High-pressure Raman spectroscopy of Al-rich and pure-Mg phase D

High-pressure Raman spectra of Al-rich phase D (Mg0.93Al0.70Si1.29O6H2.88) and pure-Mg phase D (Mg1.03Si1.71O6H3.05) were measured up to 20 GPa in diamond-anvil cells using argon as a pressure medium. The results show that the intensity of the major 777 cm- 1 band in the Raman spectra of the pure-Mg phase D exhibits a significant intensity reduction within the 18-20 GPa range during compression. However, this band displays a highly linear shift in the Raman spectra of the Al-rich phase D without notable decrease in intensity in the same pressure range. This implies that the pressure stability of the M2 octahedra in the Al-rich phase D is higher than that in the pure-Mg phase D due to the substitution of Al3+ for Si4+. The major OH band at about 2900 cm- 1 in the Raman spectra of the pure-Mg phase D sample shifts continuously toward higher frequencies with increasing pressure due to the pressure-induced transition from straight H bonds to bent ones. Whereas, this transition occurs at pressures above 10 GPa in the Al-rich phase D, indicating that Al3+ substitution in the crystal structure of phase D can also alter the high-pressure response of hydroxyl ion.