Abstract
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.
Highlights
Chovanite is one of the few lead–antimony oxysulfosalts found in nature and it has been reported from very few localities worldwide
1.69 Σ=31.03 suggested by the crystal structure study [2], they correspond to the ideal formula TlPb26 (Sb,As)31 S72 O
The crystal-chemical characterization of Tl-bearing chovanite from Monte Arsiccio allowed the distribution of monovalent Tl between two positions, with a preferential partitioning at the Me13 site, to be described
Summary
Chovanite is one of the few lead–antimony oxysulfosalts found in nature and it has been reported from very few localities worldwide. Tatra Mountains, Slovakia [1] and, later, was found in two small neighboring pyrite ± baryte ± iron oxide ore deposits from the Southern Apuan Alps, i.e., the Pollone and Monte Arsiccio mines [2] These latter findings allowed a better description of the crystal-chemistry of chovanite, whose ideal formula can be written as Pb28 Sb30 S72 O. This could correspond either content to a new mineralthe species search for crystals suitable for single-crystal X-ray diffraction studies, in order to describe the (if thallium is preferentially partitioned at one site) or to a thallium-bearing variety This could correspond either to a new mineral species disordered among several positions, without being dominant in at least one of them).
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