Contribution of fine exsolution lamellae of hematite-ilmenite to magnetic properties

Hematite-ilmenite minerals play an important role in acquisition of rock magnetism. Samples of igneous rock with hematite exsolution in ilmenite, visible by optical microscopy, have indicated high and stable NRM, although the acquisition mechanism of magnetization is still unknown (McEnroe et al. 2001a). McEnroe et al. (2001b) have identified fine scale exsolution microstructures of hematite and ilmenite in metamorphic rocks with high natural remanent magnetization (NRM) using energy-filtered TEM, and suggested that the microstructure was related to the magnetic properties. In this study samples from the Sokndal Region, Norway, provided by S. McEnroe, have been examined by SEM, TEM with EDS, high-resolution TEM (HRTEM), magnetic force microscopy, and measurements made of the temperature dependence of the magnetic susceptibility to determine the interaction between exsolution microstructure and magnetism. SEM observations indicated various scales of exsolution of hematite within ilmenite. TEM observations showed that within both the ilmenite and hematite, there are abundant fine lamellae (~ in the order of a few tens on nm in length) of hematite and ilmenite respectively. TEM-EDS analyses revealed that the chemical compositions of the hematite and ilmenite were Ilm14-19Hem81-86 and Ilm99Hem1. Our EDS results are similar to those reported by McEnroe et al. (2001b) who suggested that these phases alone cannot account for the magnitude of the NRM. The strong strain contrast around the lamellae and observation of the interface between hematite and ilmenite by HRTEM revealed that the lamellae were parallel to (001) planes and the coherency strain is partially relieved by formation of the interface dislocations. Harrison and Becker (2001) calculated that the ferrimagnetic moment of the intergrowth could be due to the arrangement of cations and spins at the interface between hematite and ilmenite. We suggest that the semicoherent interfaces of the many fine lamellae are related to acquisition of magnetization.