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Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in${\mathrm{H}}_2$O-rich setting at high pressures and temperatures ($P-T$) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between${\mathrm{S}\mathrm{i}\mathrm{O}}_2$and${\mathrm{H}}_2$O as archetypal materials for rock and ice, respectively, at high$P-T$. We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{l}}_2$-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of (${\mathrm{S}\mathrm{i}}_{1-x}{\mathrm{H}}_{4x}$)${\mathrm{O}}_2\hspace{0.17em}\left(x=0.12\right)$. At pressures above 60 GPa,${\mathrm{H}}_2$O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

Single‐crystal X‐ray diffraction and Brillouin spectroscopy experiments were performed on a natural Cr‐pyrope (Prp_71.0Alm_12.6Sps_0.7Grs_3.5Uvr_12.2) at high pressure and high temperature up to 11.0 GPa and 800 K. Fitting the collected data to the third‐order finite strain equation yields bulk modulus (K_S0), shear modulus (G₀), their pressure ((∂K_S/∂P)_Tand (∂G/∂P)_T) and temperature ((∂K_S/∂T)_Pand(∂G/∂T)_P) derivatives,K_S0 = 167.7(8) GPa,G₀ = 91.5(5) GPa, (∂K_S/∂P)_T = 4.3(1), (∂G/∂P)_T = 1.4(1), (∂K_S/∂T)_P = −0.0175(1) GPa/K and (∂G/∂T)_P = −0.0073(1) GPa/K. Using the obtained results, we examined whether the elastic properties of the Cr‐pyrope can be accurately calculated from those of endmembers including pyrope, almandine, grossular, and uvarovite assuming a linear relationship between elastic properties and composition (end‐member model). The results indicate that the end‐member model provides a sufficient approximation for the elastic properties of Cr‐pyrope in calculating the density and velocity of the subcontinental lithospheric mantle (SCLM). We modeled the densities and velocities of three typical types of SCLM (Archon, Proton, and Tecton) in order to investigate how the variation of chemical composition influences the SCLM. We obtained that the compositional change from the Archon to the Tecton increases the density of the SCLM significantly, which can be an important prerequisite for SCLM delamination. However, the compositional variation only slightly changes the velocity of the SCLM and the change is within the uncertainty of the calculation. Moreover, in comparison to the velocity,ρ/V_Pandρ/V_Sare much more sensitive to the compositional change of the SCLM.