X-RAY PHOTOELECTRON AND AUGER-ELECTRON SPECTROSCOPIC STUDIES OF PYRRHOTITE AND MECHANISM OF AIR OXIDATION

Pyrrhotite (Fe7S8) fractured under high vacuum (10(-7) Pa) and reacted with air for 6.5 and fifty hours was analyzed using X-ray Photoelectr on Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). XPS iron data from fresh surfaces indicate 32% Fe(III) and 68% Fe(II), both bon ded to sulphur. The result agrees closely with stoichiometry which sug gests 29% Fe(III) in the pyrrhotite studied. This is the first spectro scopic evidence to indicate Fe(III) in pyrrhotite. Sulphur is present primarily as monosulphide (S2-), With minor amounts of disulphide (S-2 (2-)) and polysulphide (S-n(2-)). XPS examination of 6.5 hour air-oxid ized 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 oxyg en data suggest a Fe(III)-oxyhydroxide to be the species forming. Sulp hur spectra demonstrate a range of oxidation states from S2- (monosulp hide) to S6+ (sulphate). AES compositional depth profiles of air-oxidi zed surfaces display three compositional zones. After fifty hours of a ir oxidation the outermost layer is less than 10 Angstroms, oxygen-ric h, and sulphur depleted. Immediately below the O-rich layer exists an Fe-deficient, S-rich layer that displays a continuous, gradual decreas e in S/Fe from the O-rich zone to that of the unaltered pyrrhotite. Qu antification of depth profiles utilizing the sequential layer sputteri ng model (SLS) indicate alteration trends correspond compositionally t o FeO1.5, FeS2, Fe2S3, and Fe7S8. Compositional zones develop by elect ron and iron migration towards the oxidized surface. Molecular oxygen initially taken onto the surface is reduced to O2- probably by electro n transfer from the pyrrhotite interior, and is facilitated by-rapid e lectron exchange between Fe(III) and Fe( II) of the bulk solid. Vacanc ies inherent to nonstoichiometric pyrrhotite probably promote diffusio n of iron to the surface resulting in the formation of iron oxyhydroxi de species.