Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle

Recent experiments have demonstrated the existence of previously unknown iron oxides at high pressure and temperature including newly discovered pyrite-type FeO₂ and FeO₂H_x phases stable at deep terrestrial lower mantle pressures and temperatures. In the present study, we probed the iron oxidation state in high-pressure transformation products of Fe³⁺OOH goethite by in situ X-ray absorption spectroscopy in laser-heated diamond-anvil cell. At pressures and temperatures of ~91 GPa and 1,500-2,350 K, respectively, that is, in the previously reported stability field of FeO₂H_x, a measured shift of -3.3 ± 0.1 eV of the Fe K-edge demonstrates that iron has turned from Fe³⁺ to Fe²⁺. We interpret this reductive valence change of iron by a concomitant oxidation of oxygen atoms from O^2- to O^-, in agreement with previous suggestions based on the structures of pyrite-type FeO₂ and FeO₂H_x phases. Such peculiar chemistry could drastically change our view of crystal chemistry in deep planetary interiors.