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

Abstract

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. Analysis 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.

Exploring Platinum Speciation with X-ray Absorption Spectroscopy under High-Energy Resolution Fluorescence Detection Mode

“…It can be seen in Figure 3a and Table A1 that the XANES spectrum of the K 2 Pt II Cl 4 solid has a white-line energy 0.6 eV lower than Pt II S, which is also in agreement with previous studies [16,28,29]. The Pt IV hexa-coordinated compounds (K 2 Pt IV Cl 6 and Pt IV O 2 ) have distinctly higher white-line amplitudes (by a factor of 2 to 3) and energy positions of white-line maximum (by 1 to 2 eV) than their Pt II square-planar counterparts (K 2 Pt II Cl 4 , Pt II S), in good agreement with the general tendencies for metal coordination and oxidation state observed in XANES spectra in previous studies, e.g., [81].…”

“…Our results show that Pt L III -edge XANES spectra acquired in HERFD mode (HR) show a significant gain both in the energy-position resolution and the shape and amplitude of white-line and post-absorption features, compared to conventional transmission (TR) or fluorescence (NR) modes, as has been already recognized in previous studies [39][40][41]44]. Therefore, the Pt II and Pt IV formal redox state and atomic coordination may be more clearly distinguished in Pt chloride-and sulfide-bearing systems in HR-XANES mode than in some previous works that used conventional acquisition modes [16,28,29,95]. This resolution improvement may add, in particular, more precision to the characterization of Pt-S and Pt-Cl complexes in hydrothermal fluids where a mixture of Pt IV and Pt II of different coordination may be expected, e.g., [18,96], as well as in natural mineral phases where Pt can be present in different coordination and oxidation states (e.g., Pt II and Pt IV in pyrite and pyrrhotite; [28,29]).…”

“…This gain allows a more precise determination (±0.3 eV) of the white-line maximum energy position. This gain enables, for example, a more accurate distinction between the Pt II and Pt IV redox states in sulfide and chloride solids, which was difficult in nominal-resolution spectra in some previous studies, e.g., [28,29]. The HR mode also results in a significant increase in amplitude and decrease in width of the post-edge XANES resonances, with each feature neatly emphasized (Figure 4), in particular for compounds having pronounced second shell signals evidenced by EXAFS (see below).…”

“…Therefore, the Pt II and Pt IV formal redox state and atomic coordination may be more clearly distinguished in Pt chloride-and sulfide-bearing systems in HR-XANES mode than in some previous works that used conventional acquisition modes [16,28,29,95]. This resolution improvement may add, in particular, more precision to the characterization of Pt-S and Pt-Cl complexes in hydrothermal fluids where a mixture of Pt IV and Pt II of different coordination may be expected, e.g., [18,96], as well as in natural mineral phases where Pt can be present in different coordination and oxidation states (e.g., Pt II and Pt IV in pyrite and pyrrhotite; [28,29]). It can be seen in Table A2 that the amplitude ratios between the main XANES resonances of the spectra of Pt II -Cl versus Pt IV -Cl, or Pt II -S versus Pt IV -S, types No contributions from atoms heavier than S appear in (NH 4 ) 2 Pt IV (S 5 ) 3 , either in TR or HR mode, in contrast with Pt IV S 2 -s showing strong contributions from the heavy Pt atoms in the 2nd shell.…”

“…When applied to low Pt concentrations, conventional fluorescence measurements using solid-state detectors have been widely used to study Pt in its different phases and complexes using both XANES and EXAFS analyses (e.g., [24][25][26][27]). However, conventional fluorescence-as well as transmission-mode XAS [16,[28][29][30] are rather limited as to the discrimination between Pt II and Pt IV , or among different atomic neighbors (e.g., S vs. Cl, O vs. N) in similar types of solids (e.g., Pt II S vs. Pt IV S 2 ) or aqueous complexes (e.g., Pt(HS) n vs. PtCl n ). Compared to conventional fluorescence, HERFD was commonly applied only to XANES analyses, for which it presents numerous benefits: (i) it enables a far better XANES shape definition and may reveal pre-edge and post-edge features that are poorly resolved (if detectable at all) in conventional mode (e.g., [31][32][33][34][35][36]); (ii) it is more sensitive to the presence of light elements whose presence affects HERFD-XANES spectra (e.g., H in Pt catalytic materials; [26]) and more discriminative for neighbors with close atomic numbers or monomeric vs. polymeric ligands of the same element (e.g., HS − vs. S 3 − ; [18,37]); (iii) it allows elimination of unwanted fluorescence signals from other elements with edges close to that of Pt [38], such as tungsten (W-L II edge; 11,544 eV), gold (Au L III -edge; 11,919 eV), arsenic (As-K edge; 11,867 eV), tantalum (Ta-L I edge; 11,682 eV), or rhenium (Re-L II edge; 11,959 eV).…”