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

Area of Science:

• Mineral physics
• Geochemistry
• High-pressure experimental science

Background:

• Earth's lower mantle is primarily composed of (Mg,Fe)SiO3 bridgmanite.
• Deep lower mantle conditions (>2,000 km) cause bridgmanite to decompose into an Fe-depleted bridgmanite and an iron-rich hexagonal phase (H-phase).
• The crystal chemistry and precise composition of the H-phase have remained largely unknown, hindering understanding of deep mantle processes.

Purpose of the Study:

• To determine the in situ crystal structure and chemical composition of the H-phase under deep lower mantle conditions.
• To elucidate the nature of the bridgmanite decomposition reaction.
• To assess the potential role of the H-phase as a water carrier in the deep Earth.

Main Methods:

• Utilized the multigrain method for high-pressure structure determination.
• Performed in situ structure determination at 117 GPa and after temperature quench from 2,500 K.
• Analyzed crystal structure by scaling and merging single-crystal datasets from three grains, solved in space group P63/m.

Main Results:

• Determined the crystal structure of the H-phase at 117 GPa, space group P63/m, with lattice parameters a = 5.0708(2) Å and c = 2.8214(1) Å.
• Structure refinement yielded a composition of Fe2.538O6, with estimated hydrogen content suggesting a formula of Fe2.5(OH)6.
• The H-phase Fe2.5(OH)6 exhibits a gagarinite-type structure, representing the most water-rich phase known under deep lower mantle conditions.

Conclusions:

• The identified gagarinite-type Fe2.5(OH)6 is the most water-rich phase discovered to date under deep lower mantle conditions.
• This phase is a potential significant water carrier in the deep lower mantle.
• The formation of Fe2.5(OH)6 and associated Fe-depletion in bridgmanite likely contribute to deep mantle chemical heterogeneities and complex seismic anomalies.