Crystal structure of Pb-bearing watanabeite from Pefka, Greece

Watanabeite from the Pefka epithermal deposit, northeastern Greece, was examined using single-crystal X-ray diffraction and electron microprobe analysis. The empirical formula of watanabeite is Cu3.93Fe0.10Ag0.01Pb0.23As1.55Sb0.19S4.99. This mineral is orthorhombic, space group Amm2, with unit-cell parameters a = 10.9601(5), b = 14.6498(8), c = 10.3001(5) & Aring;, V = 1653.82(14) & Aring;(3) and Z = 8. The crystal structure was solved and refined to R-1 = 0.0471 for 2108 unique reflections with F-o > 4 sigma(F-o) and 123 refined parameters. The crystal structure of watanabeite can be described as a three-dimensional framework of Cu-centred tetrahedra; cavities of the tetrahedral scaffolding host Cu6S and As-2(Pb,Sb,As)2S7 clusters. On the basis of structural data, the formula of watanabeite could be written as( [III])Cu(3)([IV])Cu(5)As(3)(Pb,Sb,As)S-10 (Z = 4), considering the three independent three-fold Cu sites and the three independent tetrahedrally coordinated Cu sites as aggregated positions. The occurrence of Pb2+ in watanabeite is probably related to the substitution Cu+ + (As,Sb)(3+) = 2Me(2+), where Me = Pb, Fe, Zn and formally divalent Cu. The relationships with tetrahedrite-group minerals are discussed on the basis of the refined structural model, highlighting possible crystal chemical implications of such relationships.