Possible H2O storage in the crystal structure of CaSiO3 perovskite

The lower mantle is believed to contain much less hydrogen (or H2O) because of the low storage capacity of the dominant mineral phases, such as bridgmanite and ferropericlase. However, possible hydrogen storage in the third most abundant mineral in the region, CaSiO3 perovskite (Ca-Pv), is not well unknown. We have synthesized Ca-Pv from different starting materials with varying H2O contents at 19–120 GPa and 1400–2200 K in laser-heated diamond-anvil cell. While cubic perovskite structure is stable at the mantle-related pressures-temperatures (P−T) in anhydrous systems, we found non-cubic diffraction peak splitting in Ca-Pv even at high temperatures when it is synthesized from hydrous starting materials. In-situ high-pressure infrared spectroscopy showed OH vibration possibly from Ca-Pv. The unit-cell volume of hydrothermally synthesized Ca-Pv is systematically smaller than that of anhydrous Ca-Pv at high pressures. These observations suggest possible H2O storage in Ca-Pv at mantle-related P−T conditions. We also found the formation of separate δ–AlOOH and Ca-Pv phases from Al-bearing CaSiO3 glass starting materials in an H2O medium at 60 GPa and 1400 K. Ca-Pv still showed non-cubic peak splitting at high temperatures in this experiment. Therefore, it is possible that hydrous phases may coexist together with hydrous Ca-Pv in the lower mantle