Phase transition and equation of state of dense hydrous silica up to 63 GPa

Although it has previously been considered to be essentially anhydrous, Al‐free stishovite can contain up to ∼1.3 wt % of H 2 O, perhaps through the direct substitution ( Si 4 + → 4 H + ), according to recent studies. Yet the stability of such substitution and its impact on the properties of silica and rutile‐structured hydrous phases (such as δ ‐AlOOH and phase H) are unknown at the conditions of the deeper mantle. We have synthesized hydrous and anhydrous Al‐free stishovite samples at 723 K and 9 GPa, and 1473 K and 10 GPa, respectively. Synchrotron X‐ray diffraction patterns show that the unit cell volume of hydrous stishovite is 1.3% greater than that of anhydrous stishovite at 1 bar, suggesting significant incorporation of OH in the crystal structure (3.2 ± 0.5 wt % H 2 O). At 300 K, we found a lower and broader transition pressure from rutile type to CaCl 2 type (28–42 GPa) in hydrous dense silica. We also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts. After the phase transition, the unit cell volume of hydrous silica becomes the same as that of anhydrous silica, showing that the proton incorporation through a direct substitution can be further stabilized at high pressure. The lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica‐rich heterogeneities, such as subducted oceanic crust.