The High-Pressure Study on the Fe - O System: Thermodynamics and Phase Transitions of Iron Ferrite (FeFe2O4)

Abstract: Knowledge about the stability of phases and their relationships in the Fe-O system at high pressures and temperatures is essential in implications of the multifarious states of iron oxides for models of the evolution of the Earth. In this respect, the iron ferrite magnetite (FeFe2O4) plays a significant role since it participates in the control of geochemistry of ferric iron, and hence oxygen fugacity in the Earth`s deep interior.High-pressure experiments on Fe3O4 were performed using the diamond anvil cell technique combined with the laser and electrical resistive heating. The approach based on the combination of the synchrotron x-ray diffraction with Raman spectroscopic measurements benefited from the complementarity of the two methods originating from the different sensitivity to a range of structural order. High-pressure transformation of magnetite to a dense polymorph of the CaTi2O4-type structure proceeds via an intermediate step of the decomposition into a mixture of oxides on a microscopic scale. The kinetic hindrance of the reaction of the decomposition effectively prevents a phase separation controlled by diffusion and restricts the formation of the daughter products to locally ordered structures in the strained lattice of magnetite.Thermodynamic analysis of the observed phase transformations along with published results on the elastic properties and pressure-induced transitions of iron oxides has led to the reassessment of the phase diagram of Fe3O4. The pressure - temperature field of its stability with respect to the breakdown to a mixture of oxides FeO and Fe2O3, and to the transition to a high-pressure form, has been accordingly modified.