New hydrous phases in the Al 2 O 3 -SiO 2 -H 2 O system under the mantle transition zone conditions

Hydrous minerals in the subducting slabs are potential water carriers into the deep mantle, and thus the synthesis of new hydrous phases is significant in our understanding of water circulation throughout the Earth’s interior. In this study, we report the two new hydrous phases, Al 2 SiO 6 H 2 and Al 5.5 Si 4 O 18 H 3.5 (hereafter referred to simply as phases Psi and Phi, respectively), which are synthesized in the Al 2 O 3 -SiO 2 -H 2 O system at 15.5 GPa, 1400°C and 17.5 GPa, 1600°C by using Sakura 2500-ton multi-anvil apparatus. The luminescence spectra of Cr 3+ show the phase Psi has characteristic peaks at 687, 693 and 705 nm, while phase Phi has characteristic peaks at 691, 696 and 708 nm. Single-crystal X-ray diffraction (SCXRD) refinements yield a monoclinic structure of both phases (space group P 2 1 ) with ideal chemical formulae of Al 2 SiO 6 H 2 and Al 5.5 Si 4 O 18 H 3.5 , respectively. The determined lattice parameters for phase Psi are a =9.4168±0.0016 Å, b =4.3441±0.0007 Å, c =9.4360±0.002 Å, and β =119.726±0.005° at ambient pressure and 300 K, while the phase Phi has a =7.2549±0.0018 Å, b =4.3144±0.001 Å, c =8.0520±0.002 Å, and β =01.740±0.009° at ambient pressure and 250 K. Electron microprobe analyses (EPMA) show the chemical compositions of phases Psi and Phi to be Al 1.99 Si 0.85 O 6 H 2.62 and Al 5.58 Si 2.81 O 18 H 8.03 , respectively, which slightly deviate from the ideal formulae inferred from SCXRD measurements. This may result from the disorder or substitution of Al and Si by H in the crystal structures under our synthesis conditions. Our study suggests that phases Psi and Phi are the two potential water carriers at the upper part of the mantle transitions zone, providing new insights into how deep water is stored in this region.