Elastic moduli ($C_{ij}$) of single-crystal stishovite and post-stishovite are determined using Brillouin light scattering, impulsive stimulated light scattering, and x-ray diffraction up to 70 GPa. The $C_{12}$ of stishovite converges with the $C_{11}$ at $\sim 55\mathrm{GPa}$, where the transverse wave $V_{S1}$ propagating along [110] also vanishes. Landau modeling of the $C_{ij}$, $B_{1g}$ optic mode, and lattice parameters reveals a pseudoproper type ferroelastic post-stishovite transition. The transition would cause peculiar anomalies in $V_S$ and Poisson’s ratio in silica-bearing subducting slabs in the mid-lower mantle.

Hydrous aluminosilicates are important deep water-carriers in sediments subducting into the deep mantle. To date, it remains enigmatic how hydrous aluminosilicates withstand extremely high temperatures in the mantle transition zone. Here we systematically investigate the crystal structures and chemical compositions of typical hydrous aluminosilicates using single-crystal X-ray diffraction, electron probe microanalyzer, and nanoscale secondary ion mass spectrometry. These single crystals are synthesized at 15.5–22.0 GPa and 1400–1700 °C, featuring pervasive structural disorders. In particular, Al and Si atoms extensively occupy new tetrahedral and octahedral sites that are nominally vacant in their ordered counterparts. High temperature activates disorders leading to variable local crystal structures and more hydrogen incorporation into the crystal structure. This result suggests that the order-to-disorder transition holds the key to the high thermal stability of hydrous aluminosilicates, significantly affecting the water cycle in the deep mantle.

δ-AlOOH is regarded as a potential water carrier that is stable in the Earth’s lower mantle down to the core-mantle boundary along the cold slab geotherm; thus, knowledge of its structural evolution under high pressure is very important for understanding water transport in the Earth’s interior. In this work, we conducted Raman scattering and luminescence spectroscopic experiments on δ-AlOOH at pressures up to 34.6 and 22.1 GPa, respectively. From the collected Raman spectra, significant changes in the pressure dependence of the frequencies of Raman-active modes were observed at ~8 GPa, with several modes displaying softening behavior. In particular, the soft A1 mode, which corresponds to a lattice vibration of the AlO6 octahedron correlated to OH stretching vibrations, decreases rapidly with increasing pressure and shows a trend of approaching 0 cm−1 at ~9 GPa according to a quadratic polynomial extrapolation. These results provide clear Raman-scattering spectroscopic evidence for the P21nm-to-Pnnm structural transition. Similarly, the phase transition was also observed in the luminescence spectra of Cr3+ in both powder and single-crystal δ-AlOOH samples, characterized by abrupt changes in the pressure dependences of the wavelength of the R-lines and sidebands across the P21nm-to-Pnnm transition. The continuous decrease in R2-R1 splitting with pressure indicated that the distortion of the AlO6 octahedron was suppressed under compression. No abnormal features were clearly observed in our Raman or luminescence spectra at ~18 GPa, where the ordered symmetrization or fully centered state with hydrogen located at the midpoint of the hydrogen bond was observed by a previous neutron diffraction study. However, some subtle changes in Raman and luminescence spectra indicated that the ordered symmetrization state might form at around 16 GPa.