Pristine pyrite fracture surfaces, exposed for 7 h to water vapour at low pressure (10(-5) Pa), display no change to their Fe(2p) or S(2p) X-ray photoelectron (XPS) spectrum, but oxygen deposition occurs as H2O, OH-, and O2- (74, 19, and 7%, respectively). An additional 24 h exposure to air (80% humidity) causes the proportions of oxygen species to change dramatically, with OH- and O2- increasing to 55 and 25%, and H2O decreasing to 20%. These changes are accompanied by development of a broad Fe(III) peak of the Fe(2p) spectrum, produced by oxidation of Fe(II) to Fe(III) and formation of Fe(III)-oxyhydroxide surface species. There is, however, no sulphate peak developed in the S(2p) spectrum during the 24 h exposure to air. The XPS data demonstrate that formation of Fe(III)-oxyhydroxides precedes sulphate formation, hence rates of redox reactions producing sulphate or other oxygen-bearing S species are initially slower than redox reactions leading to the formation of Fe(III)-oxyhydroxide surface species. Exposure to air for an additional 9 days produces no appreciable change to the O(1s) or Fe(2g) spectrum, but small amounts of sulphate are observed in the S(2p) spectrum. After production of sulphate species, Fe(III)-sulphate salts probably form at the surface of pyrite by reaction of sulphate with previously formed Fe(III)-oxyhydroxides. The Fe(2p(3/2)) spectrum of the vacuum-fractured pyrite surface reveals a high energy tail on the major Fe(lI) peak. The tail may result from the metallic character of Fe in pyrite. The binding energy and shape of the tail, however, are accurately predicted from S-bonded Fe(III) spectral peaks observed in the pyrrhotite Fe(2p) spectrum; consequently, the presence of Fe(III) bonded to sulphur in the near-surface of pyrite is another reasonable explanation for the tail. Disulphide (S-2(2-)), monosulphide (S2-), and polysulphide (S-n(2-), n > 2) are present on the vacuum-fractured pyrite surface at 85, 10, and 5%, respectively. Monosulphide decreases, and disulphide increases proportionately, during the first 24 h exposure to air. After a total of 10 days exposure, monosulphide decreases to half its original value, polysulphide increases appreciably, and sulphate and thiosulphate are present at 1.8 and 2.3 at. %, respectively. There is little change to disulphide content during the entire experiment. Two explanations are offered for the presence of the three S species on the vacuum-fractured pyrite surfaces. Approximately 5% of S is present as polysulphide (S-n(2-)) and 10% as monosulphide (S2-). This 1:2 ratio is obtained if some disulphide is ''disproportionated'' to polysulphide and monosulphide where, on average, four atoms of S are included in the polysulphide species (S-4(2-)). An alternative explanation includes ferric iron; if present, it may give rise to complex charge compensation involving ''disproportionation'' of disulphide to monosulphide (for charge balance) and polysulphide.
