Crystal structure of natural Ag–Cu–Pb–Bi sulfide

Resumo

The crystal structure of a natural sulfide Cu_3,44Ag_0,56Pb₂Bi₆S₁₃ (Сmcm, Z = 4, a = 3.973(1) Å, b = 13.370(2) Å, c = 42.182(7) Å, R = 0.059) is determined. The structure has seven cation positions: two of them (Cu and Ag) are in a tetrahedral environment of sulfur atoms; one (Pb), in a special position (mm2), has a coordination polyhedron in the form of a bicapped trigonal prism; and the other cation positions are surrounded by sulfur atoms forming distorted octahedra. The mirror symmetry plane perpendicular to the c translation causes microtwinning by cutting a layer of trigonal prisms framed by tetrahedron ribbons. These layers are divided by those composed by edge-linked octahedra with a diagonal ribbon of five octahedra (N = 5). The cation and anion positions are ordered by individual sublattices with pseudohexagonal subcells on the m planes perpendicular to the a translation, which concentrate the positions of all the atoms. Supposedly, this natural sulfide is the previously described (1885) yet unconfirmed alaskaite mineral from the lillianite–heyrovskyite homological series and may be isostructural to the ourayite mineral.