Stable iron hydrosilicates and their influence on the planetary evolution

Abstract

Wüstite (FeO), silica (SiO2) and water (H2O) are highly abundant in the interior of planets. Given their extensive participation in most geochemical reactions within the mantle of Earth and other planets, it is important to investigate the high-pressure compounds of the ternary FeO-SiO2-H2O system. Here, taking the advantage of machine learning assisted crystal structure predictions based on DFT+U calculations, we identified two thermodynamically stable iron hydrosilicate phases, the α-FeSiO4H2 and β-FeSiO4H2 at high pressures of 50-150 GPa and above 150 GPa, respectively. Both these structures exhibit superionic behavior at pressure-temperature conditions corresponding to the Earth’s interior. Most importantly, as a reservoir of water, the FeSiO4H2 can enter the lower mantle and release water through the disproportionation of ferrous iron, which occurs during the accretion process. Our findings provide a new perspective on the behavior of the Earth’s interior, and exposes more possibilities for the evolution models of terrestrial planets in our solar system and beyond.