Polyhedral characteristics of the cosalite-type crystal structures

Abstract

Abstract The crystal structure of cosalite from the Trepča orefield was refined in the orthorhombic space group Pnma [a = 23.7878 (9), b = 4.0566 (3), c = 19.1026 (8) Å, V = 1843.35 (17) Å3, Z = 2] from single-crystal data (MoKα X-ray diffraction, CCD area detector) to the conventional R1 factor 0.031 for 1516 unique reflections with I > 2σ(I). The chemical formula (Cu0.15Ag0.24)+(Fe0.19Pb7.20)2+(Bi7.06Sb1.06)3+S20, calculated on the basis of 20 S atoms per formula unit, was determined by WDX. The unit cell contains 18 + 2 symmetrically nonequivalent atomic sites: 10 occupied by S; two by pure Pb (Pb3 and Pb4); one by pure Bi (Bi1); two by a combination of Bi and small amounts of Sb (Bi2/Sb2, Bi4/Sb3); two by Pb and Bi, and in one of these also by a small amount of Ag [Me1 = Pb2 >> Bi5 > Ag1, Me3 = Pb1 >> Bi3]; and finally one site, Me2 (Bi6 >> □), is partly occupied by Bi and partly split into an additional two adjacent trigonal planar “interstitial positions”, Cu1 and Cu2, where small amounts of Cu, Ag, and Fe can be situated. All atoms are at 4c special positions at y = 0.25 or 0.75. The structure consists of slightly to moderately distorted MeS6 octahedra sharing edges, bicapped trigonal PbS8 coordination prisms, and fairly distorted Cu1S6 and Cu2S4 polyhedra. The effects of the cation substitutions, bond valence sums, and the polyhedral characteristics are compared with other published cosalite-type structures. Among known cosalite-type structures, the largest volume contraction is shown by sample 4 (Altenberg) and involves the replacement of large cations (Bi3+ and Pb2+) by the smaller Sb3+, as well as Cu+ and Ag+. These replacements are reflected in the variations of individual Me–S bond distances, which are accompanied by variations in average Me–S distances. The degree of polyhedral distortion, Δ, progressively increases for the four Bi-hosting sites of nine cosalite-type structures: Me2 < Bi2 < Bi1 < Bi4. The Bi4 and Me3 are the most and the Me1 and Me2 are the least distorted octahedral sites of the nine cosalite-type structures.