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

Angle resolved X-ray photoelectron spectroscopy (ARXPS) of air-oxidize d pyrrhotite(Fe7S8) surfaces reveals two distinctive compositional zon es. The outer most zone is composed of iron oxyhydroxide, whereas the underlying zone is sulphur-rich and depleted of Fe relative to bulk py rrhotite. Underlying this sulphur-rich zone is bulk pyrrhotite. Anger compositional depth profiles confirm that the outer most iron-oxyhydro xide layer is approximately 5 Angstrom thick. A sharp interface separa tes this layer from the underlying sulphur-rich layer (approx. 30 Angs trom thick), in which the Fe:S ratio approaches 1:2 and contains minor iron thiosulphate and iron sulphate. ARXPS and Auger data provide ins ight into the mechanism of incipient pyrrhotite oxidation. Monosulphid e of the sulphur-rich underlayer is oxidized to disulphide and polysul phides primarily. The likely reduction reaction is conversion of molec ular oxygen to oxide at the mineral surface. Iron diffuses from the in terior 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 fr om the subsurface giving rise to the sulphur-rich zone. There is no ev idence that oxygen diffuses from the oxyhydroxide layer into the sulph ur-rich layer during the initial stages of oxidation. The angle resolv ed S 2p XPS spectrum demonstrates clearly that the disulphide signal i s derived from the sulphur-rich zone beneath the oxyhydroxide layer. X -ray diffraction studies of pyrrhotite conversion to marcasite have sh own that removal of Fe atoms from the pyrrhotite structure produces ma rcasite (compositionally and structurally) on a macroscopic scale. The same conversion probably occurs in the sulphur-rich zone of pyrrhotit e, 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.