X-RAY PHOTOELECTRON AND AUGER ELECTRON SPECTROSCOPY OF AIR-OXIDIZED PYRRHOTITE: DISTRIBUTION OF OXIDIZED SPECIES WITH DEPTH

Abstract

Angle resolved X-ray photoelectron spectroscopy (ARXPS) of air-oxidized pyrrhotite (Fe7S8) surfaces reveals two distinctive compositional zones. The outer most zone is composed of iron oxyhydroxide, whereas the underlying zone is sulphur-rich and depleted of Fe relative to bulk pyrrhotite. Underlying this sulphur-rich zone is bulk pyrrhotite.Auger compositional depth profiles confirm that the outer most iron-oxyhydroxide layer is approximately 5 a thick. A sharp interface separates this layer from the underlying sulphur-rich layer (approx. 30 a thick), in which the Fe:S ratio approaches 1:2 and contains minor iron thiosulphate and iron sulphate.ARXPS and Auger data provide insight into the mechanism of incipient pyrrhotite oxidation. Monosulphide of the sulphur-rich underlayer is oxidized to disulphide and polysulphides primarily. The likely reduction reaction is conversion of molecular oxygen to oxide at the mineral surface. Iron diffuses from the interior to the surface where it combines with oxide oxygen, hydroxide, and water to form ferric oxyhydroxides. Although Fe diffuses from the interior to the surface, sulphur species do not migrate appreciably from the subsurface giving rise to the sulphur-rich zone. There is no evidence that oxygen diffuses from the oxyhydroxide layer into the sulphur-rich layer during the initial stages of oxidation.The angle resolved S 2p XPS spectrum demonstrates clearly that the disulphide signal is derived from the sulphur-rich zone beneath the oxyhydroxide layer. X-ray diffraction studies of pyrrhotite conversion to marcasite have shown that removal of Fe atoms from the pyrrhotite structure produces marcasite (compositionally and structurally) on a macroscopic scale. The same conversion probably occurs in the sulphur-rich zone of pyrrhotite, where diffusion of Fe to the oxidized surface results in formation of marcasite-like composition and structure in the sulphur-rich layer of oxidized pyrrhotite.