Rotational hysteresis study of oxidized basalts

The technique of rotational hysteresis measurements can be used as a chemically nondestructive method for determining the magnetic mineral content of rocks. With this method, rotational hysteresis (WR) is measured as a function of magnetic field (H). This method has been applied to basalts that have been classified by optical microscopy into different oxidation states. The WR-H curves show characteristic shapes for oxidation classes 1 to 5. Interpretations of the WR-H curves were made by (1) appealing to the Stoner-Wohlfarth and other theories of rotational hysteresis, (2) experimentally producing a set of calibration curves with synthetic titanomagnetites, magnetite, maghemite, and hematite, and (3) subjecting low-oxidation class basalts to laboratory heating. We conclude that the measurement of WR for fields up to 21,000 oe provides a chemically nondestructive complementary technique for studying the oxidation state of basalts. Additionally, our measurements point out that the highly oxidized basalts contain large amounts of hematite or hemo-ilmenite which are magnetically very hard. By studying the change in shape of the WR-H curves on heating the synthetic titanomagnetites, we have been able to postulate a model for the oxidation-induced phase changes in natural titanomagnetites.