Investigation on Structural Stability of $\gamma$-Al(OH)₃ under High Pressure and Shear Stress

The structural stability of gibbsite $\gamma$-Al(OH)₃ was investigated under high pressure and shear stress conditions by using a rotating diamond anvil cell (RDAC) combining with micro-laser Raman spectroscopy and micro-beam X-ray diffraction. As increasing pressure to 1.5 GPa and rotating to 180°, gibbsite triggers a new structural change at room temperature. In the range of high-wavenumbers, the four peaks of hydroxyl vibration (3363, 3434, 3524 and 3618 cm⁻¹) disappear gradually, and two new peaks with different intensities appear at 3303 and 3560 cm⁻¹. In the range of low-wavenumbers, the intensities of Raman spectra are getting weaker and weaker, but the broad peaks of amorphism are not observed. Both of the double peaks (568, 539 cm⁻¹) of Al-O-Al deformation vibration and the shoulder peaks (321 and 307 cm⁻¹) of Al-O stretching vibration merge into one peak, respectively. However, the four vibration peaks of hydroxyl deformation vibration (1052, 1018, 981 and 922 cm⁻¹) still remain. In addition, further increasing pressure to 3.5 GPa and rotating to 360°, and finally decreasing to ambient pressure, two new peaks of hydroxyl stretching vibration in high-wavenumbers, and the peaks of Al-O-Al deformation vibration and Al-O stretching vibration in low-wavenumbers are still observed. Compared to the phase transition of $\gamma$-Al(OH)₃ under quasi-hydrostatic pressure conditions, the Raman spectra and phase transition pressure in this study reveal that the $\gamma$-Al(OH)₃ take place another new structure change under high pressure and shear stress conditions. The micro-beam X-ray diffraction spectrum of the quenched product reveals that the framework of (OH)-Al-(OH) octahedra of this new phase is still remain, but has a shorter distance between the layers of (OH)-Al-(OH) and a higher symmetry. This newfound structure change of gibbsite in this study mainly due to the inhomogeneous pressure distribution in the sample chamber (from 0.5 to 4.5 GPa). The investigation on structural stability of $\gamma$-Al(OH)₃ under high pressure and shear stress conditions is vital for us to identify the stability of hydrous minerals in the cold subduction slabs, and to derive the physical and chemical properties of slab and its subduction rates.