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

The pyrite-type FeO₂and 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 FeO₂and 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.