Substitution mechanisms in In, Au, and Cu-bearing sphalerites studied by X-ray absorption spectroscopy of synthetic and natural mineralsстатья
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Аннотация:Sphalerite is the main source of In - “critical” metal which is widely used in the high-tech
electronics. In this mineral the concentration of In is often directly correlated with Cu content. Here we use X-ray absorption spectroscopy of synthetic and natural crystals in order to investigate the substitution mechanisms in sphalerites where In presents together with the group 11 metals. All the admixtures (Au, Cu, In) are homogeneously distributed within the sphalerite matrix, but their structural and chemical states are different. In all studied samples In3+ replaces Zn in the structure of sphalerite. The In-ligand distance increases by 0.12 Å and 0.09-0.10 Å for the 1st and 2nd coordination shells, respectively, in comparison with pure sphalerite. The In-S distance in the 3rd
coordination shell is close to the one of pure sphalerite. Gold in synthetic sphalerites is coordinated with sulphur (NS = 2.4 – 2.5, RAu-S = 2.35±0.01 Å). Our data suggest
that at high Au concentrations (0.03-0.5 wt%) the Au2S clusters predominate, with a small
admixture of the Au+ solid solution with Au-S distance of 2.5 Å. Therefore, the homogeneous
character of a trace element distribution, which is often observed in natural sulphides, does not confirm formation of a solid solution. In contrast to Au, Cu+
in the presence of In exists only in the solid solution state, where it is tetrahedrally coordinated with S atoms at a distance of 2.30±0.03 Å. The distant coordination shells of Cu are of disordered character. These results demonstrate that the group 11 metals (Cu, Ag, Au) can present in sphalerite in the metastable solid solution state. The solid solution forms at high temperature via the charge compensation scheme 2Zn2+↔Me++Me3+. The final state of the trace elements at ambient temperature is governed by the difference in ionic radii with the main component (Zn), and concentration of admixtures.