The crystal structure of natural cuspidine (Ca
4(F, OH)
2Si
2O
7) has been refined by full-matrix least-squares method to an
R value of 0.042 using 1923 reflections. The crystal is monoclinic,
P2
1⁄
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 Si
2O
7 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 Si
2O
7 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.
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