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

The determination of the crystal structure of benleonardite (P3İm1; R = 0.0321 for 1250 reflections and 102 parameters; refined formula Ag_15.00Cu_1.00Sb_1.58As_0.42S_7.03Te_3.97) 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 [Ag₆(Sb,As)₂S₆Te]²⁻A and [Ag₉Cu(S,Te)₂Te₂]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 Gram-Charlier development of the atom displacement factors and a split model. In the structure, two S positions are completely replaced by Te (i.e. Te3 and Te4) and one is half occupied [S1: S_0.514(9)Te_0.486], 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 benleonardite was revised according to the structural results, yielding Ag₁₅Cu(Sb,As)₂S₇Te₄ (Z = 1) instead of Ag₈(Sb,As)Te₂S₃ (Z = 2) as previously reported. Thus, the mineral must be considered a member 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.