The refinement of the crystal structure of cuspidine

Abstract

The crystal structure of natural cuspidine (Ca₄(F, OH)₂Si₂O₇) has been refined by full-matrix least-squares method to an R value of 0.042 using 1923 reflections. The crystal is monoclinic, P2₁⁄a, with a=10.906(5)Å b=10.521(6)Å, c=7.518(3)Å, β=109.30(3)°, Z=4. The structure originally determined by Smirnova et al. (1955) is basically correct. Cuspidine is a sorosilicate, in which Si₂O₇ groups are elongated parallel to the a-axis. The distances of bridging bonds Si–O are significantly longer than those of non-bridging bonds. The calcium atoms which connect these Si₂O₇ groups have six, seven and eight nearest neighbours of oxygen atoms, although Smirnova et al. considered that all calcium atoms were six coordinated. These calcium polyhedra form the columns and these columns are linked to form three-dimensional networks.