Impurities and heterogeneity in pyrite: Influences on electrical properties and oxidation products

Abstract

Macroscopic pyrite crystals originating from a variety of geologic settings were made into thick sections. Electrical properties were measured with a Hall system, and minor element composition was analyzed with laser ablation inductively coupled plasma mass spectroscopy (LA-ICPMS). Selected thick sections were oxidized in a moist air environment inside a glove chamber. The relative metal content of surface products formed during oxidation was analyzed by LA-ICP-MS. Natural pyrite exhibits a range of electrical properties corresponding to the content of the common minor elements Co, As and Ni. These properties are similar to those of synthetic pyrite doped with single elements. Pyrite enriched in Co is an n-type semiconductor with low resistivity and high carrier mobility, while arsenian pyrite tends to be p-type and have higher resistivity. The effect of Ni is weaker and tends to be obscured by Co and As in samples of mixed composition. Cobalt demonstrates the strongest effect on electrical properties. Enrichment of Co at oxidized pyrite surfaces is inversely correlated with its concentration in the underlying pyrite. Cobalt enrichment in oxidation products is also more pronounced along crystal defects such as fractures, and in crystals with heterogeneous distribution of trace elements. These observations might be explained by differences in the electronic structure of pyrite arising from the presence of impurities, and by the distribution of domains with different impurity compositions, facilitating electron transfer.