Gagarinite-type Fe_2.5(OH)₆ under the deep lower mantle conditions

Earth's lower mantle is dominated by (Mg,Fe)SiO₃ bridgmanite. It was reported that (Mg,Fe)SiO₃ decomposes into an Fe-depleted bridgmanite phase and an iron-rich phase with a hexagonal structure (H-phase) under high pressure-temperature conditions of the deep lower mantle at depth >2,000 km. The nature of the decomposition reaction remains elusive due to the lack of information on the crystal chemistry of the H-phase. Using the multigrain method for high-pressure structure determination, here we reported in situ structure determination of the H-phase at 117 GPa and after temperature quench from 2,500 K. The structure analysis was performed by scaling and merging the single-crystal datasets of three selected grains. The crystal structure has been solved in space group P6₃/m, with a = 5.0708(2) Å and c = 2.8214(1) Å at 117 GP and 298 K. We obtained Fe_2.538O₆ from the structure refinement and estimated the hydrogen content based on the volume expansion, suggesting a chemical formula Fe_2.5(OH)₆ for the H-phase. The H-phase Fe_2.5(OH)₆ resembles the crystal structure of the gagarinite-type minerals. To our knowledge, the gagarinite-type Fe_2.5(OH)₆ is the most water-rich phase reported so far under the deep lower mantle conditions. We would expect that the gagarinite-type Fe_2.5(OH)₆ is a potential water carrier in the deep lower mantle. Further, Fe-depletion in bridgmanite and formation of Fe_2.5(OH)₆ may contribute to chemical heterogeneities in the bottom 1,000 km of the mantle and explain some of the complex seismic anomalies.