The mechanism of oxidation in titanomagnetites: a magnetic study

Summary. A hypothesis for the mechanism of oxidation in titanomagnetites is first put forward on the basis of new oxidation data for pure magnetite and the new model for cation distribution i titanomagnetites. Contrary to previous claims by other workers, it is shown that single-phase pinel oxidation products of titano-magnetites can only be produced under very exacting circumstances. Low-temperature magnetic measurements on such synthetic, oxidized materials were made to show the formation of magnetite by the decomposition ofthe metastable oxidation products of titanomagnetites on prolonged heating at moderate ( ~ 390 ~ C) temperatures. This magnetic method of detection is more sensitive than that of normal X-ray diffraction. It is concluded that previous claims of an increase in magnetization due to the formation of synthetic single-phase, highly oxidized titanomagnetites must be due to the formation of undetected magnetite. LTHOUGH very little work has been done on the stabil ity of titanomagnetites, F%_xTixO 4 (0 < x < 1), to oxidation, and on the possible oxidation mechanisms, a great deal has been done on the oxidation of magnetite, F%04, particularly to maghemite, ~-F%0~, because of its commercial interest. Colombo, Fagherazzi, Gazzarini, Lanzavecchia, and Sironi (1964) studied mechanisms for the first stage of oxidation of magnetite using natural and synthetic samples made both by a high-temperature technique and by precipitation from a solu-tion. The fine-grained precipitated material produces cubic ~-F%03, but the natural samples and those synthesized at high temperature, both of which have small specific surface and low degree of crystalline imperfection, were found to produce mainly rhombohedral haematite, a-F%03, as a surface layer, and a solid solution of FeaO 4 and ~-F%0 a in the interior of the grains. The effect of hydration is also important, and water may be incorporated in the material in the course of precipita-tion or be absorbed ue to the large specific surface area of fine grain powders. Elder (1965) ground natural magnetite to a submicron grain size both in water and in dry acetone. Both samples oxidized quickl