Equation of state and phase diagram of FeO

Wüstite, Fe1 − xO, is an important component in the mineralogy of Earth's lower mantle and may also be a component in the core. Therefore the high pressure, high temperature behavior of FeO, including its phase diagram and equation of state, is essential knowledge for understanding the properties and evolution of Earth's deep interior. We performed X-ray diffraction measurements using a laser-heated diamond anvil cell to achieve simultaneous high pressures and temperatures. Wüstite was mixed with iron metal, which served as our pressure standard, under the assumption that negligible oxygen dissolved into the iron. Our data show a positive slope for the subsolidus phase boundary between the B1 and B8 structures, indicating that the B1 phase is stable at the P–T conditions of the lower mantle and core. We have determined the thermal equation of state of B1 FeO to 156 GPa and 3100 K, finding an isothermal bulk modulus K0 = 149.4 ± 1.0 GPa and its pressure derivative K0′ = 3.60 ± 0.4. This implies that 7.7 ± 1.1 wt.% oxygen is required in the outer core to match the seismologically-determined density, under the simplifying assumption of a purely Fe–O outer core.

Research highlights
► Equation of state and phase diagram of FeO was measured to 156 GPa and 3100 K.
► FeO at conditions of the Earth's interior has the B1 (not B8) structure.
► For an Fe–Ni–O core, 7.9 ± 1.1 wt% O is required in the outer core to match PREM.