Substitution mechanisms in In-, Au-, and Cu-bearing sphalerites studied by X-ray absorption spectroscopy of synthetic compounds and natural minerals

Sphalerite is the main source of In - a 'critical' metal widely used in high-tech electronics. In this mineral the concentration of In is commonly correlated directly with Cu content. Here we use X-ray absorption spectroscopy of synthetic compounds and natural crystals in order to investigate the substitution mechanisms in sphalerites where In is present, together with the group 11 metals. All the admixtures (Au, Cu, In) are distributed homogeneously within the sphalerite matrix, but their structural and chemical states are different. In all the samples investigated In3+ replaces Zn in the structure of sphalerite. The In ligand distance increases by 0.12 A and 0.09-0.10 A 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 sulfur (N-S = 2.4-2.5, RA(u-S) = 2.35 +/- 0.01 angstrom). 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 an Au-S distance of 2.5 angstrom. Therefore, the homogeneous character of a trace-element distribution, which is commonly observed in natural sulfides, does not confirm formation of a solid solution. In contrast to Au, the presence of Cu+ with In exists only in the solid-solution state, where it is tetrahedrally coordinated with S atoms at a distance of 2.30 +/- 0.03 angstrom. The distant coordination shells of Cu are disordered. These results demonstrate that the group 11 metals (Cu, Ag and Au) can exist in sphalerite in the metastable solid-solution state. The solid solution forms at high temperature via the charge compensation scheme 2Zn(2+)<-> 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.