Mechanism and kinetics of hydrothermal replacement of magnetite by hematite

The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions (140-220 ℃, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O₂(aq) environments, some of the replacement still occurred via Fe²⁺ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe²⁺ (and the oxidant) to and from the reaction front. Reaction rates are fast at ∼200 ℃, and in nature transport properties of Fe and, in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ±6 kJ mol^-1. This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions.