DRYING AND OXIDATION EFFECTS ON THE MAGNETIC-PROPERTIES OF SULFIDIC MATERIAL DURING OXIDATION

Laboratory experiments aimed at understanding the changes in magnetic properties of a sulfidic clay during its oxidation are described. Samp les of a sulfidic clay from an acid sulfate soil were slowly oxidized and their magnetic susceptibility (chi) and remanence properties (ARM and SIRM) were measured over a 28 week period. Magnetic properties wer e measured for the undried material and for subsamples which had been dried by rapid air-drying or by extraction of water with dioxane. Magn etic susceptibility decreased during the first 12 weeks of oxidation a nd then increased. Samples dried in dioxane showed similar values to t he undried material, but air drying caused decreases in chi in samples taken up to 7 weeks of oxidation. Thereafter, there was no effect of drying on chi. In contrast to susceptibility, the remanence properties decreased throughout the experiment, to 8% of their initial values. T he results were interpreted in terms of rapid chemical oxidation of a labile magnetic compound (possibly greigite) during the early stages o f 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 e ffects of changes in iron mineralogy during 28 weeks of oxidation on c hi are shown schematically.