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 P2(1)/n and unit-cell parameters a 11.5526(4), b 4.3852(1), c 15.6373(5) angstrom, beta 91.845(2)degrees, V 791.79(5) angstrom(3). 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' anions, with three at distances shorter than 2.51 angstrom and three at distances longer than 3.20 angstrom. In contrast, each As is coordinated by six Hg2+ cations, with three at distances shorter than 2.51 angstrom and three at distances longer than 3.31 angstrom. 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 A and four equatorial bonds longer than 2.98 angstrom. The structure of tvalchrelidzeite can be viewed as a sequence of sheets parallel to ((1) over bar 01). These sheets are composed of [Hg6Sb2As2S6] ribbon-like units (extending along the b axis) linked together by the short Hgl-As bonds (2.494 angstrom), The linkage between sheets is achieved through the weak Hg-S (>3.0 angstrom) and Sb-S (>3.2 angstrom) 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.