Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth

The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements. Previous studies usually focused on the current range of pressure-temperature conditions in the Earth's mantle and ignored a possible difference in the past, such as the stage of the core-mantle separation. Here, using ab initio evolutionary structure prediction, we find that only two magnesium hydrosilicate phases are stable at megabar pressures, α -Mg₂SiO₅H₂ and β -Mg₂SiO₅H₂ , stable at 262-338 GPa and >3 3 8 GPa , respectively (all these pressures now lie within the Earth's iron core). Both are superionic conductors with quasi-one-dimensional proton diffusion at relevant conditions. In the first 30 million years of Earth's history, before the Earth's core was formed, these must have existed in the Earth, hosting much of Earth's water. As dense iron alloys segregated to form the Earth's core, Mg₂SiO₅H₂ phases decomposed and released water. Thus, now-extinct Mg₂SiO₅H₂ phases have likely contributed in a major way to the evolution of our planet.