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

Processing of Zn ore accounts for >95% of production of In - a "critical" metal which is widely used in the high-tech electronics. The main source of In is sphalerite (Zn, Fe)S which also can host industrial concentrations of Au. Here we use X-ray absorption spectroscopy to investigate the coupled chemistry of In and Au in synthetic sphalerite crystals - analogues of natural minerals. The concentrations of In and Au were found to correlate with each other and reached 0.5 wt% in crystals synthesized at 850 °C. Both metals are homogeneously distributed within the sphalerite matrix. However, their positions within the mineral are different. In accord with X-ray absorption near edge structure (XANES) spectroscopy the formal oxidation state of these elements is +3 (In) and +1 (Au). Analysis of extended X-ray absorption fine structure (EXAFS) spectra revealed that In 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 spheres, respectively, in comparison with pure ZnS. The In-S distance in the 3rd coordination sphere is close to the one of pure sphalerite. The In K-edge and Au L3-edge XANES and EXAFS spectra suggest that there is no In-Au clustering. Gold in sphalerite is coordinated with 2.5±0.3 S atoms at Au-S distance of 2.35±0.01 Å in the 1st coordination sphere, whereas distant coordination spheres have disordered nature.
Our data suggest that at least two different forms of Au are present in sphalerite. At high Au concentrations (0.03-0.5 wt%) the nanosized Au2S clusters predominate, probably with small admixture of the Au solid solution characterized by higher Au-S distance. Alike Au, the other 1st group metals (Me) Cu and Ag, which often are present in high (tenths ppm to wt%) concentrations in sphalerite, can form nanosized Me-S clusters with only traces (ppm level) of metal in the solid solution state.