,

l

JOURNAL OF RESEARCH of the National Bureau of Standards - A. Physics and Chemistry

Vol. 75A, No.3, May - June 1971

The Crystal Structure of BaCa(C03)2 (barytocalcite)

B. Dickens and J. S. Bowen*

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(February 3, 1971)

Th e barytocalc ite phase of BaCa(CO,,), c rys ta llizes in the monoclini c unit ce ll a= 8 .092(1) A, b = 5.2344(6) A, c= 6.544(1) A, (3 = 106.05(1t a t 25 °C with ce ll conte nts of 2[B aCa(C O ,,), I. The Slru et ure pre viou s ly re ported by Aim is correc t in its coa rse detail s but has been rede te rmined he re and re fin ed to R w = 0 .028 , R = 0.023 in space-gro up P2 1 / m using 1652 observed re Rections. Correc tions we re made for abso rpti on, isotropic ex tin c ti on , a nd ano ma lous di s pe rs io n.

The stru c ture of ba rytocalc ite has an ... ABC ABC ... s tackin g of ca tion layers a nd re peat e ve ry 3 la yers . Th e cal cite phase of CaCO:1 has an ABC c ati on layer seque nce a nd re peats eve ry 6 laye rs . Th e orie ntations of th e CO:1 gro ups in barytoca lcite a re like th e CO:1 gro up ori entati o n in th e a rago nit e phase of CaCO:1 , and are rotate d a bout 30 ° from t he CO" group ori enta ti on in ca lcite. The cation laye r seque nce in a ragon ite is. . ABAB. . and th e s tru c ture re pea ts e ve ry 2 laye rs.

The Ca ion in barytoca lc it e is coordin at,ed to se ve n o*ygens, inc lu ding a n edge of a CO" group , with Ca .. . ° di s tances in the ra nge 2.305(2) A to 2.518(2) A. The Ba io n is coordin a ted to fiv e edges a nd one apex of the CO:1 groups with Ba . .. ° di s tances ra ngin g from 2.729(3) A to 3. 14;0(2) A. Th e di ~­l ances of th e C atom s in the CO:1 ~roups fro m the pl anes of the ° a toms a re 0.025(5) A a nd 0.022(4) A for C(l ) and C(2), res pec tivel y.

Key words : Aragonite ; barium calc ium carbonate; cal c ium carbonate; c r ys ta l stru cture; s ingle c r ys ta l x-ray diffrac tion.

1. Introduction

Th e crys tal s tructure of th e barytocalc ite phase of BaCa(CO:lh has been redetermined in our program of struc tural investigations [Ili on calc ium carbonates, calcium phos phates, associa ted hydrates, and related co mpounds. The s tru ctural features in these co m­pounds have important applications in understandin g possible epitaxy, syntaxy, and substitutional solid solution in biological min eral s such as hydroxyapatite (Ca5(P04 h OH) and calci te, aragonite and vaterite , the three phase of anhydrous CaC03 .

From a consideration of the morphologies , d'spacings and possible space·groups of barytocalcite and calcite, Gossner and Mussgnug [2J gave a structure for baryto· calcite whi ch is a rearrangement of the calcite struc· ture. They assumed the space·group to be P21 . AIm [3] used a relatively large (0.3 mm) single crystal of baryto· calcite and unfiltered Cu radiation to s:o]] ect photo· graphic data from the hOl , hll , hkO and hkh levels. He also assumed the space·group to be P2 1 , rather than P21/m , on s teri c consid eration s which are inva"ltd. The structure he gave for barytocalcite differs from tha t given by Gossner and Mussgnu g in the orientations of the CO:3 groups. However , AIm made no corrections for what mus t have been considerable absorption, gave no

*Hcscarc h Associa te of tlu' Americ an Dt 'll lal Assoc ia tion at the Na tional Burea u of S ta ndard s. \Vas h illj.!ton. D.C. 202:,4.

I Figures in b,'ae kct s ind ic at e 11 1(' lit era ture refe re nces a t the e nd of thi s pa pe r.

s tandard deviations on any para meters and used limited film data. Th e structure of barytocalcite was, therefore, poorl y known by modern s tandards, and has bee n redetermin ed he re.

2. Data Collection and Structure Refinement

Th e crys tal used in the data collection is an ap· proximate sphere , radiu s 0.094(3) mm , ground from a crystal from mineral sample R13868 (from Cumberland, England) obtained from the National Museum of Natural History, Smithsonian Institution, Washington, D.C., and supplied by J. S. White, Jr. It was mounted in our usual way [4].

formula (ideal): BaCa(CO:1}2 (barytocalcite phase). cell: monoclinic a=8.092(1) A at25°C b = 5.2344(6) A c = 6.544(1) A f3 = 106.05 (l) ~

volum e = 266.4 A3 space·group P2,/m; cell conte nts 2(BaCa(C03 h] reciprocal lattice ex tinction s, OkO :k = 2n + 1 calculated density 3.72 g' cm- 3 ; observed density

3.71 g' cm- 3 [5].

In th e determination of the unit cell and in the col· lection and processing of data , the general procedure

197

in reference [4] was followed. In the present case, 3379 reflections were collected from the ±h+k+l and ±h-k+l quadrants and merged into a unique set of 1772, of which 1652 are "observed" and 120 are "unobserved." The R factor

L IFI;kl-F~kll/LF';kl hkl

with i < j between pairs F ~kl and F I,kl of observed equivalent reflections F hkl was 0.018 calculated over 1189 pairs. i and j are the sequence numbers in the list of equivalent reflections. Absorption corrections for a sphere with fL=85.6 cm - 1 were applied. The maximum and minimum transmission factors were 0.347 and 0.317, respectively.

The 0 - 20 scans were carried out on an automated Picker t diffractometer at 2°/ min for ZIJ; backgrounds were counted for 20 s each. Because the least signifi· cant digit in all counts was dropped by the Picker hardware, standard deviations, u-hkl, of the structure factors , Fhkl, were estimated from U-II",I=Fllkd5.7 for F'lkl < 5.7; U-IIk{= 1 for 5.7 < Fhkl < 30; andu-IIkl=Fllkd30 for Fhkl > 30 where Fmax on this arbitrary scale is 113. The scattering factors used were those for the neutral atoms in reference 6 for the x·ray 67 refinements and those in references 7 and 8 for the extinction and anomalous dispersion refinements.

The quasi·normalized structure factor statistics on our barytocalcite data indicate that the structure is acentric, since < I E I > = 0.885, < £t > = 1.00 (fixed), < lEt - 11 > = 0.709; the corres ponding . theoretical values are 0.886, 1.000, 0.736 for the acentric case and 0.798, 1.000, 0.968 for the centric case. E is the quasi­normalized structure factor [9] . The fraction of E values greater than 1.0,2.0 and 3.0, respectively, was found to be 0.405 , 0.0027 and 0.0000; the corresponding theoretical values are 0.368, 0.0183 , 0.0001 for the acentric case and 0.317 , 0.0455 , 0.0027 for the centric case. The statistical procedure su~gested an average temperature factor , B , of about 2.5 At and an exponent of 1.00 for sin 8/ f... . Our experience has been that the quasi-normalized structure factor statistics are normally much closer to the theoretical values and the exponent of sin 0/ f... is closer to 2.00 than was calculated here for barytocalcite.

Because of the presence of the strongly scattering Ba ions , this indication that the space-group is the acentric P2 1 was not considered to be reliable. The structure of barytocalcite was redetermined by us from a sharpened Patterson fun ction calculated with (Et - 1) coefficients and an F 0 Fourier electron density synthesis phased from the positions of the Ba and Ca ions. The y coordinate of Ba was set equal to zero to define the origin along b. The structure was refined isotropic ally in space-group P2 1 to R w= 0.65, R = 0.057 and then anisotropically in P2 1 toRw= 0.036, R = 0.028 using the x-ray 67 system of computing programs [101. The least-squares refinements used the full matrix,

~ Cert a in commercia l equ ipment. ins trume nt s . Of mat e ri als are ide nt ified in thi s pape r in order to adequate ly specify the experime ntal procedure. In no case does such ide ntifi ca­tion impl y recomme nd ation or e ndorsement by the National Bureau of S ta ndards. nor does it impl y tha t the mate ri a ls or equipme nt ide n ti fied is necessaril y the best ava ilable for I he purpose.

minimized lw(lFol-IFcl)t, and included those un­observed reflections for which F hkl calculated more than 2u-(F hkl).

The highest peak in an electron density difference o;;ynthesis calculated after anisotropic refinement was equivalent to about 1/3 of an electron and was 0.49 A from Ba. When the space-group is assumed to be P2 1

the largest correlation coefficients are 0.90 to 0.95 between (i) x of 0(1) and x of 0(2), (ii) z of 0(1) and z of 0(2), (iii) x of 0(4) and x of 0(5) and (iv) z of 0(4) and z of 0(5); 0.80 to 0.90 between (i) B I I of 0(1) and B II of 0(2) , (ii) B 13 of 0(4) and B 13 of 0(5) , and (iii) B23 of 0(4) and B23 of 0(5). There are 48 correlation coefficients greater than 0.50.

The isotropic extinction parameter, r , where F2 = F~n c (l + f3rF~nC> and F unc is the structure factor uncorrected for extinction, was then refined together with the structural and scale parameters using the least-squares program RFINE written by L. W. Finger of the Carnegie Institution of Washington; these refine­ments included only the observed reflections. The resulting R values were R w=0.027, R=0.022. The structure obtained had essentially the symmetry P2 1 /m; subsequent anisotropic refinement in P2 1 /m gave Rw= 0.036, R = 0.028 without extinction refine­ment and R w= 0.028, R = 0.025 in refinements in which r refined to 0.000100(4) cm. All unconstrained parameters were varied. Finally, three cycles of refine­ment including corrections for anomalous dispersion and extinction gave R w=0.028, R=0.023; r became 0.000100(5) em. The largest chalJge in the other param­eters was an increase of ~ 0.1 At in allB ii parameters of Ca. In the final cycle, the average shift/error was 0.02, and the standard deviation of an observation of unit weight, [lw(Fo-Fc)2/ (1652-56)]1 /2, was 0.43.

The final Rw values for the centric and acentric cases are near the limit of the experimental data. The weight­ing scheme is arbitrary, though reasonable. Further, there are large correlation coefficients in the acentric refinement. From the first two considerations, the authors feel that the ratio test [11] on lw(Fo-Fc)2, the numerator of the RIO term, is not really applicable in this border-line case, even though it appears from this test that refinement in the acentric P2 1 is to be pre­ferred at a confidence level greater than 99.5 percent. Because refinement in the centric space-group P2 1 /m gives essentially the same result as refinement in P21

but has more restraints which remove the high cor­relation coefficients, the space-group of barytocalcite is assumed here to be P2 1/m. This is consistent with the symmetry of 2/m in the observed crystalline forms of the mineral [12]. With refinement in P2 1/m, the largest correlation coefficients were removed, and only six were greater than 0.50. The four largest were about 0.60 and were between the scale factor and the extinc­tion parameter, and between the scale factor and B 11, B22 , and B33 ofBa.

Because Sr has been reported [13] in the alstonite phase of BaCa(C03h, a refinement of barytocalcite in which the cation positions were considered to be occupied by Sr2+ in solid solution was carried out using Finger's least-squares program. The occupan-

198

TABLE 1. Atomic parameters for barytocalcite BaCa(C 0 3h

Atoms x y z B" * B22 B:,,, B' 2 B", 8 2"

Ba 0 .14740 (3) 0.25 0.28824(3) 0.883(6) 0.646(6 ) 0.652(6) .. . .. . .. ..... 0.399(4) . .. ....... .... Ca .62320(8) .25 . 19855(9) .64(2) .50 (2 ) .58(1) . . . . . . . . . . . . . . . .21(1) . . . . . . . . . . . . . . . C(1) .8972(5) .75 .2483(5) .83(8) .65 (8) .55(7) . ... ...... . . . . . 17(8) ......... ... 0 (1) l.0057(4) .75 .1393(4) 1.44(9) l.31 (9) 1.03(8) .. . ........ . . . . .77(7) .... . .. .. ... 0(2) 0.8457(3) .9620(4) .3089(3) 1.45 (6) 0.85(6) 1.25 (6) 0.50(5) .45(6) 0.03(5) C(2) .6149(4) .25 .7468(5) 0.68(8) .74 (8) 0.61 (7) ... .. .. ...... . .27(6) ....... ...... . 0(3) .6383(4) .25 .5644(4) 2.4(1 ) 1.9 (1) .80 (7) ....... ....... .97(8) ............ 0 (4) .6066 (2) .4604(4) .8474(3) 1.29 (6) 0.61 (5) 1.00 (6) . 16(4) 52(4) - .04(4)

Figures in parentheses are standard errors in the last significant figure quoted , and we re computed in th e fin al cycl e of full -mat. rix leas t· squares refinements.

*Therma l paramete rs have the form ex p [- 1/4(a*28"h2 + b*'8 22k2 + c*'8",,/2 + 2" *b*8"hk + 2a* c* B",hI + 2b*c*8 2:Jkl) ].

cies, 1.003(13) and 1.004(4), for Ba and Ca respectively, suggest that there is no solid solution of Sr ions in this sample of barytocalcite.

The atomic parameters from the final extinc tion refine ment in space-group P2dm are given in tabl e 1. The observed s tructure fac tors are give n in ta ble 2.

3 . Description of the Structure

The structure of barytocalci te (fi g. 1) consists of Ba ... C O:1 c hain s and Ca ... C0 3 c hains, both parallel to [0011 , in which the cation s are coordin ated to an edge of one neighboring C03 group in th e chain , and to an apex of the other C03 group. The C(2)0:1 group is in the CaC 0 3 c hain with its plane paralle l to (100). The C(I )O:1 group is in the BaC03 c hain , with its plane nearly parallel to (101), a nd is pus hed out of the line of th e chain because of the large ionic radius of the Ba ion. Th e chains lie in layers parallel to (210), a perfect cleavage in BaCa(C0:1h . Th e structure may also be co nsidered to co nsis t of layers of CO:1 gro ups coordinated to layers of cations, a nd is related in thi s way to the calcite [14 1 ph ase of CaCO:1, with (201 ) of barytocalcite corresponding to (001 ) of calc ite.

3 .1. The Barium Ion Environment

The Ba ion is coordin ated (fi g. 1, ta ble 3) to 11 oxygen atoms with Ba ... 0 di s tances less th an 3.2 A, i.e.,

b

L;-=, =-:;---+-+-4--~

in the normal range. Th ese oxygens consis t of 5 edges of C0 3 groups, 0 (2,2 1),0(11 ,2" ), 0(1 " ,21" ), 0 (3,4), 0 (3 1, 4 ' ) a nd one a pex, 0 (1). Th e Ba ion is more ex­te nsively coordin ated tha n it is in the witherite phase of BaC03 , where it has a coordination of 9 oxygens. The s tru cture of witherite resembles that of th e aragonite phase of CaC 0 3 .

TABLE 3. Ba environment in barytocalcite, BaCa(CO:1h

Atom s

Ba, 0(1) Ba , 0(4.4') Ba, 0(2,2') Ba . 0(2".2" ' ) Ba . 0(1'. 1") Ba. 0(3.3') Ba, CO) Ba , C(2)

Distan ce, A

2.729(3) 2.833(2) 2.847(2) 2.904(2) 2.914(1) 3. J40(2) 3. 152(3) 3.294(2)

In a ll tabl es of interatomic di stances and angles. the quant ities in pare ntheses are stand ard e rrors in the las t s ignifi ca nt figure a nd we re co mputed fro m the standa rd er rors in the atom ic pos itiona l param eters a nd in the ce ll pa ramete rs. Th ey inclu de co ntri butions from t he var iance covari a nce matri x. The atom labe ls re fe r to a toms in fi gure 1.

F IGU RE 1. The crystal structure 0/ barytocalcite , BaCa(C O:Jl2' and the environments 0/ the Ba and Ca ions.

The origin of the c rystallograph ic coordi na te sys tem is marked by *. The labe ls refer to atoms in lab l4::s 3 and 4.

199

I~ - -- ----

" 9 10 11

, 9

'0 11

, 9

" 11

, , 9

' 0

, 9

"

" '"

". eo,

,e ' ,.0 ,>'

,,' "'

Table 2. Ob8 erved structur e factors for barytocalci. te, BaCa (CO 3)2

- lO - 9 -, -, -, -, -, -, -, -, o

'" ,,,

., '" ". ," ".

-, -, -, -, -, , ,

-, -, -, o , , ,

-, -, -, -, -, -, o

-, -s

-, -, o , , ,

o , , 9

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'"' '" ", " '" '13

j39 ,'> 2()~

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2~~

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10 2~) -, -, -, -11 0 -1 0 2"" 1 - ~ 2 19 2 - 8 3D;'> J - 1 1.:2 - 6 J:.u - 5 '0<3 - '+ ~19

- 3 l 1A - 2 "J - 1 l U,""

- 0 8 " 1 - 5 1 75 - " '+ u:. - J J~J - 2 ~.. - 1

JHI 0 201 1 2 15 .2 lI D J

'" -11 - 10 l oA

- 9 2 17 - 8 7 3 - 7 209 - b 2b9 - 5 219 - 4 00.10 -J '10 - 2 7ufl -1 ':I J o 702 1 Jlq

2 :'J" j Jot,

-, -, -, o , ,

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':'" -.. 2u5 - J

8 219 - 2 9 _,

lO 0 ,

-, -7 .:A " - b 1"' 1 8 - 5 2..)5 9 -It 101 1 0 -, - 2 LO *-- 1 5"8

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216 - 9

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o , -1 0 <'

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00 9 ]09 10

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'e' '" ", ,,, '00 '" '" ,,' ,,, '0' ' Ob

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"e '" '"' "" '" '" ''1 '" " e" 20!'> ", '" 392 m ,0> no n '

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'". '90 ' " ,,, ". ,,, " '30 ,0, '" '" " <OJ!; ]1\3 ,,' ,,' ,'> '"

'00 '" 92 <0",5

' 00 ' <0 ,. '" '" '" '0' m

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'" ,., '" " ,,, '" ,,' '" " 'eo '00 '" '" 212

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292

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10

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'01 '" '51 '" 2" 292 ' 30

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2«5 n

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:~ -, -, -9 -, -, -, o , , ,

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62 -1 0 500 -9 ]86 - 8 ]75 -7 "+]4 - 6 252 - 5 .HO _4

72 - ] 196 - 2 ]4 " -1

120

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322 -1 0 187 - 9 ] 65 - a ]]] -7

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198

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m '" '" 210

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o , , ,

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11 4 - 2 - 8 296 -1 -7 ],:>: . 0 - 6 ]91 1 - 5 ~4 2 -4 ] 26 ] - ] 124 - 2 lal

- ~ 2~: 1 27R 2 22F1 ] 2:'6

'00 171 -1 0 2 22 - 9 -,

.,.7.L - 7 - 0

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o 18 0 I lJ5 2 2]0 ] la8

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19* -2 2115 -1 206 0

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28" - I 19] 0 Jb? 1 11;> 2 ]04 ]

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20 ., 0 20? 1

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1:'.'1 21 4 - ] 1~ 4 - 2 j~4 - 1 ,,, ]90

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m ]'is

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" '" '" '" ,., "" m

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Columns are 1, and lOF .. o F I S are on an absolute scale ..

o reflections are marked by

200

- 0 - 5 208 -4 1 8'9 - ] 211 - 2 7() - I 2:'~

'13 2" '" UO

>5

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-7 37" -6 1~()

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1 17 5 2 j U5

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' .0 ." 2>1 .8

21' 119

- 1 146 o l ab 1 2]1

-1 0 278 - 9 61 - 8 165 -7 21b - 6 1 0 4 - 5 ]"+4 _4 238 - ] "+21 - 2 2b9 -1 238

o 249 , 2 1 6" 6 2" .

-1 0 1'17 - 9 1 0 1 -8 209 -7 2a. - 6 3]3 - 5 173 -4 3 61 - ] 324 - 2 254 -1 ] 2"

o I b9 2B2 " . ,,, 122 ".

'" -1 0 2UI

- 9 1 ]5 - a 124 - 7 23] - 6 bb - 5 229 _4 2:11 - ] 251 - 2 2':15 -I 1 95

-, -, -, -, -. -, -, -, o

- 8 -, - 0 -, -, -, -,

o , ,

-, -, - 0 -, -. -, -, -, o

-, -, _ 9 -, -, -,

'" b8 112

" " , ,,,

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eo 201 '09 '" 259 219 m '"

, .. m

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'" '" 19' 'e9 II>~

- 9 :l ]~

- 8 6 1 - 7 27':1 - 6 111

11 0

'" -] 2 0" - 2 200 -I 117

- 9 1 0 4 -a ]20 -7 191 - 6 249 -5 2 36 - 4 "'5 -] lAS - 2 11 5 -1 16A

'99

'" " '33

-9 ]2] - 8 IJ7 -7 292 - 6 2011 - 5 212 -4 220;1 - ] 25 · - 2 172 -I 2]0 o 8b 1 ]25 , " ,

" - 8 297 -7 1 211 -6 2"5 - 5 110 -II 61 -3 Iii] - 2 1 2] -1 2 0b

-, -, - 0 - S

-, -, -, o

'" 10 0

'" ". '"

00

'" '" 15,2 '02 " . '70

'" '" 21 0

" '92

'" - 6 2"2 - 5 200 -4 9 1 - ] 1 47 - 2 121 -1 IJO o 2:> 1

9b

'00

-. -,

- 9 -, - 1 - 0 -, -, -, -, o

- 9 -, - 1 -, - S - 9 -, -, -,

o ,

-, - 0 -, -, -, -,

,., "' ' 00

'" 210 119

'" 20' '" '" '" ' 90

"

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'" '" '" '" ' 30 212 II">

"Unobserved"

'00 " 19' bO

"" - 8 29_ -7 22] - 6 11] - 5 l':l li -4 220 - 3 14t. - 2 25'5 -1 88 o 10;19 12M­,

-, - 6 204 -5 2 0 7 - 4 201 -] 269 - 2 129 -I 242 o b1

-5 186 - <0 lao - ] 192 - 2 201 -1 127

-a 211 - 7 225 - 6 192 -5 259 -II ]0. -] 149 - 2 16'1 -1 71 o 20;11

-a 142 - 7 221 - 6 145 -, -"+ 14M -3 70;1 -2 13 0 -1 19<0 o 52

'" - 6 1]] - 5 167 - 4 2]_ - 3 147 -2 1]]

-1 107 211

bO

- 6 I b8 -5 1 0 9 -4 161 - ] 5]

- 2 11 0 -1 2 0 1

o ]2.

-4 2"" - 3 12b - 2 142

- 0 -, -. -, -, -, o

- 0 -, -, -, -, -, o

-, -,

" no PO

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'01 '.8 m

9 0

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3.2. The Calcium Ion Environment

The Ca ion is coordinated (fi g. 1, table 4) to seve n oxygen atom s, including one edge, 0 (41 1, 4 111 ), of a C O:! group , with Ca . . . 0 di stances in the normal range. The five apex oxygen a toms lie in a square pyramid. T he center of the coordinated edge of the C0 3 group is near the r ema ining octahedral position. The coordination of Ca in b ar ytocalcite is thus inter· mediate between the oc tahedral coordination of six C03 apexes with no shared edges in the calcite phase of CaC0 3 and the nine-fold coordination of three shared C03 edges and three apexes in the aragonite phase of CaC0 3 .

TABLE 4. Ca environment in barytocalcite , BaCa(C O:!b

Atoms

Ca, 0(2 ' . 2") Ca, 0(4 ' . 4') Ca,0(3) Ca, 0(4" . 4"' )

Dis tance, A

2.305(2) 2.353(2) 2.364(3) 2.5 18(2)

Th e atomic labels refe r to atoms in fi ~ure 1.

3 .3. The Carbonate Groups and Their Environments

There are two crys tallogra phicall y different C0 3

groups in the s tru c ture. On e (ta ble 5, fi g. 2) is in th e Ba . .. CO:3 chains a nd the oth er (ta ble 5, fi g. 2) is in th e Ca ... COa chain s. The forme r, the C(I ) C0 3

group , coordinates with all edges to Ba ions, a nd coordi ­nates one apex, 0 (1), to a nothe r Ba ion. The two re­maining apexes 0 (2, 2t ) are coordina ted to Ca ions.

The C(2) C0 3 group coordin ates th e edge co ntaining the 0 (4,41) a toms to Ca ll a nd the re maining two edges ,

II

Y a

b~

TABLE 5. The carbonate anions and their enmron­ments in barytocalcite, BaCa(C 0 3}z

COa i!:roups Distance or a n ~ l e

C(1), 0(1) 1.275(4) A C(I) , 0 (2, 2' ) 1.286(2) 0 (1), 0(2) 2.221(3) 0 (2 ), 0 (2 ' ) 2.220(4) 0 (1), C(I), 0 (2) 120.3(2)" 0 (2), C(I ), 0 (2') 119.3(3)

C(2),0(3) 1.259(4) A C(2), 0(4, 4') 1294(2) 0(3),0(4) 2.229(3) 0 (4) . 0(4 ' ) 2.203(4) 0 (3). C(2). 0 (4) J 21.6(1)" 0 (4), C(2), 0(4') 1] 6.6(3)

o e nvironm e nt s Dista nce. A

0 (1 ), Ba 2.729(3) 0(1), (Ba'. Ba") 2.9 15(1)

0 (2), Ca'" 2.305(2) 0(2),Ba'" 2.847(2) 0(2). Ba" 2.904(2)

0(3), Ca' 2.364(3) 0 (3). (Ba". Ba') 3. 140(2)

0(4) , Ca '" 2.353(2) 0(4) , Ca" 2S I8(2) 0(4) , (BaV ) 2.833(2)

T he atom ic labe ls rcfe r to atoms in fi~urc 2.

0 (3,4) and 0 (3, 4t), to ions Ba v and Ba tV, res pectively. The average values of the C - 0 bond di s ta nce in the C(I ) a nd C(2) CO:! groups, 1.283 A a nd 1.283 A, res pec­tively, co mpa re well with the C- O bond le ngth s ob-

F IGU RE 2. The C0 3 environm.ents in bary tocalcite, BaCa(C 0 3],.

The labe ls refer to a toms in ta ble s.

201

ser ved in the calcite ().283(2) A [14]) and aragonite (1.288(2) and 1.283(1) A [4 J ) phases of CaL0 3 .

The two C03 groups are nonplanar in barytocalcite. The displacements 9f C from the pla ne of tpe oxygen atoms are 0.025(5) A for C(1) and 0.022(4) A for C(2). Both displacements are toward the two central Ca ions Calli and Cal v in fi gure 2, and probabl y ari se from polarization of the 0 atoms b y these Ca ions. Similarly di storted C0 3 groups have been found in arago nite [15, 4] and Ca2 N a2(C03h [lJ . (In calcite, the C0 3 group pQjnt symmetry is constrained to 32 by the choice of R3 c as the s pace·group.)

The differe nt C - 0 bond lengths in the C0 3 groups (table 5) are con sistent with the observed features in the structure. 0(1) is bonded to three Ba ions. 0(2) is bonded to two Ba ions and one Ca ion. Thus 0 (2) experiences s tronger interaction with its cation neigh­bors, which is compatible with the C(I )-0(2) distance being longer than C(1)-0(1 ). Similarly, 0 (4) is coordi­nated to two Ca ions and one Ba ion , wh ereas 0 (3) is coordinated to one Ca ion and two Ba ions. Thus, the C(2)- 0(4) bond distance is expected to be longer than th e C(2) - 0 (3) dis tance. Because the 0 (4 , 4 ') edge is coordinated to Ca whe reas the 0 (3, 4) and 0(3, 4') edges are coordinated to Ba, the 0 (4), C(2), 0 (4') angle is expected to be less than 120°. Both these effects are realized.

Although 0 (2) and 0 (3) have similar environments, comparison of the length s of the bonds C(1)-0(2) and C(2)- 0 (3) is not fruitful. S uc h a superficial comparison only applies when the oxyge n atoms are compe ting in their bonding to the same central atom or when the other oxygens in the C0 3 groups also have similar environments.

4. Discussion

AIm [3] showed that the s tructure given by Gossner and Mussgnug [2] is incorrect. The general features of the structure gi ':,en by AIm for barytocalcite are correct to within ~ 0.3 A. The details given here are , however , far more precise. Gossner and Mussgnug and AIm gave the space·group of barytocalcite as P2 1 in their limited analyses of the structure. Our more extensive analysis shows , howe ver , that barytocalcite can be refined equally well, within the limits of the data , in the centro­symmetric space-group P2 1 1m.

Barytocalcite is related to the calcite and aragonite phases of CaC03• There are pseudo-hexagonal layers of cations parallel to (101) in the barytQcalcite structure. If the difference between Ca and Ba is igp.ored, a pseJ!do-cell ma-Yv be defined with a ' ~c 8.7 A (along [101J) , b' = 5.2 A (along [010J ), c' ~ 9.2 A (along [WID and f3 ' ~ 95°. In this pseudo-cell , the cation layers repeat every third layer along c' as they do in calcite. The pseudo-cell of barytocalcite compare% well with the orthohexagonal cell 8.641, 4.989 , 17.062 A of calcite. The orientations of the C03 groups in the two com­pounds calcite and barytocalcite differ by a rotation of ~ 30° (the C(2)03 group in barytocalcite further differs in that it makes an angle of ~ t o° with the (101) plane), and the cell repeat , 17.062 A, perpendicular to the

layers in calcite encompasses six cation layers whereas barytocalcite repeats every three cation layers. As regards the ... ABCABC ... sequence of cation layers, barytocalcite is like calcite. However, the rotation of the C03 groups by ~ 30° relative to the C03 positions in calcite relates barytocalcite to the aragonite phase of CaC03 • Thus the detailed coordinations of the C03

groups are like those in aragonite, which also has C03 groups rotated by ~ 30° from the C03 positions in calcite, because all C03 edges in both compounds are shared in coordination to neighboring cations and all apexes are further coordinated to cations. The co­ordinations of the cations in barytocalcite are however different from the coordination of Ca in aragonite, partly because in the former there are two differently sized cations.

W. E. Brown suggested the problem and gave helpful discussions ; P . B. Kingsbury gave technical assistance. This investigation was supported in part by research grant DE- 00572 to the American Dental Association from the National Institute of Dental Research and is part of the de ntal research program conducted by the National Bureau of Standards , in cooperation with the American Dental Association; the United States Army Medical Research and Development Command; the Dental Sciences Division of the School of Aerospace Medicine, USAF; the National Institute of Dental Research and the Veteran s Administration.

5. References

[11 Dickens, B. , Hym an, A., and Brown , W. E., The crystal structure of Ca,Na,(C 0 3)3, (shortite), 1. Res . Nat. Bur. S tand. (U.S.), 75A Whys. and Chem.), No.2, 129-135 (Mar.- Apr. 1971).

[21 Coss ner, B. , and Mussgnug, F., Uber Alstonite und Milarit. Zbl. Mineral. Ceol Palaonl. A1930, 220 (1930); Beitrag zur Kenntnis des Baryto·calc ites und seiner strukturellen Beiziehungen zu anderen Stoffen. , Ibid. A1930. 321 (930).

[3] Aim , K. F. , Th e crystal structure of barytocalcite, BaCa(C 0 3 )z, Arki v fo r Mineralogi och Geologi 2 , (1960) 399-410.

[4] Dickens, B. , and Bowen, 1. 5. , Refinement of the crystal struc· ture of the aragonite phase of CaCO". J. Res. Nat. Bur. Stand. 75A (phys. a nd Che rn. ), No.1 , 27-32 (Jan .- Feb. 1971).

[5] Kreutz,S., P arallel growths of different subs tances, Min. MaO'. 15, 232- 237 (1909). '"

[6] International T ables for X-ray Crys tallograph y, 3, (The Kynoch Press, Birmingham, England , 1962) p. 202.

[7] Cromer, D. T. , and Mann, J. B., X·ray scattering factors com· puted fro m numerical Hartree·Fock wave functions, Acta Crys t. A24, 321-324 (1968) .

[8] Cromer, D. T. , Anomalous dispersion corrections computed from self·cons istent fi e ld relativistic Dirac-Sla ter wave func· tions, Acta Cryst. 18, 17- 23 (1965).

[9] Dickinson, c., Stewarl , 1. M., and Holden, 1. R. , A direc t determination of the crystal structure of 2,3,4,6·tetranitro· aniline, Acta Cryst. 21 ,663- 670 (1966) .

[10] Stewart , J. M., editor, X· ray 67 program sys te m for x·ray crysta llography, Technical report 67- 58, University of Maryland, College Park , Maryland 20742 , Dece mber 1967.

[11] Hamilton, W. C., Significance tests on the crys tallographic R fac tor, Ac ta Cryst. 18, 502- 510 (1965) .

[1 2] Palache, c. , Berman, H. , and Frondel, c., Dana's system of mmeralogy, vol. II, 7th edilion (John Wil ey and Sons, Inc., New York , 1963) pp. 220 and 221.

202

[13] Chang, L. L. Y., Subsolidus phase relations in the sys tems BaC03 ·SrC0 3 , SrC0 3 ,CaC03 and BaC03 ,CaC0 3 , J . Ceo!. 73, 346- 368 (1965).

[14] Chess in , H., Hamilton, W. c., and Post, B. , P osition and therm al parameters of oxyge n atoms in calc ite, Acta Crys t. 18, 689- 693 (1965).

[15] de Villiers, J. P . R. , The crystal st ru ctures of aragonite, stron, ionite and witherite, Ph.D . Thesis, Universi ty of Illinois (1969).

(P aper 75A3-663)

203