The crystal structure of tvalchrelidzeite, Hg₃SbAsS₃, and a revision of its chemical formula

We have determined the crystal structure of tvalchrelidzeite, Hg₃SbAsS₃, for the first time with single-crystal X-ray diffraction. It is monoclinic, with space group P2₁In and unit-cell parameters a 11.5526(4), b 4.3852(1), c 15.6373(5) Å, β 91.845(2)°, V 791.79(5) ų. There are eight symmetrically distinct sites in the structure, three occupied by Hg, one by Sb, one by As, and three by S. There is no disorder between Sb and As. Each Sb is surrounded by six S^2- anions, with three at distances shorter than 2.51 Å and three at distances longer than 3.20 Å. In contrast, each As is coordinated by six Hg²⁺ cations, with three at distances shorter than 2.51 Å and three at distances longer than 3.31 Å. All three independent Hg ions are situated in considerably distorted octahedral sites, with two opposite bonds (one Hg-S and one Hg-As) shorter than 2.51 Å and four equatorial bonds longer than 2.98 Å. The structure of tvalchrelidzeite can be viewed as a sequence of sheets parallel to (′101). These sheets are composed of [Hg₆Sb₂ As₂S₆] ribbon-like units (extending along the b axis) linked together by the short Hg 1-As bonds (2.494 Å). The linkage between sheets is achieved through the weak Hg-S (>3.0 Å) and Sb-S (>3.2 Å) bonds, accounting for the observed perfect cleavage in one direction. Tvalchrelidzeite represents one of very few sulfosalt minerals that contain both Sb and As, with the latter behaving as an anion.