Hidden Oceans? Unraveling the Structure of Hydrous Defects in the Earth's Deep Interior

High-pressure silicates making up the main proportion of the earth's interior can incorporate a significant amount of water in the form of OH defects. Generally, they are charge balanced by removing low-valent cations such as Mg²⁺. By combining high-resolution multidimensional single- and double-quantum ¹H solid-state NMR spectroscopy with density functional theory calculations, we show that, for ringwoodite (γ-Mg₂SiO₄), additionally, Si⁴⁺ vacancies are formed, even at a water content as low as 0.1 wt %. They are charge balanced by either four protons or one Mg²⁺ and two protons. Surprisingly, also a significant proportion of coupled Mg and Si vacancies are present. Furthermore, all defect types feature a pronounced orientational disorder of the OH groups, which results in a significant range of OH···O bond distributions. As such, we are able to present unique insight into the defect chemistry of ringwoodite's spinel structure, which not only accounts for a potentially large fraction of the earth's entire water budget, but will also control transport properties in the mantle. We expect that our results will even impact other hydrous spinel-type materials, helping to understand properties such as ion conduction and heterogeneous catalysis.