Old defined minerals with complex, still unresolved structures: the case of stutzite, Ag5-xTe3

The crystal structure of the mineral stutzite, a relatively common silver telluride, was solved using intensity data collected using a crystal from the cotype material from the May Day mine, La Plata District, CO (USA). The study revealed that the structure is hexagonal, space group P (6) over bar, with cell parameters: a = 13.454(2), c = 8.459(1) angstrom and V = 1326.0(5) angstrom(3). The refinement of an anisotropic model led to an R index of 0.0421 for 1950 independent reflections. In the crystal structure of stutzite there are eight independent Te sites and eight Ag sites with Z = 7. Only Ag5, Ag6, Ag7 and Ag8 form classic metal-anion polyhedra: Ag8 is in linear coordination, Ag5 and Ag6 are triangularly coordinated, whereas Ag7 is in tetrahedral coordination. The case for Ag1, Ag2, Ag3 and Ag4 is more complicated as there are Ag-Ag contacts beside the typical bonds with the chalcogen. These positions correspond to low coordination (two, three and four) sites, in agreement with the silver preference for such environments. d(10) silver ion distribution has been evidenced by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split model. A discussion on the possible existence of different polytypes in stutzite-like compounds and on the use of the non-harmonic approach based upon a Gram-Charlier development of the atomic displacement factors for the determination of disordered structures is also presented.