Deep mantle hydrogen in the pyrite-type FeO₂-FeO₂H system

The pyrite-type FeO₂ and FeO₂H were synthesized at the pressure-temperature conditions relevant to Earth's deep lower mantle. Through the water-iron reaction, the pyrite-phase is a good candidate to explain the chemical heterogeneities and seismological anomalies at the bottom of the mantle. The solid solution of pyrite-type FeO₂ and FeO₂H, namely the FeO₂Hx (0 ≤ x ≤ 1), is particularly interesting and introduces puzzling chemical states for both the O and H atoms in the deep mantle. While the role of H in the FeO₂-FeO₂H system has been primarily investigated, discrepancies remain. In this work, we summarize recent progress on the pyrite-phase, including FeO₂, FeO₂H, and FeO₂Hx, which is critical for understanding the water cycling, redox equilibria, and compositional heterogenicities in the deep lower mantle.