Scientists long believed that Earth’s lower mantle was composed of Bridgmanite (Mg,Fe)SiO3 and magnesiowüstite (Mg,Fe)O, in which Fe2+ dwells. This view changed when experiments showed that Fe2+ simply can’t exist at the pressure and temperature of the lower mantle. What is present is Fe3+. The two phases (Mg,Fe)SiO3 and (Mg,Fe)O both shed Fe2+ and, in turn, MgSiO3 and MgO remain. However, what mineral hosts Fe3+ had remained unknown.