Reactivity of pyrrhotite (Fe9S10) surfaces: Spectroscopic studies

Abstract

Synthetic hexagonal pyrrhotite (Fe₉S₁₀) etched in hydrochloric acid solution and then dried in air has been studied using ex situ XPS, X-ray fluorescence, Mössbauer, solid-state NMR and EPR spectroscopies. The metal-deficient non-equilibrium, up to several micrometres thick, layer (NL) formed on pyrrhotite under non-oxidative conditions has been found to be composed predominantly of low-spin Fe²⁺, nearly equal quantities of di- and polysulfide sulphur (probably, chains of 3–5 atoms) and no or low oxygen. When pyrrhotite with the NL is kept in air, singlet ferrous iron converts into high-spin Fe²⁺ and Fe³⁺, oxygen is incorporated into the layer and the surface enrichment in sulfur over iron decreases. A Mössbauer signal with an isomer shift of 0.36 mm s⁻¹ and negligible quadruple splitting has been detected for the etched sample, desiccation in air gives rise to a quadruple split of up to 0.65 mm s⁻¹ and a minor decrease in the isomer shift. The application of variable X-ray tube accelerating voltage has made it possible to obtain depth-resolved Fe-Lα,β spectra of the NL and to find several alteration zones which include different forms of iron. Slow oxidative dissolution of the material in 1 M HCl+0.01 M FeCl₃ electrolyte produces only a thin NL with mostly O-bonded Fe³⁺ and polysulfide prevailing over mono- and disulfide species. Subsequent air-drying of this sample results in an increase in the concentrations of oxygen, S-bonded Fe, and mono- and disulfide species, along with S⁰ formation. No unpaired electron spins have been registered in any of these NL.