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The Crystal Structure of Baca(CO3)2 (Barytocalcite)

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The Crystal Structure of Baca(CO3)2 (Barytocalcite) JOURNAL O F 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) The barytocal c ite phase of BaCa(CO,,), c rysta llizes in the monoclinic unit cell a= 8.092(1) A, b = 5.2344(6) A, c= 6.544(1) A, (3 = 106.05(1t at 25 °C with cell conte nts of 2[B aCa(CO,,), I. The Slru et ure pre viou s ly reported by Aim is correct in its coarse details 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. Correcti ons we re made for absorption, isotropic extin cti on, a nd ano ma lous di s pe rsio n. The struc ture of barytocalc ite has an ... AB C ABC ... s tackin g of cati on layers a nd re peat e very 3 la yers. The cal cite phase of CaCO:1 has an AB C c ati on layer seque nce a nd re peats every 6 layers. The orie ntations of th e CO:1 gro ups in barytocalcite a re like th e CO:1 gro up ori entati o n in th e a ragonit e phase of CaCO:1 , and are rotate d a bout 30 ° from t he CO" group ori entati on in calcite. The cation layer seque nce in a ragonite is. ABAB. and th e s tructure re peats eve ry 2 laye rs. The Ca ion in barytocalcit e is coordinat,ed to seve n o*ygens, in c 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 ated to fiv e edges a nd one apex of the CO:1 groups with Ba . .. ° di stances ra ngin g from 2.729(3) A to 3. 14;0(2) A. The di ~­ lances of th e C atoms in the CO:1 ~r o up s fro m the pl anes of the °a toms are 0.025(5) A a nd 0.022(4) A for C(l ) and C(2), respectivel y. Key words : Aragonite; barium calcium carbonate; calcium carbonate; crys ta l structure; single crys tal x-ray diffraction. 1. Introduction standard deviations on any para meters and used limited film data. The structure of barytocalcite was, The crystal s tructure of th e baryto calcite phase of therefore, poorl y known by modern s tandards, and BaCa(CO:lh has been redetermined in our program of has been redetermined he re. structural investi gations [Ili on calcium carbonates, calcium phos phates, associated hydrates, and related 2. Data Collection and Structure Refinement compounds. The structural features in these com­ pounds have important applications in understanding The crystal used in the data collection is an ap· possible epitaxy, syntaxy, and substitutional solid proximate sphere, radius 0.094(3) mm , ground from a solution in biological min erals such as hydroxyapatite crystal from mineral sample R1 3868 (from Cumberland, (Ca5(P04 h OH) and calcite, aragonite and vaterite, England) obtained from the National Museum of the three phase of anhydrous CaC03 . Natural History, Smithsonian Institution, Washington, From a consideration of the morphologies, d'spacings D.C., and supplied by J. S. White, Jr. It was mounted and possible space·groups of barytocalcite and calcite, in our usual way [4]. Gossner and Mussgnug [2J gave a structure for baryto· calcite whi ch is a rearrangement of the calcite struc· formula (i deal): BaCa(CO:1}2 (barytocalcite phase). ture. They assumed the space·group to be P21 . AIm [3] cell: monoclinic used a relatively large (0.3 mm) single crystal of baryto· a=8.092(1) A at25°C calcite and unfiltered Cu radiation to s:o]] ect photo· b = 5.2344(6) graphic data from the hOl , hll, hkO and hkh levels. He A c = 6.544(1) A also assumed the space·group to be P21 , rather than f3 = 106.05 (l) ~ P21/m , on steri c consid erations which are inva"ltd. The volume = 266.4 A3 structure he gave for barytocalcite differs from that space·group P2,/m; cell contents 2(BaCa(C03 h] given by Gossner and Mussgnu g in the orientations of reciprocal lattice extinctions, OkO :k= 2n + 1 the CO:3 groups. However, AIm made no correcti ons for calculated density 3.72 g' cm- 3 ; observed density what mu s t have been considerable absorption, gave no 3.71 g' cm- 3 [5]. *Hcscarch Associate of tlu' Americ an Dt 'll lal Associa tion at the National Bureau of S ta ndard s. \Va s hillj.!ton. D.C. 202:,4. In the determination of the unit cell and in the col· I Fi gures in b,'aekct s indic at e 11 1(' lit erature refe re nces a t the e nd of thi s pa pe r. lection and processing of data, the general procedure 197 , l in reference [4] was followed. In the present case, minimized lw(lFol-IFcl)t, and included those un­ 3379 reflections were collected from the ±h+k+l and observed reflections for which F hkl calculated more ±h-k+l quadrants and merged into a unique set of than 2u-(F hkl). 1772, of which 1652 are "observed" and 120 are The highest peak in an electron density difference "unobserved." The R factor o;;ynthesis calculated after anisotropic refinement was equivalent to about 1/3 of an electron and was 0.49 A L IFI;kl-F~kll/LF';kl hkl from Ba. When the space-group is assumed to be P2 1 the largest correlation coefficients are 0.90 to 0.95 with i < j between pairs F ~kl and F I,kl of observed between (i) x of 0(1) and x of 0(2), (ii) z of 0(1) and z of equivalent reflections F hkl was 0.018 calculated over 0(2), (iii) x of 0(4) and x of 0(5) and (iv) z of 0(4) and 1189 pairs. i and j are the sequence numbers in the z of 0(5); 0.80 to 0.90 between (i) B I I of 0(1) and B II of list of equivalent reflections. Absorption corrections 0(2), (ii) B 13 of 0(4) and B 13 of 0(5), and (iii) B23 of 0(4) for a sphere with fL=85.6 cm- 1 were applied. The and B23 of 0(5). There are 48 correlation coefficients maximum and minimum transmission factors were greater than 0.50. 0.347 and 0.317, respectively. The isotropic extinction parameter, r , where The 0 - 20 scans were carried out on an automated F2 = F~n c (l + f3rF~nC> and F unc is the structure factor Picker t diffractometer at 2°/ min for ZIJ; backgrounds uncorrected for extinction, was then refined together were counted for 20 s each. Because the least signifi· with the structural and scale parameters using the cant digit in all counts was dropped by the Picker least-squares program RFINE written by L. W. Finger hardware, standard deviations, u- hkl, of the structure of the Carnegie Institution of Washington; these refine­ factors , Fhkl, were estimated from U-II",I=Fllkd5.7 for ments included only the observed reflections. The F'lkl < 5.7; U-IIk{= 1 for 5.7 < Fhkl < 30; andu-IIkl=Fllkd30 resulting R values were R w=0.027, R=0.022. The for Fhkl > 30 where Fmax on this arbitrary scale is 113. structure obtained had essentially the symmetry The scattering factors used were those for the neutral P21 /m; subsequent anisotropic refinement in P2 1 /m atoms in reference 6 for the x·ray 67 refinements and gave Rw= 0.036, R = 0.028 without extinction refine­ those in references 7 and 8 for the extinction and ment and R w= 0.028, R = 0.025 in refinements in anomalous dispersion refinements. which r refined to 0.000100(4) cm. All unconstrained The quasi·normalized structure factor statistics on parameters were varied. Finally, three cycles of refine­ our barytocalcite data indicate that the structure is ment including corrections for anomalous dispersion acentric, since < IE I > = 0.885, < £t > = 1.00 (fixed), and extinction gave R w=0.028, R=0.023; r became < lEt - 11 > = 0.709; the corres ponding . theoretical 0.000100(5) em. The largest chalJge in the other param­ values are 0.886, 1.000, 0.736 for the acentric case and eters was an increase of ~ 0.1 At in allB ii parameters 0.798, 1.000, 0.968 for the centric case. E is the quasi­ of Ca. In the final cycle, the average shift/error was normalized structure factor [9] . The fraction of E 0.02, and the standard deviation of an observation of values greater than 1.0,2.0 and 3.0, respectively, was unit weight, [lw(Fo-Fc)2 / (1652-56)]1/2, was 0.43. found to be 0.405 , 0.0027 and 0.0000; the corresponding The final Rw values for the centric and acentric cases theoretical values are 0.368, 0.0183 , 0.0001 for the are near the limit of the experimental data.
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