Topotaxial reactions during oxidation of ilmenite single crystal

The mechanism of ilmenite-rutile transformation during oxidation of natural ilmenite crystal was studied at elevated temperatures in air. The progress of oxidation with annealing time was studied in the temperature range between 600 and 900 degrees C. 2.5 mm cubes were cut from the single Mn-ilmenite crystal in two special orientations, [001](ILM) and , that allowed determination of crystallographic relations among the reaction products. Using X-ray diffractometry, energy-dispersive spectroscopy, and electron microscopy (SEM, TEM) techniques, we determined that the ilmenite to rutile and hematite transformation is triggered by surface oxidation of divalent cations (Fe, Mn) from the starting ilmenite and their crystallization in the form of hematite and bixbyite on the surface of the single crystal. Surface oxidation and out-diffusion of Fe2+ and Mn2+ ions opens paths for exsolution of rutile within the pseudo-hexagonal oxygen sublattice of the parent ilmenite, following simple topotaxial orientation relationship . With this transformation, new channels for fast out-diffusion of divalent cations to the oxidation surface are opened along the c-axis of the rutile structure. The volume difference of the reaction products causes cracking of the single crystal, which opens additional free surfaces for accelerated oxidation. The results of this study contribute to better understanding of the recrystallization processes during pre-oxidation of ilmenite.