Experimental determination of the effects of pressure and temperature on the stoichiometry and phase relations of wüstite

When quenched metastable wüstite (Fe_.924O and Fe_.947O) is held at 300°C at pressures up to 200 kbar in a diamond anvil cell, a mixture of magnetite, metallic iron and wüstite is found. We interpret this to indicate that magnetite plus metallic iron constitute the stable phase assemblage at pressures and temperatures below this boundary is stoichiometric FeO ($\text{a}{}_0\text{= 4.332 ± 0.001}\text{A}\text{?}$) at pressures below 110 kbar at 300°C. However, just below the boundary in the pressure range 110 kbar to 200 kbar at 300°C, the residuál wüstite is non-stoichiometric ($\text{a}{}_0\text{< 4.332}\text{A}\text{?}$). Data collected at pressures and temperatures above the boundary indicate that non-stoichiometric wüstite (Fe_xO) plus metallic iron constitute the stable phase assemblage and that the value of x in Fe_xO increases as pressure is increased isothermally to 100 kbar and then decreases as pressure is increased above 100 kbar.