Ferdowsiite from the Monte Arsiccio mine, Apuan Alps, Tuscany (Italy): occurrence and crystal structure

A new finding of ferdowsiite, ideally Ag8Sb5As3S16, from the Monte Arsiccio mine, Apuan Alps, Tuscany (Italy) is reported. Ferdowsiite occurs as black euhedral crystals, up to 0.3 mm in size, associated with arsiccioite, boscardinite, and chabournéite in small vugs of baryte + dolomite veins embedded in the pyrite-rich metadolostone close to the contact with the baryte + pyrite ore body. Electron microprobe data gave (wt% - mean of nine spot analyses): Ag 39.03(36), Pb 0.24(3), As 10.63(18), Sb 26.57(9), S 23.71(14), Cl 0.04(1), total 100.22(41). On the basis of ΣMe = 16 apfu, the chemical formula is Ag8.01(5)Pb0.03(1) Sb4.83(2)As3.14(4)S16.36(15)Cl0.03(1). The crystal structure of ferdowsiite has been refined in the space group P21/n, with unit-cell parameters a = 8.6573(4), b = 5.7896(3), c = 13.7866(7) Å, β = 96.122(2)°, V = 687.08(6) Å3, Z = 1. The R1 factor converged to 0.024 on the basis of 1898 reflections with Fo > 4σ(Fo) and 75 parameters.