Phase transitions in ε-FeOOH at high pressure and ambient temperature

Abstract:

Constraining the accommodation, distribution, and circulation of hydrogen in the Earth’s interior is vital to our broader understanding of the deep Earth due to the significant influence of hydrogen on the material and rheological properties of minerals. Tomographic evidence supports the idea that subducting lithospheric plates pierce the transition zone, potentially ushering water into the Earth's lower mantle¹ and hydrous inclusions in ultradeep diamonds indicate that the Earth's transition zone and lower mantle, at least locally, host significant quantities of hydrogen^2,3,4. Recently, a great deal of attention has been paid to the high-pressure polymorphs of FeOOH (space groups P2₁nm and Pnnm). These phases could form a hydrogen-bearing solid solution with AlOOH and phase H (MgSiO₄H₂) that may transport water (OH^-) deep into the Earth’s lower mantle. Additionally, the pyrite-type polymorph of FeOOH and its potential dehydration have been linked to phenomena as diverse as the introduction of hydrogen into the outer core⁵, the formation of Ultra Low Velocity Zones⁶, and the Great Oxidation Event⁷. In this study, the high-pressure evolution of FeOOH was re-evaluated up to ~75 GPa using a combination of synchrotron-based X-ray diffraction, Fourier transform infrared spectroscopy, and optical absorption spectroscopy. Based on these measurements, we report three principal findings: (1) pressure-induced changes in hydrogen bonding (proton disordering or hydrogen bond symmetrization) occur at substantially lower pressures in ε-FeOOH than previously reported and are unlikely to be linked to the high-spin to low-spin transition, (2) ε-FeOOH undergoes a 10% volume collapse coincident with an isostructural Pnnm → Pnnm transition at approximately 45 GPa, and (3) a pressure-induced band gap reduction is observed in FeOOH at pressures consistent with the previously reported spin transition (40 to 50 GPa).

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