by 0(2) and 0(2')

,

the C03

anion is hydrogen bonded by all the water molecules if the

assumed hydrogen positions are approximately correct. Carbonate

oxygen 0(1) is hydrogen bonded by H(6) to the 0(5) water and by

H(3) to the 0(4) water; 0(2) is hydrogen bonded by H(l) to

the 0(3) water and by H(5) to the 0(5) water. The carbonate groups

are all parallel to a plane containing b and inclined to _c by

=*12°. This is in reasonable accord with the observation that

the highest index of refraction seen down b is 17° from c: in

acute 3. Carbonate oxygen 0(1) accepts 4 hydrogen bonds from

four water molecules while 0(2) accepts only 2 hydrogen bonds.

This appears reasonable since 0(2) is coordinated to Ca,

but 0(1) is not
-18

Table 5

The Carbonate Anion

Atoms distance, i

0,0(1) 1.283(7)

0,0(2) 1.287(5)

0(1) ,0(2) 2.226(5)

CM O

CM O 2.228(6)

Coordinated atoms Angle, deg

0(1) , C, 0 (2) 120.0(2)

0(2) , C, 0 (2 '

)

119.9(4)

0(1) environment

Atoms distance, 1

0(1) ,0(5) 2.700(5)

0(1) ,H(6) 1.75

0(1) ,0(4) 2.854(4)

0(1) , H ( 3) 1.92

0(2) environment

Atoms distance, i

0(2) , Ca 2.429(3)

0(2) ,0(3) 2.749(5)

0(2) ,H(1) 1.80

0(2) ,0(5) 2.764(6)

0(2) , H ( 5) 1.81
-19-

The water environments and proposed hydrogen bonding

scheme. —The environments of the water molecules 0(3) , 0(4)

and 0(5) are summarized in Figures 1, 2 and 3 and in Table 6.

The water molecules are all coordinated to Ca and also provide

extensive hydrogen bonding. The following hydrogen bonding

scheme is proposed. Water oxygen 0(3) has two oxygen near

neighbors, 0(2) (2.749 K) and 0(4) (2.867 A); all other oxygens

are more than 3 A away. The 0 (2) . . .0 (3) „ » .0 (4) angle of 115 0 3°

suggests that approximately linear hydrogen bonds are formed

from 0(3) to these two oxygens. Thus, H(l) may form a hydrogen

bond from 0(3) to 0(2); similarly, H(2) may hydrogen bond 0(3)

to 0 (4) .

Water-oxygen 0(4) has four near oxygen neighbors. One

of these, 0(3), is already the donor in a hydrogen bond to

0(4). Further, a hydrogen bond from 0(4) to 0(5) (2.864 A)

would put the hydrogen near Ga (2.2 A) . Therefore, the probable

scheme is that 0(4) is the donor of H(3) to 0(1) and of H(4)

to 0(5'); the 0...0 distances for these bonds are 2.854 and

2.781 A, respectively. Since the 0 ( 5) -0 (4) -0 ( 1) angle is 126.8°,

deviations from linearity in the hydrogen bonds will be small
- 20-

Table 6

The Water

0(3) environment

Atoms

0(3) , Ca

0(3) ,0(2)

0(3) ,0(4)

H ( 1) ,0(2)

H (2) ,0(4)

Coordinated atoms

Environments

distance, &

2.397 (4)

2.749(5)

2.867 (4)

1.80

1.92

angle, deg 0

0(2) ,0(3), 0(4)

0(3) , H ( 1) ,0(2)

0(3) , H ( 2) ,0(4)

0(4) environment

Atoms

115.3(2)

172°

172°

distance,

OA

0(4) , Ca

0(4) ,0(1)

0(4) ,0(5)

0(4) ,0(3)

0(4) , H ( 2)

0(4) ,0(5)

H ( 3) ,0(1)

H(4) ,0(5')

2.576(3)

2 o 854 (4)

2.781(5)

2.867(4) * 1.92

2.864(4)

1.92

1.85

Coordinated atoms angle, deg.

0(1) ,0(4), 0(5) 126.8(1)

0(1) ,0(4), 0(3) 94.9(2)

0(1) ,0(4) ,0(5') 89.2(2)

0(5) ,0(4) ,0(3) 97.7(1)

0(5) ,0(4) ,0(5') 119.7(1)

0(3) ,0(4) ,0(5') 175.6(2)

0(4) ,0 (4) ,H (3 ) , 0 ( 1) 163°

0(4) ,0(4) ,H(4) ,0(5') 163°
- 21-

Table 6

(continued)

0(5) environment

Atoms

0(5) ,0(1)

0(5) ,0(2)

0(5) ,0(4)

0(5) ,H(4)

H ( 5) ,0(2)

H (6) ,0(1)

Coordinated atoms

H ( 5) ,0(5) , H ( 6)

0(1) ,0(3), 0(2)

0 (1) ,0(5 ), 0(4)

0(2) ,0(5 ), 0(4)

0(5) ,0 (5 ) ,B (5 ) 0(5) ,0 (5 ) ,H (6)

distance, A

2.700(5)

2.764(6)

2 o 781 ( 5) * 1.85

1 o 81

1.75

angle, deg 0

116 0 112 o 0 (1)

99.9(1)

106 o 4(l)

174°

174°

*. Suggested hydrogen bonds
-22-

Water-oxygen 0(5) has three near neighbor oxygens suitable

for hydrogen bonding, 0(1) at 2 , 700 , 0(2) at 2,764 and 0(4) at

has been

2.781 A. One, of these, 0 (4) , /assigned as a donor in a hydrogen

with 0(5). Therefore, 0(5) may be considered to be the donor

in hydrogen bonds to 0(2) using H(5) and to 0(1) using H(6),

The 0 (1) -0 (5) -0 (2) angle, 112.0°, suggests that these hydrogen

bonds will be almost linear.

Assuming that this hydrogen bonding scheme is correct,

we calculated possible positions for the hydrogen atoms in

a way identical to the calculation for the hydrogen position in

CaNa2 (C03

) 2

» 5H2 0 and CaNa 2 (C0 3 ) 2

• 2H2 0. 7 The water molecules,

with the geometry of free water (H-O-H angle = 104.5°, 0-H

distance = .96 A) , were oriented so that their hydrogen bonds

were as linear as possible. The resulting hydrogen positions are

given in Table 3. The short H...0 distance and the nearlinearities of the 0-H. „.0 angles lend credence to this scheme.
-23-

DISCUSSION

The outstanding feature in the structure is the presence

of isolated (CaC03

°) ion pairs, each surrounded by water

molecules* This is the first instance, to our knowledge, of

the existence of this ion pair in a crystal structure, and

provides insight into its structure and its relationship with

the water environment. The (CaC03 °) ion pair may have a significant concentration in solutions saturated with respect to

calcium carbonates1

. 6 Its lack of ionic charge suggests that

it may play an important role in diffusion through membranes

in the same way that has been suggested for the (CaS04 °) ion

pairs2;” The

The ion pair found in CaC03 °6H2 0 may be contrasted to

the Ca(C03 ) 2 triplets found in CaNa2 (C03

) 2

* 5H2 0

7 and

CaNa2 (C03 ) 2

• 2H2 0. 7 ' 8 In all three salts, the calciums are bonded

to the

/edges of C03 groups c In CaC03 °6H2 0, the ion pairs are

completely isolated by water envelopes? in CaNa2 (C03 ) 2

• 5H2 0,

the triplets are isolated from one another by sodium ions and

water molecules,* in CaNa2 (C03 ) 2

• 2HS 0, which contains the

least water, there is bonding between the calcium of one triplet

and the apex oxygens of the carbonates of adjacent triplets.

We have discussed the structures of CaNa2 (C03 ) 2

• 5H2 0 and

CaNa 2 (C0 3 ) 2 °2H2 0 in more detail in reference 7.
-24-

The hydrated calcium carbonates may be compared with the

anhydrous forms as follows*

The eventual conversion in aqueous environments of the

hydrated calcium carbonates to anhydrous modifications is

proof that the hydrated salts are the most soluble forms. The

observation that CaC03 »H2 0 spherulites dissolve with concommittant

formation of CaC03 «6H2 0, as noted in the description of the

preparation of CaC03

# 6H2 0 crystals, is similar proof that,

in dilute solutions (i.e., when activity of water 1) and

near 0°C, CaC03 *6H2 0 is less soluble and hence more stable than

CaC03•

H

2 0

•

At atmospheric pressure, the reactions

CaC03 (calcite) + 6H2 0 (liq.) = CaC03 -6H2 0 (solid)

CaC03 (aragonite) + 6H2 0 (liq.) = CaC03 »6H2 0 (solid)

CaNa2 (C03 ) 2 (solid) + 5H2 0 (liq.) = CaNa2 (C03 ) 2» 5H2 0 (solid)

would be accompanied by 20, 18 and 13 percent decreases in volume

respectively. This suggests that at high hydrostatic pressures

(such as those that occur in the abyssal layer of the oceans

or in hydrostatic vessels) CaC03 ®6H2 0 in particular, and

hydrated salts in general, may act as nucleating particles in

crystallization for the reasons mentioned earlier and under

these conditions they may be the most stable forms. This is

possibly related to the observations that most deep sea animals

have little or no calcareous materials in their skeletons* 8
-25-

and that calcium carbonate is essentially absent from sediments collected at depths greater than 5000 m.

19 Any enhancement of the stability of the hydrated forms might prevent the

formation of normal skeletal material. Although it has been

suggested20 from electron microscopic examinations that almost

all the calcium carbonate in deep sea sediments is of organic

origin and has not been changed after deposition, the possibility

must be considered that at great depths the calcium carbonates

in such sediments may have been converted in part into hydrated

forms which decompose rapidly to the anhydrous forms at normal

temperatures and pressures during sample recovery.

ACKNOWLEDGMENTS

F, Balestrieri visually estimated the intensities and

R. Rosenthal helped in writing and using the data processing

computer programs, D« E. Ernst of the Naval Ordnance Laboratory, White Oak, Maryland 20910, collaborated in writing

the oscillation photograph indexing program. J. S* Bowen

and P. B. Kingsbury gave technical help in he use of the ORTEP

program of C. K. Johnson, Oak Ridge National Laboratory,

Oak Ridge, Tennessee, for drawing the figures.
-26

This investigation was supported in part by research

grant DE-00572-09 to the American Dental Association from the

National Institute of Dental Research and is part of the

dental research program conducted by the National Bureau ©f

Standards, in cooperation with the Council on Dental Research

of the American Dental Association? the Army Dental Corps?

the Dental Sciences Division of the School of Aerospace

Medicine, USAF the National Institute of Dental Research?

and the Veterans Admin is tr alien. The structure and some of the

considerations in this paper were presented at the 46th General

Meeting of the International Association for Dental Research,

San Francisco, California, March 21 , 1968 and at the Symposium

on the Structural Properties of Hydroxyapatite and Related

Compounds, National Bureau of Standards, Gaithersburg, Maryland,

September 12 -14 , 1968 .

USCOMM-NBS-DC
References

K. M. Wilbur, Chap. 8, "Shell Formation and Regeneration"

in Physiology of Mollusca, K. M. Wilbur and C. M. Yonge,

Eds, , Vol. 1, Academic Press, New York. (1964), and

references therein.

T. Pobeguin, Ann. Des Sc. Nat., Bot., _15, 29 (1954).

D. J* Sutor and S. E. Wooley, Science, 159 , 1113

(1968) and references therein.

R. Brooks, L. M. Clark and E, F. Thurston, Phil. Trans.

Roy. Soc. , A243 , 145 (1951) and references therein.

J. Johnston, H. E. Merwin and E. D. Williamson, Amer.

J. Sci. , 41, 473 (1916)

.

F. Lippmann, Naturwissen, 4_6, 553 (1959) .

B. Dickens, and W. E. Brown, submitted for publication

to Inorganic Chemistry.

E. Corazza and C. Sabelli, Acta Cryst., 23_, 763 (1967).

B. Dickens, Report 66-9, U. S. Naval Ordnance Station,

Indian Head, Maryland 20640 (1966)

.
10. J. Hume and B. Topley, J„ Chem. Soc., 2932 (1926).

11. J. Schilling and C. E. Nordnam, private communication.

12. E. G, Cox and W. F a B. Shaw, Proc. Roy. Soc. , A127

71 (1930) .

13. See C. Dickinson, J. M. Stewart and J. R. Holden, Acta

Cryst. , 2JL, 663 (1966) for the method used in estimating

the quasi-normalized structure factors.

14. The International Tables for Crystallography, The

Kynoch Press, Vol.XIX, Birmingham, England, p. 202

(1962)

.

15. R. McWeeny, Acta Cryst., _4, 513 (1951).

16. X. Greenwald, J. Biol. Chem. , 141 , 789 (1941).

17. M. Walser, Amer. J. Physiol., 200 , 1099 (1961).

18. H. U. Sverdrup, M. W. Johnson and R. H. Fleming,

"The Oceans", p. 808, 853, Prentice-Hall (1942).

19. G. Arrhenius in "The Sea", M. N. Hill, Ed., 3, p.

689, 691, Interscience (1963)

.

20 W. R. Riedel, reference 18, p. 869
Legend of Figures

Figure 1. A stereoscopic pair of the CaC03 ®6H

2 0

structure. The contents of the unit cell is shown.

Figure 2. The calcium environment ion pair geometry

and a proposed hydrogen bonding scheme in CaC03 «6H2 0.

Figure 3. The carbonate environment and a proposed

hydrogen bonding scheme in CaC03 ®6H£ 0«
_/

~-c

C\J o

CNJ o

ol

1II1