Chemical Remanent Magnetization of Ferromagnetic Minerals and Its Application to Rock Magnetism

Experimental studies have been carried out on the remanent magnetization generated by chemical reactions (which may properly be called chemical remanent magnetization or C. R. M. in abbreviation) for both natural and synthetic specimens. Remanent magnetization generated during two kinds of reaction processes, that is, reduction of α-Fe2O3 to Fe3O4 and oxidation of Fe3O4 to maghemite is firstly examined (Part I). It may be safely concluded that the remanent magnetization thus generated has an intensity which is intermediate between isothermal remanent magnetization and thermo-remanent magnetization, and that its magnetic and thermal stability is similar to that of thermo-remanent magnetization, much higher than that of isothermal remanent magnetization.In Part II, magnetic properties of several natural rocks and ore deposits containing maghemite are systematically examined. These natural specimens can be classified into two groups with respect to the magnitude of Qn-ratio; the one has large magnitude of remanence and large Qn-value amounting to 100, while Qn of the other is quite small. Greatest parts of natural remanent magnetization of the specimens having large Qn-value are attributable to the chemical remanent magnetization of maghemite, which is considered to result from oxidation of magnetite probably by weathering at nearly atmospheric temperature. On the other hand, the natural remanent magnetization of the specimens belonging to the second group with low Qn are found to be merely isothermal remanent magnetization. These two groups are also compared with each other from the petrological point of view.The results of these investigations suggest that various magnitude of remanence can be generated by chemical processes possibly according to the mode of precipitation of ferromagnetic minerals.