An excited oxygen gas has been used to oxidize pure magnetite at temperatures between 50°C and 200°C. The oxidation products are either maghemite or hematite, depending on the water content of the initial material. In all cases the intensity of remanent magnetization, ARM or TRM, is reduced on oxidation with only small changes in the microscopic coercive force spectra. A chemical remanent magnetization, which is relatively weak with respect to the initial remanence, is always acquired during oxidation. These results are useful in the interpretation of intensity changes in some linear magnetic anomalies of ocean floors and in the interpretation of changes in direction and intensity of remanence in land outcrops that have undergone low-temperature oxidation.
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Johnson et al. (1972) studied this question.