A complete mechanism for the transformation goethite to hematite based upon the results of thermogravimetric, transmission electron microscope and X-ray diffraction investigations is presented. A porous microstructure and hematite crystallites in twin orientation are found to develop during transformation. For the main part of the transformation, and at higher temperatures, the reaction is controlled by a two-dimensional phase boundary. Activation energies of 169 +_ 8 k J/mole (for an ore mineral) and 154_+ 15 kJ/mole (for a recent sedimentary goethite) were obtained for this part of the transformation. At early stages and lower temperatures, the mechanism is one of proton/iron transfer across the reaction interface. Important goethite characteristics are grain size, shape, crystallinity and excess water content. The activation energy is found to depend upon temperature and degree of dehydration.

Abstract. This study has characterised the oxidation products of a fine-grained single domain magnetite which was made synthetically by a colloidal method. Changes in the intrinsic magnetic properties (saturation magnetisation, saturation remanence, and coercive force) during progressive oxidation are correlated with lattice parameter changes and an oxidation mechanism. It is proposed that magnetite oxidises to hematite via at least two metastable maghemites. The first of these, formed on low temperature oxidation by the formation of a magnetite/maghemite solid solution, is a face centered maghemite with lattice parameter a= 8.3419 +0.0006 A.. A second maghemite, produced on oxidation at higher temperatures, has a primitive cubic structure and a lattice parameter a= 8.3505 + 0.0005 A.. Maghemite cation distributions are derived to explain the reduced saturation magnetisations of between 56 and 74 Am 2 kg-1 observed, and a maghemite structure containing an increase in tetrahedral Fe a + ions and up to 3 octahedral vacancies per 32 oxygen unit cell is proposed.

Self-reversed chemical remanent magnetization (CRM) has been observed in haematite formed on heating maghemite which has been produced by the dehydration of acicular crystals of synthetic lepidocrocite, 1 to 2 p m in length. Our experimental evidence suggests that self reversal of haematite remanence only occurs when the parent maghemite is still blocked at the temperature of its transformation to haematite; when the transformation temperature is above the blocking temperature of the parent maghemite and it is unblocked, the resulting haematite remanence is normally magnetized. It is suggested that the strong dependence on remanent state supports exchange control of the self-reversal process. We propose that the self reversal is probably a general feature of the maghemite to haematite transition, and the significance of the source lepidocrocite in our experiments is that it produces maghemite of a suitable grain size so that much of it remains blocked at the elevated temperatures required to make the transformation to haematite occur over the short time scale of the laboratory experiments. In nature, transformation of maghemite to haematite will occur at much lower temperatures due to the much longer time scales involved, and self reversal of the resulting CRM may occur over a much less restricted grain size range of parent maghemite.