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 paramters 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 2Å. 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 – are discussed.