Pyrite was oxidised under O-18(2) gas in H-2 O-16 solutions, with and without added ferric ion, and the sulfate produced was analysed by vibrational spectroscopy to determine the relative amounts of sulfate isotopomers ((SOnO4-n2-)-O-16-O-18) formed. At 70-degrees-C and pH 1, with no added Fe3+, the majority of the sulfate formed was that which derived all four oxygen atoms from water (i.e., (SO42-)-O-16), but significant amounts of two other isotopomers, (SO3O22-)-O-16-O-18 and (SO2O22-)-O-16-O-18, which derive one or two oxygen atoms from molecular oxygen were observed. When Fe3+ was added at the start under identical conditions, no (SO2O22-)-O-16-O-18 was observed. The major isotopomer formed was still (SO42-)-O-16, with (SO3O2-)-O-16-O-18 present as a minor product. Experiments which were performed at initial pH 7 yielded similar results, as did others performed at 20-degrees-C, although the amounts of the minor isotopomers formed vary with temperature. All of the results were confirmed by performing identical experiments with the source of the oxygen isotopes reversed, that is, by oxidising pyrite under air in H-2 O-18 solutions and obtaining the same products in isotopic reverse.
