Abstract Manganese is an important minor element in the Earth’s mantle, and its metal–silicate partitioning behavior has previously been used to constrain the conditions of planetary core formation. Therefore it is important to understand the properties of manganese in lower mantle phases like (Mg,Mn)O, including its electronic spin state. We performed optical transmittance spectroscopy on a sample of Mg0.8Mn0.2O up to 84 GPa, finding a change in the spectra at ∼58 GPa, beyond which the sample gradually became opaque. We also performed synchrotron X-ray diffraction measurements on Mg0.8Mn0.2O up to 136 GPa. The compression data exhibit discontinuities at 58 GPa and 102 GPa, which we interpret as the onset and completion of a gradual spin crossover in Mn2+ from high spin to low spin, analogous to that of Fe2+ in (Mg,Fe)O. We fit equations of state to our pressure–volume data for both the high spin and low spin phases of Mg0.8Mn0.2O, finding a similar V0 but different K0 and K0′ for the two phases. Using 1 bar thermodynamic data and the equations of state of the endmembers, combined with constraints from our new data, we modeled the Gibbs free energy of a ternary solid solution of MgO, high spin MnO, and low spin MnO, allowing construction of pressure–temperature–composition phase diagrams across the Mn2+ spin transition in (Mg,Mn)O. These calculated phase diagrams suggest that this transition should occur more gradually (over a wider range of pressures) for lower XMnO and higher temperatures, such that all (Mg,Mn)O in Earth’s mantle may be mixed spin, with its low spin fraction increasing gradually but considerably with depth along a geotherm. If the Mn2+ spin state evolves similarly with pressure–temperature–composition in other phases, it could affect the partitioning behavior and other properties of manganese, which may be useful to consider when interpreting and especially extrapolating experimental data.
Fischer et al. (Wed,) studied this question.