The crystal structure of BaCa(CO3)2 (barytocalcite)

Abstract

The barytocalcite phase of BaCa(CO3)2 crystallizes in the monoclinic unit cell a = 8.092(1) ?, b = 5.2344(6) ?, c = 6.544(1) ?, ? = 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 Rw = 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) ? to 2.518(2) ?. The Ba ion is coordinated to five edges and one apex of the CO3 groups with Ba . . . O distances ranging from 2.729(3) ? to 3.140(2) ?. The distances of the C atoms in the CO3 groups from the planes of the O atoms are 0.025(5) ? and 0.022(4) ? for C(1) and C(2), respectively.