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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 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 sulfur (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 an Au–S distance of 2.5 Å. 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 Å. 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 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.

Pyrite (FeS2) is a typical container of Pt in ores of magmatic and hydrothermal origin and in some carbonrich ores of sedimentary-diagenetic origin. Knowledge of the state of Pt disseminated in the matrix of pyrite, including local atomic environment (type of atoms in the nearest and distant coordination shells, coordination numbers, interatomic distances) and oxidation state, is necessary for physical-chemical modeling of platinum group element mineralization and for the improvement of Pt ore extraction and processing technologies. Here we report results of an investigation of local atomic structure of synthetic Pt-bearing pyrites by means of X-ray absorption spectroscopy (XAS). Synthesis experiments, performed at 580° and 590°C in a Pt-saturated system by means of salt-flux method, yielded crystals of pyrite with concentrations of Pt up to 4 wt %. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) showed that the distribution of Pt within the pyrite grains is of zonal character, but within the distinct zones Pt is distributed homogeneously. Negative correlation between the concentrations of Pt and Fe was observed in the synthesized pyrite grains. The slope of the correlation line corresponds to the formation of the solid solution in the Pt-Fe-S system and/or to the formation of PtS2. The XAS experiments revealed the existence of two forms of Pt in pyrite. The main form is the solid solution Pt(IV), which isomorphically substitutes for Fe. The Pt-S distance in pyrite is ~0.1 Å longer than that of Fe-S in pure pyrite. The distortion of the pyrite crystal structure disappears at R >2.5 Å. The second Pt-rich form was identified by means of high-resolution transmission electron microscopy (HRTEM) as nanosized inclusions of PtS2. Heating experiments with in situ registration of X-ray absorption spectra resulted in partial decomposition (dissolution) of PtS2 nanosized inclusions with the formation of the solid solution (Fe1–xPtx)S2. Therefore, the PtS2 nanosized particles can be considered as a quench product. Our data demonstrate that both Pt solid solution and PtS2 nanosized inclusions (at high Pt content) can exist in natural Pt-bearing pyrites.

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