Magnetic Susceptibility and Anisotropy of Hematite Single Crystals Determined by Increasing Low-fields and High-field Instruments

Hematite is one of the most important carriers of magnetic remanence in natural rocks. The magnetic properties of hematite bearing rocks have been intensively studied, particularly in the last decade, as instrumentation has become more sensitive, e.g., squid magnetometers and sophisticated vibrating sample magnetometers. The anisotropy of hematite and hematite-bearing rocks is more difficult to evaluate since its magnetic susceptibility is lower than other magnetic minerals such as magnetite. The magnetic anisotropy of hematite, however, is often responsible for inclination shallowing of the remanence in hematite-bearing rocks. It is important to estimate this degree of flattening, so that remanence shallowing can be compensated when making paleoreconstructions of terrains. . The anisotropy of magnetic susceptibility has been measured on a collection of hematite single crystals in increasing low fields between 2 and 450 A/m in high fields up to 1.5 T using magnetic torque . Magnetic torque at 77K has also been measured on the largest crystals of the collection. The magnetic susceptibility ellipsoid and magnetic structure of the crystals have been studied with emphasis on the magnetic properties within the basal plane. Additional rock magnetic characterisation has been obtained from IRM acquisition curves, thermomagnetic curves and magnetic in order to constrain the composition of the samples. Results reveal that the magnetic susceptibility variation for hematite is out of the Rayleigh region for applied fields in commercial instruments. At high fields, the magnetic structure within the basal plane is a combination of biaxial and triaxial structures. The triaxial structure has different intensities probably due to different accumulations of stress within the crystal lattice as it is derived by torque measurements at low temperature.