Conductive Mg sub-silicate mantles dictate the dynamo of super-Earths

While bridgmanite and the post-perovskite MgSiO ₃ dominate Earth-sized terrestrial mantles, the extreme pressures within larger terrestrial exoplanets may disrupt the archetypal ionic balance of rock-forming elements, forming non-stoichiometric magnesium silicates. Here, we predict the stabilization of magnesium (Mg) sub-silicates such as Mg ₂ SiO ₃ , Mg ₃ SiO ₄ and Mg ₄ SiO ₃ in exoplanetary interiors. These phases are thermodynamically stable as low as 100 GPa, and become gravitationally favorable at multi-megabar pressures, corresponding to the mantle depth of silicate super-Earths. We further reveal that Si atoms donate electrons to localized sites, reducing valence states to 2 + and even anionic 1- and substantially narrowing band gaps. Mg sub-silicates start to form layer in exoplanet with > 2 Earth’s mass, and potentially become the major constituents in the basal mantle of larger super Earth. Their high electrical conductivity creates an electrically conductive layer that suppresses core-mantle electromagnetic coupling and strongly attenuates the dynamo-generated magnetic field, including the massive super-Earth Kepler-725c.