FEOOH POLYMORPHS IN THE DEEP EARTH: COMBINING THEORETICAL AND EXPERIMENTAL APPROACHES

Our understanding of the Earth’s interior hinges on knowing where and how hydrogen is stored in the deep Earth because hydrogen has an outsized influence on the chemical, material, and rheological properties of high-pressure phases. A flurry of recent studies has honed in on the importance of two high-pressure polymorphs of FeOOH (ε-FeOOH and pyrite-type FeOOH) as potential hosts of hydrogen in the Earth’s upper and lower mantles—a region that cumulatively represents roughly 84% of the Earth’s volume. Although both FeOOH polymorphs are minor mantle phases, their stability and ability to form partial or solid solutions with similarly structured Al- and Mg- rich phases mean these FeOOH polymorphs likely play an important role in the hydrogen cycle of the Earth’s interior. Goethite (ɑ-FeOOH), a component of rust and common hydrous phase at the Earth’s surface, is transported in subducting plates and transforms to ε-FeOOH at ~5 GPa. Once transformed, CaCl₂-structured ε-FeOOH is stable at the pressures of the upper mantle, transition zone, and the uppermost mantle. At these pressures, CaCl₂-structured ε-FeOOH forms a solid solution with isostructural δ-AlOOH and phase H (MgSiO₄H₂). At even higher pressures, both ε-FeOOH and δ-AlOOH transform into a pyrite-type crystal structure. This presentation will highlight recent computational and experimental efforts to evaluate the structural evolution and seismic properties (density, sound velocities) of ε-FeOOH and pyrite-type FeOOH as a function of pressure and show how by combining these efforts we can better constrain the distribution of these phases. By determining the elastic properties of intermediate compositions in the CaCl₂-structured FeOOH–AlOOH–MgSiO₄H₂ system and pyrite-type (Al,Fe)O₂H, we find that iron-bearing oxyhydroxides may contribute to the unusual seismic properties of both Large Low Shear Velocity Provinces (LLSVPs) and Ultra Low Shear Velocity Zones (ULVZs). Furthermore, we find that the CaCl₂-type → pyrite-type structural transition pressure is compositionally driven, forming a broad phase loop such that Fe-rich pyrite-type (Al,Fe)OOH may coexist with Al-dominant CaCl₂-type δ-(Al,Fe)OOH in the deep Earth.