Handbuch der Physik-Geophysik III/6: G.M. Von Nikol'skij,K. Rawer,P. Stubbe,L. Thomas and T. Yonezawa. Springer-Verlag, 1982, 124 Figs., VI + 385 pp., US $126.60, ISBN 3-540-07080-X

A model mechanism has been derived for the evolution of the cation distribution in titanomaghemites, which have formed by the progressive removal of iron from the spinel lattice. According to the model, the cation distribution of a titanomaghemite is not a unique function of composition but depends on the path by which maghemitization has taken place. Cation distributions (and consequent magnetic properties) have been calculated and compared for titanomaghemite produced by the “removal of iron” and “addition of oxygen” mechanisms. For a given composition, the Curie point temperature is almost invariant, being, according to molecular field theory, more sensitive to composition than cation distribution. Therefore, the model cannot explain the variations observed in synthetic titanomaghemite of different provenances. According to the model, the spontaneous magnetization is significantly lower for the removal of iron mechanism, which may be helpful in explaining a decrease in the remanence of submarine basalts with age.Structurally metastable titanomaghemites invert to an intergrowth of phases on heating above approximately 350°C and it is expected that the inversion product would be independent of the mechanism by which maghemitization had previously taken place. Available X-ray, and other data, have been used to produce a simple model of the inversion process, and values of the Curie point of the inversion product and the ratio of the magnetization before and after the inversion derived. These model values compare favourably with experimental values for a synthetic titanomaghemite in which inversion was induced by progressive slow heating to approximately 600°C. The inversion process therefore has potential in determining the compositions of naturally occurring titanomaghemites.