Water (OH-) within the Earth’s interior is known to influences a wide range of processes inside the Earth including melting temperatures (e.g., Inoue 1994), rheology (Karato et al., 1986), electrical conductivity (Wang et al., 1991), atomic diffusivity (Karato, 1990) and trace element partitioning during partial melting (e.g., Tiepolo et al. 2000). Yet there remains a substantial gap in our understanding of which mantle phases host water (OH-) and what the geophysical properties of these phases are. Refining our understanding of which phases host water is a critical step in determining the hydrogen carrying capacity of the Earth’s mantle, while determining the geophysical properties of these phases (e.g. density and sound velocities) provides important parameters that allow geophysicists and geodynamicists to constrain the distribution of these phases in the Earth.,This thesis traces a potential path of hydrogen transport through the Earth’s lower mantle to the outer core. Specifically, I explore the stability and properties of hydrous lower mantle phases including Al-bearing phase D [(Mg,Al)(Si,Al)2O4(OH)2], ε-FeOOH, and pyrite type (Al,Fe)O2H, as well as potential core constituent phase fcc FeHX. While stable at the pressure-temperature (P-T) conditions of a subducting slab in the Earth’s uppermost lower mantle (i.e., approximately 660-1200 km depth), at more extreme P-T conditions phase D transforms into phase H [MgSiO2(OH)2], which forms a solid solution with ε-FeOOH and δ-AlOOH (Nishi et al., 2014). At even higher pressures (>60 and >190, respectively), ε-FeOOH and δ-AlOOH transform to a pyrite structure, with intermediate solid solution compositions [(Al,Fe)O2H] potentially stable at core-mantle boundary conditions (Nishi et al., 2017; Tsuchiya and Tsuchiya, 2011). The high-pressure, high-temperature reaction of hydrous phases and iron have been shown to result in the formation of iron hydrides (Yagi and Hishinuma, 1995), a means by which hydrogen may be incorporated into the Earth’s outer core. This thesis concludes by investigating the geophysical properties of fcc FeHX, concluding the subduction path of our hypothetical parcel of hydrogen-bearing,material. In addition to probing the properties of idealized endmember compositions, this x,research aims to probe the influence of solid-solutions on the stability and properties of the hydrous deep Earth phases, allowing a more realistic assessment of hydrogen accommodation in the Earth’s interior.