Pristine and reacted surfaces of pyrrhotite and arsenopyrite as studied by X-ray absorption near-edge structure spectroscopy

Fe L-, S L-, and O K-edge X-ray absorption spectra of natural monoclinic and hexagonal pyrrhotites, Fe_1-xS, and arsenopyrite, FeAsS, have been measured and compared with the spectra of minerals oxidized in air and treated in aqueous acidic solutions, as well as with the previous XPS studies. The Fe L-edge X-ray absorption near-edge structure (XANES) of vacuum-cleaved pyrrhotites showed the presence of, aside from high-spin Fe²⁺, small quantity of Fe³⁺, which was higher for a monoclinic mineral. The spectra of the essentially metal-depleted surfaces produced by the non-oxidative and oxidative acidic leaching of pyrrhotites exhibit substantially enhanced contributions of Fe³⁺ and a form of high-spin Fe²⁺ with the energy of the 3d orbitals increased by 0.3–0.8 eV; low-spin Fe²⁺ was not confidently distinguished, owing probably to its rapid oxidation. The changes in the S L-edge spectra reflect the emergence of Fe³⁺ and reduced density of S s–Fe 4s antibonding states. The Fe L-edge XANES of arsenopyrite shows almost unsplit e_g band of singlet Fe²⁺ along with minor contributions attributable to high-spin Fe²⁺ and Fe³⁺. Iron retains the low-spin state in the sulphur-excessive layer formed by the oxidative leaching in 0.4 M ferric chloride and ferric sulphate acidic solutions. The S L-edge XANES of arsenopyrite leached in the ferric chloride, but not ferric sulphate, solution has considerably decreased pre-edge maxima, indicating the lesser admixture of S s states to Fe 3d orbitals in the reacted surface layer. The ferric nitrate treatment produces Fe³⁺ species and sulphur in oxidation state between +2 and +4.