Pyrrhotite Fe1–xS is the main sulfide component of platinum-group element (PGE) ores and commonly contains from a few tenths to a few dozen ppm of disseminated Pt. Here we report an X-ray absorption spectroscopy investigation into the state of Pt in synthetic pyrrhotite in combination with theoretical spectra modelling. The pyrrhotite crystals were obtained by means of the salt flux technique, using a eutectic mixture of alkali metal halides as the transport media. Analysis of the chemical composition of synthesised crystals showed that an increase of the temperature and sulfur fugacity yields higher concentrations of Pt in pyrrhotite. The Pt content reached 0.6 wt.% at the maximum temperature and sulfur fugacity (t = 720°C, log $f_{{\rm S}_ 2}$ = –0.1) achieved in a Pt-saturated system. X-ray absorption near-edge structure (XANES) analysis of Pt L3-edge spectra revealed that Pt is present in pyrrhotite in the 4+ and 2+ ‘formal’ oxidation states. Theoretical modelling of XANES and interpretation of extended X-ray absorption fine structure (EXAFS) spectra showed that Pt4+ substitutes for Fe in the crystal lattice of pyrrhotite, whereas Pt2+ forms PtS-like clusters disseminated in the pyrrhotite matrix. Atoms of isomorphous Pt4+ are surrounded by 6 S atoms at a distance of 2.39 ± 0.02 Å. According to theoretical simulations using the FDMNES program, the second coordination sphere of the solid-solution Pt contains one vacancy in the Fe sublattice within the Fe-layer. The Pt2+S-like clusters can be considered as a quench product. High sulfur fugacity stabilises 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 plot, it increases from 1 ppm at 350°C to 3 wt.% at 900°C.

The compositions of coexisting and individual cooperite (ideally PtS) and braggite (ideally (Pt,Pd)S) grains from the Merensky Reef of the Bushveld Complex, as well as cooperite, braggite and vysotskite (ideally PdS) grains from the UG-2 of the Bushveld Complex were investigated. There is a clearly defined miscibility gap between cooperite and braggite, but no evident gap between braggite and vysotskite. Partition coefficients between cooperite and braggite are determined on coexisting phases. The KDbraggite/cooperite in atomic ratios are estimated to be 0.54 for Pt, 15.81 for Pd and 5.93 for Ni. For Rh and Co the KDbraggite/cooperite are estimated to be > 1.40 and > 1.46 respectively. No systematic behaviour is detected for Fe and Cu. Coupled substitutions of Pd + Ni for Pt in cooperite and braggite/vysotskite are indicated. Within the cooperite of the Merensky Reef, the Pd:Ni ratio is approximately 9:11. The substitution trend in braggite, which extends to vysotskite in the UG-2, is dependent on the base-metal sulphide (BMS) association. If pentlandite is the dominant Ni-bearing BMS, the Pd:Ni ratio is about 7:3 in the Merensky Reef and in the UG-2. Millerite as the dominant Ni-bearing BMS in the UG-2 changes this ratio to 3:1. It is concluded that the Ni-content in braggite/vysotskite from BMS assemblages does not depend on the NiS activity, but rather on temperature of formation.

Idiomorphic crystals (up to 3.5 mm) of ferroan platinum, cooperite and mertieite-II were found in a heavy-mineral concentrate from stream sediments of the Darya river in the Aldan Shield, Russia. Pt-Fe crystals display cubic and thin platy habits; occasionally they are twinned. The chemical composition ranges from Pt2.64Fe1.00 to Pt2.88Fe1.00 with Os, Ru, Ir, Rh and Pd below the analytical detection limit of the electron microprobe. X-ray diffractometry of Pt-Fe crystals suggests a F-centred cubic lattice, characteristic of ferroan platinum. Some of the ferroan platinum crystals have large (about 100 μm wide) cooperite overgrowth rims or are covered by a Au-Ag alloy. Cooperite also occurs as large euhedral crystals (up to 3 mm across, partly twinned). Crystals of mertieite-II are speckled with μm-sized (2–5 μm) inclusions of sperrylite and intergrown with minerals of cooperite-braggite solid solution, Pt-Pd-Hg alloy, keithconnite and Au-Ag alloy. Fractures along crystallographic planes of the mertieite-II crystals are filled with Pd-Pt-Fe-Sb-As-Hg-Te-Bi-bearing oxides. The coarse-grained PGM from the Darya have a geological setting similar to the Kondyor PGE placer 75 km to the northeast and are probably related to clinopyroxenite-hornblende-magnetite units of Alaskan-/Uralian-type intrusions.