Drying and oxidation effects on the magnetic properties of sulfidic material during oxidation

Abstract

Laboratory experiments aimed at understanding the changes in magnetic properties of a sulfidic clay during its oxidation are described. Samples of a sulfidic clay from an acid sulfate soil were slowly oxidized and their magnetic susceptibility (χ) and remanence properties (ARM and SIRM) were measured over a 28 week period. Magnetic properties were measured for the undried material and for subsamples which had been dried by rapid air-drying or by extraction of water with dioxane. Magnetic susceptibility decreased during the first 12 weeks of oxidation and then increased. Samples dried in dioxane showed similar values to the undried material, but air drying caused decreases in χ in samples taken up to 7 weeks of oxidation. Thereafter, there was no effect of drying on χ. In contrast to susceptibility, the remanence properties decreased throughout the experiment, to 8% of their initial values. The results were interpreted in terms of rapid chemical oxidation of a labile magnetic compound (possibly greigite) during the early stages of oxidation and by air drying, and the biological oxidation of pyrite during later stages of oxidation. Associated with these reactions was the formation of a paramagnetic compound, probably ferrihydrite. The effects of changes in iron mineralogy during 28 weeks of oxidation on χ are shown schematically.