Constraining the phase relations and valence state of iron-bearing oxides is crucial to understanding the chemistry of Earth's mantle. In this issue of American Mineralogist, Uenver-Thiele et al. (2017a) present an experimental study on the phase diagrams of magnesioferrite-magnetite solid solutions at high pressures and high temperatures. By analyzing the compositions of the quenched products, Uenver-Thiele et al. were able to constrain the phase diagram of the (Mg,Fe2+)Fe23+O4 series, and they identified several new phases with non-conventional stoichiometry. From the phase diagrams of (Mg,Fe2+)Fe23+O4 spinels determined in...

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