American Geophysical Union (AGU) Acta Medica Okayama 0094-8276 53 18 2026 Hydrogen-Bond Symmetrization in Hydrous Aluminous Silica Enhances Deep-Mantle Water Cycle e2026GL123823 EN Chaoshuai Zhao State Key Laboratory of Precision Geodesy, University of Science and Technology of China Zhu Mao State Key Laboratory of Precision Geodesy, University of Science and Technology of China Yingxin Yu Department of Earth Sciences, University of Oxford Ningyu Sun State Key Laboratory of Precision Geodesy, University of Science and Technology of China Yunhua Fu SKLab]DeepMinE, MOEKLab]OBCE, School of Earth and Space Sciences, Peking University Jianbo Zhang Center for High]Pressure Science and Technology Advance Research Hanns]Peter Liermann Deutsches Elektronen]Synchrotron DESY Renbiao Tao Center for High]Pressure Science and Technology Advance Research Takayuki Ishii Institute for Planetary Materials, Okayama University (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. No potential conflict of interest relevant to this article was reported.