HIGH-PRESSURE PHASE OF MAGNETITE

Mossbauer spectroscopy and x-ray diffraction were employed to investig ate the magnetic and structural properties of the high-pressure monocl inic phase of magnetite. Measurements were performed to 66 GPa at 300 K using diamond anvil cells. Based primarily upon the Mossbauer parame ters, the following features of the high-pressure phase were deduced. With increasing pressure the high-pressure phase evolves at P greater- than-or-equal-to 25 GPa and its abundance increases monotonically at t he expense of the low-pressure cubic phase. The high-pressure phase is not magnetic at 300 K and its monoclinic structural features resemble that of the low-pressure phase. The tetrahedral and octahedral sites characteristic of the inverse spinel structure, albeit distorted, rema in the building blocks of the high-pressure phase. The fast electron h opping between Fe3+ and Fe2+ at the octahedral sites prevails to the h ighest pressure. It is suggested that the cubic --> monoclinic --> cub ic hysteretic cycle involves a mild displacive phase transition not af fecting the coordination number of any of the iron cations.