Lead-antimony sulfosalts from tuscany (Italy). XXIV. crystal structure of thallium-bearing chovanite, TLPB26 (Sb,As)31 S72 o, 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)31 S72 O. The structural role of thallium has been investigated using single-crystal X-ray diffraction using synchrotron radiation (? = 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, ? = 106.290(4)o, and V = 13143(2) Å3. The crystal structure was refined to a final R1 = 0.083 for 12,052 reflections with Fo > 4?(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