Abstract
Pellouxite, the second natural oxy-chloro-sulfosalt after pillaite, has been discovered in cavities of calcite veins in the small Fe-Ba deposit of Buca della Vena mine, Apuan Alps (Tuscany, Italy). The new mineral name honours professor Alberto Pelloux (1868–1948), curator of the mineralogical Museum at the University of Genova. Pellouxite occurs as black acicular flattened crystals with metallic lustre, up to 1 mm long and less than 0.05 mm thick. Main reflectance values [λ (nm), Rair/Roil %]: 470, 38.3/23.8; 546, 37.3/22.4; 589, 36.9/21.5; 650, 35.2/19.6. It has monoclinic symmetry, space group C2/m, with a = 55.824(11), b = 4.0892(8), c = 24.128(5) Å, β = 113.14(3)° and V = 5065(2) Å3. The eight strongest lines of the X-ray powder diffraction pattern are [d(Å), Iobs (hkl)]: 4.002, 38 (606, 406); 3.878, 24 (206, 112); 3.562, 31 (804, 1202); 3.423, 100 (807, 1604, 407); 3.009, 25 (808, 912, 713); 2.948, 27 (1313, 1111); 2.265, 19 (409); 2.048, 20 (020). Electron probe microanalysis gives (mean of 15 spots; wt.%, error σ): Pb 47.17(20), Sb 31.16(23), Cu 0.89(5), Ag 0.59(5), S 19.08(6), Cl 0.33(3), O 0.39(15), Total 99.60. The unit formula, on the basis of Pb + Sb = 22 at. according to the crystal structure study, is (Cu0.64Ag0.25)Pb10.36Sb11.64S27.07Cl0.42O1.11 (Z = 4). Taking into account a 2b superstructure, the crystal chemical formula is (Cu,Ag)2-xPb21-xSb23+xS55ClO (x = 0.12; dcalc. = 5.97 g/cm3), derived from the stoichiometric one (Cu,Ag)2Pb21Sb23S55ClO. Pellouxite, structurally related to scainiite, is an expanded monoclinic derivative of synthetic hexagonal Ba12Bi24S48. It belongs to the zinkenite group of lead sulfosalts with cyclically twinned structures and their derivatives. Like for pillaite, its formation is the result of a complex sulfide deposition from hydrothermal brines, at relatively high temperature, and fO2/fS2 conditions controlled by the pyrite-hematite-magnetite buffer.
