A chemical and structural re-examination of fettelite samples from the type locality, Odenwald, southwest Germany

The crystal structure and chemical composition of two samples of fettelite from the type locality, including a portion of the holotype material, was investigated to verify if a previously proposed revision of the chemical formula was applicable, and to study the role of cation substitution for Hg that would suggest new members of the fettelite family. The crystal structure of fettelite from the type locality was found to be equivalent to that reported previously for the Chilean occurrence, and consists of an alternation of two kinds of layers along c: layer A with general composition [Ag₆As ₂S₇]^2- and layer B with general composition [Ag₁₀HgAs₂S₈]²⁺. In this structure, the Ag atoms occur in various coordination configurations, varying from quasi-linear to quasi-tetrahedral, the AsS₃ groups form pyramids as are typically observed in sulfosalts, and Hg links two sulfur atoms in a linear coordination. The refined compositions for the crystals in this study, [Ag ₆As₂S₇][Ag₁₀(Fe _0.53Hg_0.47)As₂S₈] (R100124) and [Ag₆As₂S₇][Ag₁₀(Hg _0.79Cu_0.21)As₂S₈] (R110042), clearly indicate that new mineral species related to fettelite are likely to be found in nature.