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Raman spectra of two synthetic, noncubic garnet samples rich in MgSiO 3  component were measured in a diamond-anvil cell to 200 kbar at room temperature. Sample 1 was pure MgSiO 3  majorite; sample 2 was a solid solution with 93.4 mol% MgSiO 3  and 6.6 mol% Mg 3 Al 2 (SiO 4 ) 3 . Although the X-ray patterns of both samples can be indexed assuming tetragonal symmetry, it cannot be completely ruled out that sample 2 is orthorhombic. High-pressure Raman spectra of sample 1 show a continuous shift of all bands to higher frequency with increasing pressure; there are no indications of phase transitions. Sample 2, however, undergoes a phase transition between 51 and 85 kbar on compression; this phase transition is completely reversible between 54 and 38 kbar on decompression. In the frequency range between 300 and 800 cm -1 , several bands that are present in the low-pressure phase disappear or merge in the high-pressure spectra, whereas some of the remaining bands intensify. The high-pressure spectrum of sample 2 is very similar to the room-pressure spectrum of sample 1. The phase transition could possibly be the result of a symmetry change from an orthorhombic to a tetragonal space group.

LETTERS. A pressure-induced phase transition in MgSiO 3 -rich garnet revealed by Raman spectroscopy