Magnetic Interaction between Ferromagnetic Materials Contained in Rocks

Abstract

The detailed mechanism of the self-reversal of thermo-remanent magnetism of rocks was studied. The ferromagnetic mineral grains responsible for this phenomenon were determined as A′B-grains, which consist of two constituents, A′ and B. The magnetic and crystallographic examinations of Haruna rocks showed that the constituent with higher Curie point, in the two-constituent model, should be estimated to be a solid solution between ilmenite and hematite, and not the Ti-poor titanomagnetite as was estimated to be in the previous studies. In other words, the magnetic interaction causing the self-reversal of thermo-remanent magnetism is between two ilmenite-hematite solid solutions with different Curie points. Electron-microscopic observations showed that the constituent with higher Curie point is intergrown into the constituent with lower Curie point as fine lamellae. The observed configurations of these two constituents indicated that the phenomenon of reverse thermo-remanent magnetism could be explained theoretically, provided that the grains as fine as the observed lamellae have very strong thermo-remanence.Similar examinations were conducted on ferromagnetic minerals of several other rocks with the same mineral assemblage as the Haruna ferromagnetic minerals. The results of these examinations indicated that there exist, in natural ferromagnetic minerals, various degrees of magnetic interaction which are governed by their state of co-existence.