The state of platinum in pyrrhotite: X-ray absorption spectroscopy study and implications for the role of Fe sulphides as platinum carriers

Olga N. Filimonova, Alexander L. Trigub, Maximilian S. Nickolsky, Dmitriy A. Chareev, Kristina O. Kvashnina, Elena V. Kovalchuk, Ilya V. Vikentyev, Vladimir L. Reukov, Boris R. Tagirov
2021 Mineralogical magazine  
Pyrrhotite Fe 1-x S is the main sulphide component of platinum group elements (PGEs) ores where it commonly contains from a few tenths to a few dozen ppm of disseminated Pt. Here we report an investigation of the state of Pt in synthetic pyrrhotite performed by X-ray absorption spectroscopy (XAS) in combination with theoretical spectra modelling. The pyrrhotite crystals were obtained by means of salt flux technique, using an eutectic mixture of alkali metal halides as a transport media.
more » ... of the chemical composition of synthesized crystals showed that an increase of the temperature and sulphur fugacity yields higher concentrations of Pt in pyrrhotite. The Pt content reaches 0.6 wt% at the maximum achieved temperature and sulphur fugacity (t = 720°C, log f(S 2 ) = -0.1) in Pt-saturated system. Analysis of Pt L 3 -edge XANES spectra revealed that Pt presents in pyrrhotite in the 4+ and 2+ "formal" oxidation states. Theoretical modelling of XANES and interpretation of EXAFS spectra showed that Pt 4+ substitutes for Fe in the crystal lattice of pyrrhotite, whereas Pt 2+ forms PtS-like clusters disseminated in the pyrrhotite matrix. Atoms of isomorphous Pt 4+ are surrounded by 6 S atoms at a distance of 2.39±0.02 Å. According to theoretical FDMNES simulations, the 2 nd coordination sphere of the solid solution Pt contains one vacancy in the Fe sublattice within the Fe-layer. The Pt 2+ S-like clusters can be considered as a quench product. High sulphur fugacity stabilizes the solid solution Pt and prevents the formation of the PtS-like clusters during cooling. The maximum content of the solid solution Pt in pyrrhotite is ca. 50 times lower than in pyrite and can be approximated by a straight line in the log C(Pt) vs. 1/T coordinates, it increases from 1 ppm at 350°C to 3 wt% at 900°C.
doi:10.1180/mgm.2021.76 fatcat:glyyv5klnbfkbmax6bln65voua