We have heated magnesiowüstites (Mg0.5Fe0.5)O and (Mg0.8Fe0.2)O to temperatures of over 1000 K at pressures of over 80 GPa simulating the stability of the solid solution at physical conditions relevant to the Earth's lower mantle. The X-ray study of the externally heated samples in diamond-anvil cells shows that magnesiowüstite may dissociate into a magnesium-rich and an iron-rich oxide component. The result is important because the decomposition of magnesiowüstite into lighter and heavier phases will cause dynamic effects that could lead to mantle heterogeneity.

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