Hydrogen-Bond Symmetrization in Hydrous Aluminous Silica Enhances Deep-Mantle Water Cycle

(Al, H)-bearing CaCl2-type SiO2 serves as a key water carrier in Earth's deep-water cycle. Evolution of hydrogen-bond symmetrization in this phase was measured up to 62 GPa and 800 K by synchrotron infrared and Raman spectroscopy, and X-ray diffraction. It undergoes symmetrization at ∼30 GPa and 300 K, while elevated temperature to 800 K lowers the onset pressure to ∼12 GPa. This implies that the symmetrized phase will form directly near the base of the transition zone in (Al, H)-rich subducted slabs, and deeper in (Al, H)-poor slabs via transformation from stishovite. More importantly, this symmetrization state stiffens the crystal lattice and immobilizes hydrogen within the structure, enhancing water retention and preventing hydrogen loss during subduction to the lower mantle. Our findings suggest that this phase acts as an important deep-water reservoir capable of transporting water to the lower mantle, thereby linking the deep-water cycle from Earth's surface to core-mantle boundary.