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

The determination of the crystal structure of benleonardite (P3m1; R=0.0321 for 1250 reflections and 102 parameters; refined formula Ag (sub 15.00) Cu (sub 1.00) Sb (sub 1.58) As (sub 0.42) S (sub 7.03) Te (sub 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 [Ag (sub 6) (Sb,As) (sub 2) S (sub 6) Te] (super 2-) A and [Ag (sub 9) Cu(S,Te) (sub 2) Te (sub 2) ] (super 2+) B layer modules in which (Sb,As) forms isolated SbS (sub 3) 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 (sub 0.514(9)) Te (sub 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 Ag (sub 15) Cu(Sb,As) (sub 2) S (sub 7) Te (sub 4) (Z=1) instead of Ag (sub 8) (Sb,As)Te (sub 2) S (sub 3) (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.