HIGH-PRESSURE PHASE-TRANSITION IN BRUCITE, MG(OH)(2)

Brucite, Mg(OH)(2), was investigated by Raman spectroscopy to pressures of 36.6 GPa under nonhydrostatic conditions and to 19.7 GPa under quasi-hydrostatic conditions. Several new Raman lines are first observed at 4 GPa, demonstrating the existence of a high-pressure structural change. One of the new lines grows over a broad pressure interval, and this growth can be explained by a resonant interaction with the E(g) translational mode. Raman data are compared with recent infrared spectroscopy, X-ray diffraction, and neutron diffraction studies of brucite. The structural change is likely to involve displacement or disordering of the H atoms, consistent with neutron diffraction results. The Raman-active O-H stretching vibration in brucite decreases with pressure at the rate of similar to 7 cm(-1)/GPa, larger than the pressure dependence of the infrared-active stretching vibration by more than a factor of ten. The primary differences in the Raman spectra of Mg(OH)(2) and Ca(OH)(2) are that the O-H vibrational frequencies in Mg(OH)(2) vary linearly with pressure, and the O-H stretching vibration band width increases with pressure at a rate that is an order of magnitude lower for Mg(OH)(2) than for Ca(OH)(2).