X-ray photoelectron and Auger electron spectroscopic studies of pyrrhotite and mechanism of air oxidation

Abstract Pyrrhotite (Fe7S8) fractured under high vacuum (10−7 Pa) and reacted with air for 6.5 and fifty hours was analyzed using X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). XPS iron data from fresh surfaces indicate 32% Fe(III) and 68% Fe(II), both bonded to sulphur. The result agrees closely with stoichiometry which suggests 29% Fe(III) in the pyrrhotite studied. This is the first spectroscopic evidence to indicate Fe(III) in pyrrhotite. Sulphur is present primarily as monosulphide (S2−), with minor amounts of disulphide (S22−) and polysulphide (Sn2−). XPS examination of 6.5 hour air-oxidized surfaces indicates 58% Fe(III) and 42% Fe(II). Fe(III) is bonded to oxygen and most Fe(II) remains bonded to sulphur. XPS iron and oxygen data suggest a Fe(III)-oxyhydroxide to be the species forming. Sulphur spectra demonstrate a range of oxidation states from S2− (monosulphide) to S6+ (sulphate). AES compositional depth profiles of air-oxidized surfaces display three compositional zones. After fifty hours of air oxidation the outermost layer is less than 10 Angstroms, oxygen-rich, and sulphur depleted. Immediately below the O-rich layer exists an Fe-deficient, S-rich layer that displays a continuous, gradual decrease in S Fe from the O-rich zone to that of the unaltered pyrrhotite. Quantification of depth profiles utilizing the sequential layer sputtering model (SLS) indicate alteration trends correspond compositionally to FeO1.5, FeS2, Fe2S3 and Fe7S8. Compositional zones develop by electron and iron migration towards the oxidized surface. Molecular oxygen initially taken onto the surface is reduced to O2− probably by electron transfer from the pyrrhotite interior and is facilitated by rapid electron exchange between Fe(III) and Fe(II) of the bulk solid. Vacancies inherent to nonstoichiometric pyrrhotite probably promote diffusion of iron to the surface resulting in the formation of iron oxyhydroxide species.

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