Lead–Antimony Sulfosalts from Tuscany (Italy). XXIV. Crystal Structure of Thallium-Bearing Chovanite, TlPb₂₆(Sb,As)₃₁S₇₂O, from the Monte Arsiccio Mine, Apuan Alps

A thallium-bearing variety of the lead⁻antimony oxysulfosalt chovanite from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy) has been reexamined. It occurs as thin, ribbon-like crystals, black in color, up to 5 mm in length in vugs of dolomite ± baryte ± quartz veins embedded in the metadolostone of the Sant’Olga level. Associated minerals are rouxelite, robinsonite, sphalerite, valentinite, baryte, dolomite, quartz, and Ba-rich K-feldspar. Chemical analysis pointed to contents of Tl up to 0.86 apfu, corresponding to the ideal chemical formula TlPb26(Sb,As)31S72O. The structural role of thallium has been investigated using single-crystal X-ray diffraction using synchrotron radiation (λ = 0.59040 Å). Thallium-rich chovanite is monoclinic, space group P21/c, with unit-cell parameters a = 34.280(3), b = 8.2430(7), c = 48.457(4) Å, β = 106.290(4)°, and V = 13143(2) Å3. The crystal structure was refined to a final R1 = 0.083 for 12,052 reflections with Fo > 4σ(Fo) and 1210 refined parameters. The general features of thallium-rich chovanite agree with those of chovanite. Thallium is present as Tl+; it is disordered among two mixed (Pb/Tl) positions, with a Tl/Pb atomic ratio below 1, that precludes this compound to be a new species