The Crystal Structure of BaCa(CO₃)₂ (barytocalcite)

The barytocalcite phase of BaCa(CO₃)₂ crystallizes in the monoclinic unit cell a = 8.092(1) Å, b = 5.2344(6) Å, c = 6.544(1) Å, β = 106.05(1)° at 25 °C with cell contents of 2[BaCa(CO₃)₂]. The structure previously reported by Alm is correct in its coarse details but has been redetermined here and refined to R = 0.028, R = 0.023 in space-group P2₁/m using 1652 observed reflections. Corrections were made for absorption, isotropic extinction, and anomalous dispersion. The structure of barytocalcite has an … ABCABC … stacking of cation layers and repeat every 3 layers. The calcite phase of CaCO₃ has an ABC cation layer sequence and repeats every 6 layers. The orientations of the CO₃ groups in barytocalcite are like the CO₃ group orientation in the aragonite phase of CaCO₃, and are rotated about 30 ° from the CO₃ group orientation in calcite. The cation layer sequence in aragonite is … ABAB … and the structure repeats every 2 layers. The Ca ion in barytocalcite is coordinated to seven oxygens, including an edge of a CO₃ group, with Ca … O distances in the range 2.305(2) Å to 2.518(2) Å. The Ba ion is coordinated to five edges and one apex of the CO₃ groups with Ba … O distances ranging from 2.729(3) Å to 3.140(2) Å. The distances of the C atoms in the CO₃ groups from the planes of the O atoms are 0.025(5) Å and 0.022(4) Å for C(l) and C(2). respectively.