Kinetics of the magnetite–maghemite–hematite transformation, with special reference to hydrothermal systems

Abstract

Maghemite, prepared in the usual way (but with exclusion of silica, e.g., from glassware) by precipitation of non-stoichiometric magnetite Fe3−xO4−x in aqueous MOH (M = Na, K) followed by air oxidation, picks up moisture from the air to reach the limiting composition MδH1−δFe5O8, where δ ~ 0.02–0.03 for fresh material, but changes under hydrothermal conditions because of ion exchange. Despite the role of absorbed moisture in stabilizing maghemite, formation of the latter from Fe3−x O4−x is markedly retarded, and its decomposition to α-Fe2O3 greatly accelerated, under hydrothermal conditions relative to the rates of the corresponding reactions of the dry solids. The rate of hydrothermal decomposition of maghemite is strongly retarded by silica. Over the range 160–187 °C at least, silica-free maghemite decomposes in water according to the empirical equation −ln (1 − α) = (kt)n, where α is the fractional extent of decomposition, and n ~ 2.5 for neutral water (with k = 1.3 × 10−5 s−1 at 160 °C and 3.1 × 10−5 s−1 at 175 °C) but approaches unity, without major effects on the overall time-scale of reaction, at high [MOH]. The mechanistic significance of these and previous results are considered; the hydrothermal conversion of maghemite to hematite evidently proceeds by a dissolution–reprecipitation sequence.