Oxidation of magnetite to maghemite or hematite

In the dry state magnetite is readily oxidized to maghemite by air. Ultrafme crystals of magnetite change (over years) from black to the brown of maghemite even at room temperature (Murad Schwertmann, 1993). At temperatures 300°C, the transformation proceeds further to hematite (see section 14.2.7). 

No porosity develops, ho vever, and the sample surface area does not change (Sidhu et ah, 1977). 

A feature of this transformation is the influence of magnetite crystal size on the nature of the reaction products (Feitknecht, 1964 Gallagher et ak, 1968 Gillot et ak, 1978). At 200-250 °C, crystals smaller than 300 run transformed via the mixed phase to maghemite which in turn transformed to hematite at temperatures above 500 °C. 

Sidhu et ak (1981 a) compared the oxidation upon heating, of natural and synthetic magnetites. The coarse, natural magnetites were much more resistent to oxidation and higher temperatures or longer times were needed for it to take place. Hematite was the only oxidation product, which is in agreement with the results quoted above. 

Magnetite transforms to maghemite (and thence to hematite) in water or alkali under hydrothermal conditions. Conversion to maghemite also involves outward migration of cations via cation vacancies (Swaddle Oltmann, 1980). The hydrothermal transformation is slower than that in air at the same temperature (180 °C) and it has been suggested that this is because the cation vacancies which assist cation diffusion are reduced or eliminated by the large excess of water. 

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