Identification of magnetite and hematite in rocks by magnetic observation at low temperature

A method of identification of coarse-grained hematite and magnetite based on the recognition of their distinctive low-temperature transitions is presented. Specimens are magnetized to saturation, first at room temperature and then at −196°C. The transitions are observed as discontinuities in the curve of the change of isothermal remanent magnetization (IRM) with increasing temperature, from −196°C to 20°C. At −140°C the low-temperature IRM of magnetite is almost completely lost. At −20°C about one-half of the room-temperature IRM of hematite is recovered owing to the memory effect of hematite. The basic characteristics of the memory effects, upon which the method of identification is based, have been investigated with a vibration magnetometer. The memory of room-temperature magnetization has been described previously for hematite and magnetite. It has also been found that under the conditions of the present experiments there is a reciprocal relationship across the transition temperature between the low- and ordinary-temperature IRM of magnetite. On passing through the transition from high to low temperature, the remanence from the preceding cycle of high-temperature events is alone memorized; all earlier magnetizations are lost. Similarly, the last low-temperature IRM is alone memorized when magnetite is heated through the transition temperature. The method of identification has revealed the presence of hematite in a lightly metamorphosed red sediment and of magnetite in a red sediment baked by an igneous intrusion and in a red arkose, but neither coarse hematite nor magnetite has yet been found in unmetamorphosed red sandstones. The detection of both hematite and magnetite by this method is limited by the dependence of the magnetic transitions upon grain size and the presence of impurities.