Elasticity of ε-FeOOH: Seismic implications for Earth's lower mantle

We have calculated the structure and elasticity of low-spin ferromagnetic epsilon-FeOOH to 140 GPa using density functional theory calculations with a Coulombic self-interaction term (U). Using these data, the elastic moduli and sound velocities of epsilon-FeOOH were calculated across the pressure stability of the hydrogen bond symmetrized structure (30 to 140 GPa). The obtained values were compared with previously published values for phase H (MgSiH2O4) and delta-AlOOH, which likely form a solid solution with e-FeOOH. In contrast to these Mg and Al end-members, epsilon-FeOOH has smaller diagonal and larger off-diagonal elastic constants, leading to an eventual negative pressure dependence of its shear wave velocity. Because of this behavior, iron-enriched solid solutions from this system have smaller shear wave velocities than surrounding mantle and therefore are a plausible contributor to large low-shear velocity provinces (LLSVPs) which exhibit similar seismic properties. Additionally, epsilon-FeOOH has substantial shear wave polarization anisotropy. Consequently, if iron-rich solid solutions from the FeOOH-AlOOH-MgSiH2O4 system at the core-mantle boundary exhibit significant lattice-preferred orientation due to the strong shear stresses which occur there, it may help explain the seismically observed S-H > S-V anisotropy in this region.