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 FeO(2) and FeO(2)H(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 Fe(3+)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 FeO(2)H(x), a measured shift of −3.3 ± 0.1 eV of the Fe K‐edge demonstrates that iron has turned from Fe(3+) to Fe(2+). 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 FeO(2) and FeO(2)H(x) phases. Such peculiar chemistry could drastically change our view of crystal chemistry in deep planetary interiors.