Raman and infrared spectra to monitor the phase transition of natural kyanite under static compression

The stability of kyanite Al2SiO5 at high pressure is of significance in understanding the fate of the subducted crust and how they recycle within the Earth. Thus, in situ high-pressure vibrational spectra of natural kyanite are presented. The shift amplitudes of the measured modes dependent on pressure manifest the contraction of the Al-O bond bears the most compression of kyanite. At 9.7 GPa, a first-order phase transition to kyanite-II occurs, which is distinguished by changes occurred in the entire spectral region, including the pressure dependences of the modes, the wavenumber jumps of some peaks, the splitting of a Si-O symmetric stretching band, and the emergence of new modes. A Raman mapping image is collected illustrating kyanite and kyanite-II in coexistence at 10.0 GPa. Emphasize that, as is revealed by the infrared spectra, a part of H-O center dot center dot center dot O distance becomes elongated after the transformation. At higher pressure up to 18.6 GPa, the Raman spectra disclose little effects associated with further transformations. Finally, releasing pressure to ambient leads to recovery of pure kyanite. Our results mark a lowest pressure limit of the change in the intermolecular and intramolecular bonding of kyanite and shed new lights on the crystal chemistry of aluminosilicate under mantle pressures.