Structure refinement of an untwinned single crystal of Ag-excess fizélyite, Ag_5.94Pb_13.74Sb_20.84S ₄₈

The crystal structure of Ag-excess fizélyite from the Van Silver mine, just north of Squamish, British Columbia, Canada, was studied on the basis of X-ray-diffraction data collected from an untwinned single crystal. The structure was refined to R₁ = 0.041 in space group P2₁/n, with unit-cell parameters a 19.2767(6), b 13.2345(4), c 8.7230(3) Å, β 90.401(2)°, and V 2225.3(1) ų. The chemical composition of the sample, determined by electron-microprobe analysis, is Ag_5.94Pb_13.74Sb_20.84S₄₈, which reveals nearly one more Ag atom per formula unit than in the ideal composition, Ag₅Pb₁₄Sb₂₁S₄₈, presently proposed by the IMA for fizélyite. Except for an additional partially occupied Ag site (Ag2) and two split cation sites (Ag1 and M2) in our sample, fizélyite is isostructural with ramdohrite, CdAg_5.5Pb ₁₂Sb_21.5S₄₈. The structure of fizélyite contains chains of edge-sharing SbS₆ octahedra connected by chains of alternating M1S₆, (M1 = Pb + Sb) and SbS₆ octahedra, forming slabs parallel to (100). The slabs of octahedra are linked by distorted AgS₄ tetrahedra, M2S₆ octahedra, and PbS₈ polyhedra. The excess Ag in our fizélyite is mostly situated at the Ag2 site, and is likely responsible for the splitting of the Ag1 and M2 sites.