Lead-antimony sulfosalts from Tuscany (Italy). XV. (TI-Ag)-bearing rouxelite from Monte Arsiccio mine: occurrence and crystal chemistry

摘要

A third world occurrence of rouxelite, ideally Cu_2HgPb_(22)Sb_(28)S_(64)(O,S)_2, has been identified from the baryte-pyrite-Fe oxides ore of Monte Arsiccio mine, near Sant'Anna Hi Stazzema (Apuan Alps, Tuscany, Italy). Rouxelite occurs as mm-sized acicular crystals, black in colour, with bluish-violet iridescence, in vugs of carbonate + baryte + quartz veins embedded in dolostones from the Sant'Olga tunnel. It is associated with Tl-bearing chovanite, sphalerite and valentinite. Its X-ray powder diffraction pattern gives unit-cell parameters a = 43.10(2), b = 4.060(2), c = 37.88(2) A, β = 117.33(2)°, V = 5889(5) A3. Eleetron-microprobe data reveal a complex chemistry, with additional minor elements (wt.%): Tl (0.6-1.7), Ag (0.4-0.6), As (0.2-0.6) and Bi (≤0.05). This indicates a widespread substitution of Hg by Ag, according to Hg + Pb = Ag + Sb and incorporation of Tl, with some Ag, according to 2Pb = Sb + (Tl, Ag). The occurrence of mixed (Hg, Ag) and (Hg, Cu) sites in natural sulfides and sulfosalts is briefly reviewed. The Tl-content of the samples studied is a characteristic fingerprint agreeing with the Tl-rich nature of the mineral assemblage from Monte Arsiccio. Rouxelite therefore constitutes a new example of a Tl-bearing sulfosalt.