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A SOLUTION TO THE CRYSTAL STRUCTURES OF BISMUTITE AND BEYERITE

Joel D. Grice

Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada

E-mail address: jgrice{at}mus-nature.ca

The crystal structures of bismutite, Bi₂(CO₃)O₂, a 3.865(2), b 3.862(2), c 13.675(6) Å, V 204.1(2) ų, space group Imm2, Z = 2, and beyerite, CaBi₂(CO₃)₂O₂, a 3.7729(5), b 3.7742(7), c 21.726(4) Å, V 309.4(1) ų, space group Immm, Z = 2, have been refined to R indices of 0.0206 and 0.0145 on the basis of 184 and 258 unique reflections. Although the cell and the distribution of diffraction intensities obey tetragonal symmetry, for both these minerals, attempts to solve the structure with tetragonal symmetry led to stereochemical problems. Introduction of a twin plane via reflection on {110} for orthorhombic symmetry simulates tetragonal diffraction-intensity distribution for the pseudo-merohedral twin, and gives a structure with sensible stereochemistry. All large cations, Ca and Bi, have [8]-coordination, and Bi³⁺ shows stereoactive lone-pair behavior. The structures are layered with Bi–O layers, a Ca–O layer in beyerite and (CO₃) layers with the plane of the (CO₃) group orthogonal to the plane of the layer. Bismutite and beyerite have crystal structures typical of "Sillén" phases, and they are closely related structurally to the only other Bi oxycarbonate mineral, kettnerite.

Keywords: bismutite, beyerite, crystal structure, pseudo-merohedral twin, stereoactive lone-pair of electrons, oxycarbonate.

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