Crystal structure and crystal chemistry of biehlite, Sb1.79As0.21MoO6

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2000
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  • The new mineral biehlite was found in the Tsumeb mine, Namibia and has the composition Sb1.79As0.21MoO6. It crystallizes monoclinic, space group C2/c with lattice parmeters a = 18.076(5) Å, b = 5.920(5) Å, c = 5.083(5) Å, β = 96.97(1)°. Refinement led to a final R(F) value of 0.065 for 679 symmetrically independent reflections. [MoO6]6− octahedra share common edges and form zigzag chains which run parallel to the c direction. The octahedra are flanked by a [Sb2O2]2+ group on each side and infinite ribbons of composition [Sb4Mo2O12] are obtained. Neighbouring ribbons are only held together by weak Sb-O bonds in the directions of a and b. The [MoO6]6− octahedra are strongly distorted with Mo-O distances ranging from 1.700(5) to 2.248(6) Å and O-Mo-O angles between 72.1(2)° and 105.3(4)°. The common O-O edge (2.492(8) Å) is shortened with respect to the remaining edges of the octahedra (2.704(8) Å-2.899(8) Å). The Sb-cation forms three short bonds with oxygen at distances slightly smaller than 2 Å. Six further oxygen atoms are located at distances ranging from 2.910(6) to 3.470(6) Å. Calculated distance for the lone-pair is 1.18 Å. Part of the Sb3+ is substituted by the smaller As3+ ion. The relationship of the new mineral to similar structures — especially to the mineral stibivanite and the synthetic compound Sb2MoO6 — is discussed. © 2000, Oldenbourg Wissenschaftsverlag GmbH. All rights reserved.
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