Oxidation of magnetite to maghemite

Equation 5 describes the oxidation of magnetite to form maghemite and equation 6 describes the reductive dissolution of magnetite (22). The decoupled half cell reactions describing ilmenite oxidation (equation 3) are... 

In the synthesis of y-Fe203, careful oxidation of magnetite in a spinel lattice to a superstructure of crystallizing ferrimagnetic y-Fe203 (maghemite) under well-defined conditions is necessary ... 

In a solid state reaction at low temperatures a solid reactant may be entirely converted to a solid product that keeps the original structure of the reactant. In such a thermal process new nonequilibrium solids can be made that cannot form when the system would be close to equilibrium. The temperature cannot be too low so that the reaction remains possible (sufficient mobility), and it should not be so high as to prevent crystallization to equilibrium structures. This procedure is called chimie douce or soft chemistry. An example of chimie douce from industrial chemistry is the oxidation of magnetite (a mixed valence form of iron oxide, FCjOJ to the ferromagnetic y-Fe203 (maghemite) in a microstructure that renders it suitable for... 

Litter et al. (1991) found that the dissolution of maghemite was also considerably speeded up, once a dissolved Fe "-oxalate or Fe" -EDTA complex was reduced to an Fe" complex by UV irradiation 1 = 254 nm). This system also showed an induction period which could be eliminated by addition of Fe " (see Fig. 12.28). In a study concerned with dissolution of corrosion oxide, electrons from viologen radicals produced by y-radiation ( Co) were used to dissolve hematite and goethite (Mulvaney et al., 1988) it was observed that the Fe " appearing in solution could only account for a fraction of the electrons consumed. The remainder was involved in conversion of the Fe " oxide into magnetite. 

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