An anomaly in the oxidation rate of magnetite at its Curie temperature

The rate of oxidation of Fe3O4 to Fe2O3 was measured in oxygen over the range of 500–720°C by following isothermal weight changes. Rate constants were evaluated at 22–28% conversion using numerous kinetic rate laws. Arrhenius plots of the rate data exhibit a significant perturbation in the region of the Curie temperature, Tc. This perturbation is relatively independent of the particular mechanistic model or rate law used. There are no significant changes in the Arrhenius parameters (activation energy, 1.2 eV) well above and below Tc. The specific heat and thermal expansion of Fe3O4 were also measured in the vicinity of Tc. Implications of the irregularity in the rate are discussed.