Influence of crystallite size on the oxidation kinetics of magnetite

The oxidation of magnetite yields the lacunar phase γ-Fe₂O₃, for sizes less than 5000 Å and the rhombohedral phase, α-Fe₂O₃, for sizes above 10 000 Å. For intermediate sizes, oxidation kinetics and X-ray analysis have confirmed that the γ-Fe₂O₃ phase forms at the beginning of the reaction, followed by phase α-Fe₂O₃ forming from γ-Fe₂O₃ and then directly from the still-unoxidized magnetite. Influence of size could be accounted for in terms of structure and stresses at the crystal lattice level.