A new hydrous silicate, a water reservoir, in the upper part of the lower mantle

We synthesized a new hydrous silicate in the pressure range from 20 GPa to 24 GPa at 800–1300°C. This phase, named tentatively as phase G, has a hexagonal unit cell, a=4.790 (3) Å and c=4.344 (3) Å, and V=86.3 (2) ų and the atomic ratio Mg/Si=0.66±0.03. SIMS analysis revealed that it contains 14.5±2.0wt% water. Phase G has a chemical formula of Mg 1.14 Si 1.73 H 2.81 O 6 and a density of 3.37g/cm³. Phase G coexists with periclase and Mg‐perovskite under the lower mantle conditions, and thus it can be a reservoir of water in cold slabs penetrating into the lower mantle.