The transformation of γ Fe 2O 3 to α Fe 2O 3: thermal activation and the effect of elevated pressure

Abstract

The transformation of acicular γ Fe 2O 3 particles to α Fe 2O 3 has been monitored using magnetic properties as a proxy for γ Fe 2O 3 concentration during the inversion process. The transformation is thermally activated, the height of the barrier opposing inversion being 3.7 eV at atmospheric pressure and 0.5 eV at a pressure of about 100 MPa. The barrier arises from the combination of a term representing the reduction in lattice energy in an inverted region, and the strain energy associated with the interface between the inverted and non-inverted phases. The sensitivity of the inversion process to pressure can be understood in terms of the dependence of these energy terms, and the energy barrier, on interatomic spacing. Extrapolation of these laboratory data to the conditions of the submarine crust at Site 504B of the Deep Sea Drilling Project is consistent with the inferred magnetic mineralogy of the recovered material.