The dry oxidation of tetragonal FeS1-x mackinawite

The gradual oxidation of dry mackinawite (tetragonal FeS1-x) has been studi ed using X-ray diffraction (XRD), transmission electron microscopy (TEM), t ransmission Mossbauer spectroscopy (TMS) and X-ray photoelectron spectrosco py (XPS). The initial material and samples exposed to the air (5 min to 6 m onths) have been analysed. Diffraction patterns showed the slow disappearan ce of mackinawite with time with concomitant appearance of greigite (Fe3S4) and elemental sulphur (S(0)) as well as iron (oxyhydr)oxides, i.e. magneti te (Fe3O4) and probably goethite (alpha -FeOOH). After 6 months' air exposu re, mackinawite and also greigite were entirely converted into elemental su lphur and iron (oxyhydr)oxide(s), indicating that greigite was an intermedi ate reaction product. Mossbauer spectra of samples oxidized in air appeared rather complex for interpreting what was easily conceivable in view of the association of several phases, as revealed by the diffraction patterns. Th e low-temperature Mossbauer spectrum obtained after 6 months air exposure w as attributed to magnetite, although a mixture of magnetite and goethite wa s not completely excluded. XPS iron and oxygen data confirmed the formation of Fe(III) (oxyhydr)oxides at the surface after an induction period. Sulph ur spectra demonstrated various oxidation states from S(II) (monosulphide) to S(VI) (sulphate) for the longest experiments. Mackinawite in these exper iments reacted mainly with adsorbed O-2 to form elemental sulphur and magne tite. Additionally, sufficient sulphur was generated to react stoichiometri cally with mackinawite to produce greigite. Finally, greigite, in the longe st experiments, was transformed into elemental sulphur and magnetite.