Magnetite transformation into maghemite

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

This transformation is characteristic of the small size of the particles and observed to a much lower extent in micrometer-size particles. It occurs in two consecutive steps. The first step, elimination of surface ferrous ions (i.e. formation of cation vacancies near the surface), is almost immediate. The. second Step is slower and must be limited by the diffusion coefficient of ferric ions in the lattice. Although diffusion takes place between nearest neighbors by a cooperative process, it is a relatively slow phenomenon. In small particles of high surface/volume ratio, the net diffusion involves the addition of rather short distances. The conversion of magnetite into maghemite is therefore compatible with reasonable observation time-frames (a few days at room temperature), without a measurable reduction in particle size. 

Monodispersed spindle-type maghemite can be produced by first transforming spindle-type hematite (see chap. 10) into magnetite (by heating the hematite in a H2 gas stream at 340-400 °C for 1-3 h) and then oxidizing the magnetite to maghemite with air at 240 °C for 1-2 h (Ozaki and Matijevic, 1985). 

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