Abstract
Summary The oxidation of finely divided (≲ 1000 A) synthetic ferromagnesian olivines (FexMg1-x)2SiO4 has been studied using gravimetric and magnetic techniques. It is found that olivines oxidize in air at temperatures appreciably lower than formerly thought possible. Magnetite, Fe3O4, is produced at temperatures ≲ 300°C. At higher temperatures this oxidizes to haematite, α– Fe2O3. Above 550°C a second spinel phase, magnesium ferrite, MgFe2O4, is produced together with further quantities of haematite. The kinetics of the magnetite production have been studied gravimetrically and magnetically in the temperature range 300°–420°C yielding values for the activation energy (1.6–1.9 eV mole–1) typical of binding energies of cations in oxides. The development of CRM at 345°C has been observed in the magnetite, showing the characteristic transitions from superparamagnetism to the stable monodomain and to the multidomain state.
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