CONTROLS ON SPATIAL DISTRIBUTION OF OXIDATION PRODUCTS IN NATURAL PYRITE

Pyrite oxidation plays an important role in production of acid mine drainage. Since oxidation is an electrochemical process, there is expected to be a correlation between the electrical properties and the oxidation rates of pyrite. Previous studies of synthetic and natural pyrite show that it behaves as a semiconductor, and the electrical properties can be correlated with the trace element content in the crystals. The samples used in this study come from mines in Colorado, California, Missouri, and Elba (Italy). The concentrations and distribution of trace elements in these crystals were determined using laser ablation ICP-MS. The main trace elements are As, Ni, and Co. As is present up to ~670ppm (Black Cloud Mine, CO), Co up to ~940ppm (Elba) and Ni up to ~320ppm (location unknown). LA-ICP-MS results also indicate that the samples from Elba and Missouri are homogeneous while the ones from California have heterogeneous trace element distributions. A four probe van der Pauw and Hall measurement system were used to determine the resistivity, mobility, carrier concentration and carrier type of the pyrite crystals. The crystals with high Co concentration have the highest Hall mobilities, lowest resistivities and the highest carrier concentrations. High Co concentrations correspond to n-type conductivity while high As concentrations correspond to p-type conductivity. The crystals in which the As concentration was comparable with the Co+Ni concentration, as well as the crystals with low impurity concentrations, exhibit both p-type and n-type conductivity. The oxidation experiments are conducted on powders and thick sections under controlled temperature, humidity, and oxygen levels. The spatial distribution of the oxidation products will be analyzed, and relationships with defects in the crystals such as voids or cracks, and with local concentrations of trace elements, will be described.