CRYSTAL CHEMISTRY, STABILITY, AND FORMATION CONDITIONS OF SULFIDES WITH SPHALERITE-LIKE CRYSTAL STRUCTURES

Abstract

Compositional and structural characteristics of ore minerals were studied in detail using a combination of physical and chemical methods with wide application of modern equipment and sophisticated programs for data processing. A comprehensive analysis of types of inclusions in chalcopyrite and possible mechanisms of their formation was conducted on the example of the sphalerite-chalcopyrite system. It was shown that regular intergrowths do not prove high-temperature genesis but could be formed under specific crystallization rates and composition of the hydrothermal solution. The valence state and coordination of atoms were studied in the structures of stannite-group compounds. It was found that iron substitutes for copper and zinc in the isomorphic series kuramite-stannite (Cu3SnS4-Cu2FeSnS4) and kesterite-stannite (CU2ZnSnS4-CuFeSnS4 , filling octahedral voids and forming tetrahedral vacancies in the closest packing of sulfur atoms. It was demonstrated by Mössbauer analysis that iron occurs in both di- and trivalent states in the two series. There is a limiting concentration of Fe atoms, Fe(x) - 0.5, below which all iron is ferric and octahedrally coordinated by S atoms. Also considered were the reasons for difficulties in the application of regularities in the partitioning of elements and stable isotopes between coexisting minerals (on the example of sphalerite and stannite). Temperatures calculated by various geothermometers were compared, and an explanation was proposed for the observed discrepancy. The investigation of isomorphism in ore systems indicated the block structure of mineral segregations and their synthetic analogues. The compositional heterogeneity of ore minerals is mostly caused by the presence of tiny inclusions of a second phase or of intergrowths on micro- and submicro- (nano-) levels. The achievements of nanomineralogy, a rapidly developing branch of mineralogy, demonstrate that one of the major reasons for the observed deviations from the expected behavior of minerals and their aggregates is the size factor.