New thermo-elastic and crystal-chemical insights on 11.5 Å phase: a major water carrier to the mantle transition zone

In the past three decades, experimental studies have demonstrated the stability of new hydrous silicates in the MgO-Al¬2O3-SiO2-H2O (MASH) system, known as "Dense Hydrous Magnesium Silicates" (DHMS). These phases, like the DHMS in the simpler MgO-SiO2-H2O (MSH) system (e.g., phase A), can form after the destabilization of layer hydrous minerals (serpentine, chlorite). Recent research has studied their stability under mantle pressures and temperatures, highlighting their role in the deep-water cycle and their impact on the mantle's water budget (Hermann et al., 2021). Among these phases, the 11.5 Å phase, with chemical formula Mg6Al(SiO4)2(OH)7, has been identified as stable in the region between 7-11 GPa and 500-1000°C, and plays a potential role in storing and transporting water to the mantle transition zone (Cai et al., 2019; Hermann et al., 2021). In subduction contexts, the 11.5 Å phase has been associated with the destabilization of chlorite in Al-rich systems and can store up to 13wt% of water, a notably high content compared to other DHMS under similar conditions. Its crystal structure was determined by Gemmi et al. (2016) using standard precession-assisted electron diffraction tomography (PEDT), revealing a layered structure consisting of TOT groups and double di-octahedral layers of face-sharing octahedra (O-O), with unit cell parameters a = 9.012(1)Å b = 5.201(1)Å c = 23.202(5)Å β = 97.8(1)° and space group C2/c. We performed multi-anvil syntheses under different mantle P-T conditions (6-10 GPa and 700-900 °C) to assess the effect of iron and chromium on the crystal-chemistry and stability of the 11.5 Å phase, starting from natural Fe-bearing chlorite sample and synthetic Cr-bearing systems. The results reveal that solid solutions from pure Mg-Al term can form with Fe-Al and Mg-Cr end-members. The synthesis performed at the highest pressures crystallized single crystals up to 50x50x50 µm3. Single crystal diffraction allowed ab-initio structure determinations and refinements, confirming the structural description reported by Gemmi et al. (2016). In addition, we report the determination of PV and VT equations of state, which allow the density estimation for the 11.5 Å phase at mantle conditions. We observed also an interesting single crystal to single crystal phase transition at high temperature, at 400 °C, upon partial dehydration, with noticeable 5% volume expansion and a partial unpacking of the double Mg-octahedral layers. References: Cai N. & Inoue T. (2019) – High-pressure and high-temperature stability of chlorite and 23-Åphase in the natural chlorite and synthetic MASH system. CR Geoscience, 351(2-3), 104-112. Gemmi M. et al. (2016) – Electron diffraction determination of 11.5Å and HySo structures: Candidate water carriers to the Upper Mantle. American Mineralogist, 101(12), 2645-2654. Hermann J. & Lakey S. (2021) – Water transfer to the deep mantle through hydrous, Al-rich silicates in subduction zones. Geology, 49(8), 911-915.