Publication Date:
2019
Description:
〈span〉〈div〉Abstract〈/div〉The crystal structure of cosalite from the Trepča orefield was refined in the orthorhombic space group 〈span〉Pnma〈/span〉 [〈span〉a〈/span〉 = 23.7878 (9), 〈span〉b〈/span〉 = 4.0566 (3), 〈span〉c〈/span〉 = 19.1026 (8) Å, 〈span〉V〈/span〉 = 1843.35 (17) Å〈sup〉3〈/sup〉, 〈span〉Z〈/span〉 = 2] from single-crystal data (Mo〈span〉Kα〈/span〉 X-ray diffraction, CCD area detector) to the conventional 〈span〉R〈/span〉〈sub〉1〈/sub〉 factor 0.031 for 1516 unique reflections with 〈span〉I〈/span〉 〉 2σ(〈span〉I〈/span〉). The chemical formula (Cu〈sub〉0.15〈/sub〉Ag〈sub〉0.24〈/sub〉)〈sup〉+〈/sup〉(Fe〈sub〉0.19〈/sub〉Pb〈sub〉7.20〈/sub〉)〈sup〉2+〈/sup〉(Bi〈sub〉7.06〈/sub〉Sb〈sub〉1.06〈/sub〉)〈sup〉3+〈/sup〉S〈sub〉20〈/sub〉, 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 [〈span〉Me〈/span〉1 = Pb2 〉〉 Bi5 〉 Ag1, 〈span〉Me〈/span〉3 = Pb1 〉〉 Bi3]; and finally one site, 〈span〉Me〈/span〉2 (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 4〈span〉c〈/span〉 special positions at 〈span〉y〈/span〉 = 0.25 or 0.75. The structure consists of slightly to moderately distorted 〈span〉Me〈/span〉S〈sub〉6〈/sub〉 octahedra sharing edges, bicapped trigonal PbS〈sub〉8〈/sub〉 coordination prisms, and fairly distorted Cu1S〈sub〉6〈/sub〉 and Cu2S〈sub〉4〈/sub〉 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 (Bi〈sup〉3+〈/sup〉 and Pb〈sup〉2+〈/sup〉) by the smaller Sb〈sup〉3+〈/sup〉, as well as Cu〈sup〉+〈/sup〉 and Ag〈sup〉+〈/sup〉. These replacements are reflected in the variations of individual 〈span〉Me〈/span〉–S bond distances, which are accompanied by variations in average 〈span〉Me–〈/span〉S distances. The degree of polyhedral distortion, Δ, progressively increases for the four Bi-hosting sites of nine cosalite-type structures: 〈span〉Me〈/span〉2 〈 Bi2 〈 Bi1 〈 Bi4. The Bi4 and 〈span〉Me〈/span〉3 are the most and the 〈span〉Me〈/span〉1 and 〈span〉Me〈/span〉2 are the least distorted octahedral sites of the nine cosalite-type structures.〈/span〉
Print ISSN:
0008-4476
Electronic ISSN:
1499-1276
Topics:
Geosciences
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