New structural data reveal benleonardite to be a member of the pearceite-polybasite group

摘要

The determination of the crystal structure of benleonardite (P (3) over bar m1; R = 0.0321 for 1250 reflections and 102 parameters; refined formula Ag15.00Cu1.00Sb1.58As0.42S7.03Te3.97) obtained using data from a gem-quality, untwinned crystal recovered from the type material, revealed that benleonardite exhibits the structure observed for minerals of the pearceite-polybasite group. The structure consists of the stacking of [Ag-6(Sb,As) 2S6Te](2) A and [Ag9Cu(S,Te)(2)Te-2](2+) B layer modules in which (Sb, As) forms isolated SbS3 pyramids typically occurring in sulfosalts; Cu links two (S, Te) atoms with linear coordination, and Ag occupies sites with coordination geometries ranging from quasi-linear to almost triangular. The silver ions are found in the B layer module along two-dimensional diffusion paths and their electron densities are evidenced by means of a combination of a GramCharlier development of the atom displacement factors and a split model. In the structure, two S positions are completely replaced by Te (i.e. Te-3 and Te-4) and one is half occupied [ S1: S0.514(9) Te0.486], whereas S2 is completely filled by sulfur. This distribution reflects the crystal-chemical environments of the different cations. On the basis of information gained from this characterization, the crystal-chemical formula of benleonarditewas revised according to the structural results, yielding Ag15Cu(Sb,As)(2)S7Te4 (Z = 1) instead of Ag-8(Sb, As)Te2S3 (Z = 2) as previously reported. Thus, the mineralmust be considered amember of the pearceite-polybasite group. A recalculation of the chemical data listed in the scientific literature for benleonardite according to the structural results obtained here leads to excellent agreement.