Parapierrotite from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy): occurrence and new data on its crystal-chemistry

A new finding of parapierrotite from the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy, is reported here. Parapierrotite was identified in two different samples, as striated prismatic crystals, black in colour, ranging from 0.05 to 0.5 mm in length. In the studied specimens, parapierrotite is associated with sphalerite, zinkenite, tetrahedrite, and routhierite or with stibnite, protochabourneite, and routhierite. Chemical analysis indicated the presence of a Pb-bearing variety of parapierrotite; the electron microprobe data are (average of 15 spot analyses - in wt%): Tl 17.06(33), Pb 3.64(55), As 0.09(7), Sb 56.07(38), S 23.94(13), total 100.80(34). On the basis of Sigma Me = 6 atoms per formula unit, the chemical formula of Pb-bearing parapierrotite from the Monte Arsiccio mine is Tl0.89(1)Pb0.19(3)(Sb4.91(13)As0.01)(Sigma 4.)S-92(7).96(36), ideally Tl0.9Pb0.2Sb4.9S8. Its crystal structure study gave a monoclinic unit cell, space group Pn, with a = 8.1005(14), b = 19.4385(9), c = 9.0359(4) angstrom, beta = 92.041(3)degrees, V = 1421.9(3) angstrom(3), Z = 4. The crystal structure was refined to R-1 = 0.046 on the basis of 10 461 reflections with F-o > 4 sigma(F-o) and 265 refined parameters. Parapierrotite belongs to the pierrotite-parapierrotite homeotypic pair. The refinement of the crystal structure of this Pb-bearing variety allowed the description of the mechanism favouring the hosting of Pb in one of the two (010) walls of (Tl,Sb) atom pairs of this Tl sulfosalt.