Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle

Abstract

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.