The crystal structure of kudriavite, (Cd,Pb)Bi2S4

The crystal structure of kudriavite, (Cd,Pb)Bi2S4, a new mineral species, was solved from single-crystal X-ray-diffraction data and refined to R = 4.9% (4.3% for a model with split mixed-cation sites). Lattice parameters are a 13.095(1), b 4.0032(3), c 14.7 11 (1) angstrom. beta 11 5.59(1)degrees, V 695.6(1) angstrom(3). The structure is equivalent to that of synthetic CdBi2S4, space group C2/m, Z = 4, and represents a pavonite homologue, N = 3. It is built of three-octahedron-thick columns of (3 1 1)PbS-like slabs combined by "unit-cell twinning" in a quasi-mirror-glide succession. The slabs, which are intrinsically of the same topology, differ in the coordination state of bordering cations because of the relative positions of the adjacent layers. In the slabs of type I (the "non-accreting" slab common to all pavonite homologues), the central columns of octahedra are flanked by half-octahedral (square-pyramidal) coordinations completed to split-octahedra by the addition of two S atoms from the adjacent slab. The central columns of octahedra in the slabs oft pe 11 (of varying thickness, i.e., "accreting" in pavonite homologues) are flanked by octahedral coordinations involving a S atoin from the adjacent slab. The observed chemical Cd <-> Ph and Bi <-> III Substitutions are, according to the crystal -chemical analysis. structurally limited to Bi-containing sites. They are distributed predominantly in the boundary coordinations of the type-I slabs, and the central octahedra of the type-II slabs, with Ph preferentially entering the former, and lit the latter. The marginal octahedral sites of the type-if slabs, and the central octahedral sites of the type-I slabs, have been refilled as pure Bi and Cd sites. respectively. The crystal -chemical analysis suggests that the observed 1:1 Pb:Cd ratio in natural kudriavite represents the upper limit for the substitution of Ph in the structure. A limit of the lit substitution is not possible to determine from the present data. but it most probably is less than In:Bi = 1:3. The ratio observed in type kudriavite is less than 1:10.