THE CRYSTAL CHEMISTRY OF COSALITE BASED ON NEW ELECTRON-MICROPROBE DATA AND SINGLE-CRYSTAL DETERMINATIONS OF THE STRUCTURE

Crystal-structure refinements done on six samples of cosalite with different types and degrees of copper and silver substitutions, as well as electron-microprobe studies of 33 different samples of cosalite, both enlarged and clarified the spectrum of cation substitutions and the crystal chemistry of this species. The Ag + Bi ↔ 2Pb scheme of substitution acts at the structural site Me 1. The 2(Cu + Ag) ↔ Pb scheme is a result of two combined mechanisms: creation of vacancies in the Bi-containing octahedral site Me2 accompanied by a progressive occupancy of two triangular faces of this octahedron by Cu + Ag [ i.e. , Bi ↔ 2(Cu + Ag)], which is combined with progressive replacement of Pb in the adjacent Me 1 octahedron by Bi; their combination gives the resulting closely followed chemical relationship. The structural mechanism of the 3Cu ↔ Bi substitution observed in thin zones in cosalite from Felbertal remains unknown. Cosalite offers a unique case of solid solution based on a combined omission – interstitial insertion substitution.