Water-bearing, high-pressure Ca-silicates

- Water-bearing Ca-silicates are found at mantle transition-zone conditions.
- A new HP Ca3SiO5 polymorph, post-hartrurite, occurs in anhydrous and hydrous forms.
- Larnite (Ca2SiO4) also has a hydrous counterpart.
- Textural and EPMA evidence suggest hydrous pv. may occur in Ca-rich environments.
- Ca-silicates have a high affinity for H2O at HP and high dehydration temperatures.

Water-bearing minerals provide fundamental knowledge regarding the water budget of the mantle and are geophysically significant through their influence on the rheological and seismic properties of Earth's interior. Here we investigate the CaO-SiO2-H2O system at 17 GPa and 1773 K, corresponding to mantle transition-zone condition, report new high-pressure (HP) water-bearing Ca-silicates and reveal the structural complexity of these phases. We document the HP polymorph of hartrurite (Ca3SiO5), post-hartrurite, which is tetragonal with space group P4/ncc, a=6.820(5), c=10.243(8) Å, V=476.4(8) Å3, and Z=4, and is isostructural with Sr3SiO5. Post-hartrurite occurs in hydrous and anhydrous forms and coexists with larnite (Ca2SiO4), which we find also has a hydrous counterpart. Si is 4-coordinated in both post-hartrurite and larnite. In their hydrous forms, H substitutes for Si (4H for each Si; hydrogrossular substitution). Fourier transform infrared (FTIR) spectroscopy shows broad hydroxyl absorption bands at ∼3550 cm−1 and at 3500-3550 cm−1 for hydrous post-hartrurite and hydrous larnite, respectively. Hydrous post-hartrurite has a defect composition of Ca2.663Si0.826O5H1.370 (5.84 weight % H2O) according to electron-probe microanalysis (EPMA), and the Si deficiency relative to Ca is also observed in the single-crystal data. Hydrous larnite has average composition of Ca1.924Si0.851O4H0.748 (4.06 weight % H2O) according to EPMA, and it is in agreement with the Si occupancy obtained using X-ray data collected on a single crystal. Superlattice reflections occur in electron-diffraction patterns of the hydrous larnite and could indicate crystallographic ordering of the hydroxyl groups and their associated cation defects. Although textural and EPMA-based compositional evidence suggests that hydrous perovskite may occur in high-Ca-containing (or low silica-activity) systems, the FTIR measurement does not show a well-defined hydroxyl absorption band for this phase, implying the water content, at least in the quenched glass, is below the limit of detection (100-1000 ppm). We conclude that at high pressure, as at ambient pressure, some calcium silicates have a high affinity for H2O and high dehydration temperatures. The thermal stability of these hydrous phases suggests that they could exist along a typical mantle geotherm and thus they might be relevant for understanding the mineralogy and water content of Earth's mantle.

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