Abstract

The new mineral pillaite, Pb9Sb10S23ClO0.5, an oxy-chloro-sulfosalt, has been found at Buca della Vena mine in the Apuan Alps, northern Tuscany, Italy. It crystallized in small fractures of dolomitic lenses in massive Fe-Ba ore, together with many Pb-Sb sulfosalts, among which the oxy-sulfosalt scainiite. Pillaite occurs as black acicular crystals with metallic lustre, elongated // [010], up to 1 cm long and less than 0.1 mm thick. The mineral is brittle, opaque; VHN50 = 175 kg/mm2, dcalc = 5.77 g/cm3. In reflected light it shows a weak anisotropy and bireflectance, and rare red internal reflections. Electron-microprobe analysis (wt. %): Pb 49.07, Sb 30.36, Cu 0.16, S18.73, Cl 0.98, O (crystal structure) 0.21, Sum 99.51. Empirical formula: Pb9.30Sb9.80Cu0.10S22.94Cl1.06O0.5; ideally: Pb9Sb10S23ClO0.5. Pillaite is monoclinic, space group C2/m, a = 49.65(3) Å; b = 4.150(4) Å; c = 21.91(1) Å; β = 99.76(5)° V = 4449(10) Å3; Z = 4. The strongest lines of the X-ray powder diffraction pattern are (d (Å), Iobs, hkl): 4.14 (27) (205), 3.88 (20) (1201), 3.621 (26) (406,1202), 3.548 (40) (1204, 1005), 3.480 (100) (206), 3.249 (24) (1205), 2.956 (47) (515, 1601, 1206, 514), 2.780 (22) (1310, 515). Pillaite belongs to the zinkenite group. Like for dadsonite and other chloro-sulfides, the formation of this mineral is related to a high chlorinity of the hydrothermal solution, but also to the oxygen fugacity, as for scainiite.

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