High-pressure study of stability of magnetite by thermodynamic analysis and synchrotron X-ray diffraction

[1] Thermodynamic analysis and synchrotron X-ray diffraction experiments aimed at the investigation of stability of magnetite were carried out in the pressure-temperature ranges 0-36 GPa and 300-800 K, respectively. Thermodynamic assessment shows that at 298 K, the equilibrium pressure for the breakdown of magnetite to hematite and wustite is 13.3 GPa, while pressure for the backward synthesis FeO + Fe2O3 double left right arrow h-Fe3O4 is between 35 and 47 GPa, depending on the choice of equation of state. The stability field for the mixture of oxides Fe2O3 + FeO narrows with temperature increase, reaching a maximum temperature of only similar to850 K at pressure 14.6 GPa. The calculations predict that the high-pressure phase of magnetite (h-Fe3O4) becomes unstable with respect to h-Fe2O3 + FeO at pressures higher than 50 GPa. The 298 K pressure-volume isotherm of magnetite derived from the compression experiment is given by K-T = 217(7) GPa for the fixed K'(0) = 4. The 1sigma confidence ellipsoid shows large and negative correlations for the fit parameters K-T, K'(0), and V-0. The thermodynamically predicted breakdown of magnetite to hematite and wustite was not observed, but the trace amounts of hematite detected in the sample assemblage may signify a presence of nucleation centers of breakdown products, the growth of which is kinetically hindered because of the energetic requirements for the reaction. The transformation of magnetite to a dense polymorph evolves gradually above 19 GPa. Rietveld refinement of the diffraction pattern of h-Fe3O4 is consistent with the CaTi2O4-type structure. The values of Gibbs formation energies at standard conditions for h-Fe3O4 and h-Fe2O3 were estimated to be -962 and -610 kJ mol(-1), respectively. The standard state entropy of h-Fe3O4 is 172.4 J K-1 mol(-1).