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 Cu 3.93 Fe 0.10 Ag 0.01 Pb 0.23 As 1.55 Sb 0.19 S 4.99 . This mineral is orthorhombic, space group Amm 2, with unit-cell parameters a = 10.9601(5), b = 14.6498(8), c = 10.3001(5) Å, V = 1653.82(14) Å 3 and Z = 8. The crystal structure was solved and refined to R 1 = 0.0471 for 2108 unique reflections with F o > 4σ( 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 Cu 6 S and As 2 (Pb,Sb,As) 2 S 7 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 Pb 2+ in watanabeite is probably related to the substitution Cu + + (As,Sb) 3+ = 2 Me 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.