RARE SULFOSALTS FROM VULCANO, AEOLIAN ISLANDS, ITALY. VI. VURROITE, Pb20Sn2(Bi,As)22S54Cl6, A NEW MINERAL SPECIES

Vurroite, ideally Pb20Sn2(Bi,As)22S54Cl6, is a new mineral species from the modern Pb–Bi epithermal deposit of La Fossa crater, on Vulcano Island, Italy. It occurs as a volcanic encrustation deposited around high-temperature fumaroles (400–600°C) located both on the rim and on the inner slope of the active crater. The mineral consists of slender, needle-shaped crystals up to 0.4 mm long and 0.01 mm across, which generally form fibrous aggregates. Associated minerals are bismuthinite, lillianite, kirkiite, heyrovsk´yite, galena, and other less-well-characterized sulfochlorides of Pb(Bi), some of which contain traces of Tl, Sn or As. The color of vurroite is silver-gray, with a metallic luster. In reflected plane-polarized light, it is white, without distinct bireflectance or pleochroism. Anisotropism is weak, without color effects. The measured values of reflectance in air for a single grain are 34.25 (470 nm), 32.95 (546 nm), 32.60 (589 nm), 31.05% (650 nm); values for 400–800 nm are tabulated. Electronmicroprobe analyses yield the following ranges of concentrations (wt.%): Pb 39.92–41.80 (ave. 40.92), Bi 27.51–29.11 (ave. 28.30), As 7.59–9.03 (ave. 8.38), Sn 1.33–1.78 (ave. 1.60), Tl 0.00–0.34 (ave. 0.06), S 17.64–8.64 (ave. 18.28), Se 0.09–0.25 (ave. 0.18), Cl 1.74–2.02 (ave. 1.85), Br 0.23–0.25 (ave. 0.24). The empirical chemical formula, calculated on the basis of 44 cations, is Pb18.95Sn1.30(Bi12.99As10.73Tl0.03)23.75(S54.71Se0.22Cl5.00Br0.29)60.22. Single-crystal X-ray-diffraction data reveal a pseudo-orthorhombic symmetry, space group F2mm, with a 45.824(6), b 8.368(2), c 53.990(6) Å, V 20702(6) Å3 and Z = 8. The calculated density is 6.15 g/cm3. The strongest powder-diffraction lines [d in Å(I)(hkl)] are: 2.07(100)(18.2.8), 3.34(80)(8.2.2), 2.85(80)(16.0.2), 2.69(80)(12.2.6), 2.10(70)(0.4.0), 3.17(60)(18.2.2), 2.17(60)(8.2.6), and 2.04(50)(20.0.12). Vurroite is the second Pb–As–Bi sulfosalt mineral known (kirkiite, Pb10Bi3As3S19, is the other), and it is the first mineral of the system PbS–Bi2S3–As2S3 to contain Cl and Sn. The name of this new mineral species (IMA 2003–027) honors of Filippo Vurro, Professor of mineralogy at the University of Bari, specialist in the mineralogy and geochemistry of modern volcanic deposits