Oxidation of pyrite grains: A Mossbauer spectroscopy and mineral magnetism study

Fe2+ in pyrite is found in a low-spin d(6) configuration, a necessary condition for diamagnetic and semi-conducting properties of material. The semi-conducting property of pyrite has been studied since the time when pyrite was used as a rectifier in early radios. Pyrite posses the highest possible crystal field stabilisation energy and offers a better altemative as solar material compared to Si- based materials. Unfortunately, pyrite is difficult to study due to its inherent deviation from stoichiometry and its ease of oxidation. Since pyrite and its oxidation products are all Fe-bearing phases, combining Mossbauer spectroscopy with mineral magnetic methods provides enough information to monitor the oxidation of pyrite in air and identify the different phases produced and their relation to different experimental parameters. For mm-sized grain samples, heating FeS2 at temperatures between 450 degrees C and 650 degrees C five different mineral assemblages are identified. FeS2 is oxidized to alpha-Fe2O3 along two separate routes: FeS2 -> FeSO4 -> epsilon-Fe2O3 -> alpha-Fe2O3; and FeS2 -> FeSO4 -> Fe-2 (SO4)(3) -> ss-Fe2O3 -> alpha-Fe2O3