Vibrational spectroscopy of dense oxyhydroxides: hydrogen bond disorder and symmetrization at high pressure

Earth’s lower mantle is often considered to be ‘dry’ on the basis of hydrogen storage capacities of nominally anhydrous minerals. Recent investigations of dense oxyhydroxides in the (δ-AlOOH)-(MgSiO₂(OH)₂)-(ε-FeOOH) system have revealed that these phases may carry hydrogen into the lowermost mantle and contribute to heterogeneity of seismic velocities. To retain hydrogen at the high temperatures of the lower mantle, it is likely that hydrogen bonds must become symmetric, and consequently strong, at high pressure. In this work, we use Raman and infrared spectroscopy to assess the hydrogen bonding environment of (Al,Fe)-phase H (δ-Al_0.84Fe³⁺ Mg_0.02Si_0.06OOH) and δ-Fe13 (δ-Al_0.87Fe_0.13OOH) at high pressure. We conclude that (Al,Fe)-phase H has occupationally disordered hydrogen atoms due to cation substitutions at ambient pressure. Hydrogen bonds in this phase become dynamically disordered at approximately 10 GPa and symmetric at 35 GPa. Our results are commensurate with previous observations of hydrogen-bond disorder in δ-(Al,Fe)OOH at 10 GPa.