X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY OF A PRISTINE PYRITE SURFACE REACTED WITH WATER-VAPOR AND AIR

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 expo sure to air (80% humidity) causes the proportions of oxygen species to change dramatically, with OH- and O2- increasing to 55 and 25%, and H 2O 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 specie s. There is, however, no sulphate peak developed in the S(2p) spectrum during the 24 h exposure to air. The XPS data demonstrate that format ion of Fe(III)-oxyhydroxides precedes sulphate formation, hence rates of redox reactions producing sulphate or other oxygen-bearing S specie s are initially slower than redox reactions leading to the formation o f Fe(III)-oxyhydroxide surface species. Exposure to air for an additio nal 9 days produces no appreciable change to the O(1s) or Fe(2g) spect rum, 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 pyrit e. The binding energy and shape of the tail, however, are accurately p redicted from S-bonded Fe(III) spectral peaks observed in the pyrrhoti te Fe(2p) spectrum; consequently, the presence of Fe(III) bonded to su lphur in the near-surface of pyrite is another reasonable explanation for the tail. Disulphide (S-2(2-)), monosulphide (S2-), and polysulphi de (S-n(2-), n > 2) are present on the vacuum-fractured pyrite surface at 85, 10, and 5%, respectively. Monosulphide decreases, and disulphi de increases proportionately, during the first 24 h exposure to air. A fter a total of 10 days exposure, monosulphide decreases to half its o riginal value, polysulphide increases appreciably, and sulphate and th iosulphate are present at 1.8 and 2.3 at. %, respectively. There is li ttle change to disulphide content during the entire experiment. Two ex planations 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 i s obtained if some disulphide is ''disproportionated'' to polysulphide and monosulphide where, on average, four atoms of S are included in t he polysulphide species (S-4(2-)). An alternative explanation includes ferric iron; if present, it may give rise to complex charge compensat ion involving ''disproportionation'' of disulphide to monosulphide (fo r charge balance) and polysulphide.