Oxidation kinetics of fayalite and growth of hematite whiskers

Abstract

The oxidation of fayalite to hematite and silica by oxygen and oxygen-nitrogen or oxygen-water vapour mixtures has been studied by TGA, SEM and X-ray diffraction. In the temperature range 690 to 950° C, the isotherms of oxidation are pseudo-parabolic. The Arrhenius plot shows a break near 840° C, related to the quartz-cristobalite transformation, the activation energy being about 230 kJ mol⁻¹. The first stage of oxidation leads to the formation of a covering layer constituted of silica plus iron oxide (mainly hematite). Solid state diffusion of oxygen then takes place through this layer, which progressively evolves with the crystallization of silica and a relaxation of stress due to fracture. After the fracture of the covering layer, whiskers of hematite grow, usually on a silica substrate, and align themselves along linear defects. Under specific conditions, their growth is periodic.