A PRESSURE-INDUCED PHASE-TRANSITION IN MGSIO3-RICH GARNET REVEALED BYRAMAN-SPECTROSCOPY

Raman spectra of two synthetic, noncubic garnet samples rich in MgSiO3 component were measured in a diamond-anvil cell to 200 kbar at room t emperature. Sample 1 was pure MgSiO3 majorite; sample 2 was a solid so lution with 93.4 mol% MgSiO3 and 6.6 mol% Mg3Al2(SiO4)(3). Although th e X-ray patterns of both samples can be indexed assuming tetragonal sy mmetry, it cannot be completely ruled out that sample 2 is orthorhombi c. High-pressure Raman spectra of sample 1 show a continuous shift of all bands to higher frequency with increasing pressure; there are no i ndications of phase transitions. Sample 2, however, undergoes a phase transition between 51 and 85 kbar on compression; this phase transitio n is completely reversible between 54 and 38 kbar on decompression. In the frequency range between 300 and 800 cm(-1), several bands that ar e present in the low-pressure phase disappear or merge in the high-pre ssure spectra, whereas some of the remaining bands intensify. The high -pressure spectrum of sample 2 is very similar to the room-pressure sp ectrum of sample 1. The phase transition could possibly be the result of a symmetry change from an orthorhombic to a tetragonal space group.