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Garnets rich in the majorite (MgSiO₃) component are a major constituent of the transition zone of the Earth´s mantle. While these garnets are tetragonal at ambient conditions, they may have different symmetry at high pressures and temperatures. We have therefore measured the Raman spectra of two synthetic non-cubic garnets rich in the MgSiO₃ component in a diamond-anvil cell to pressures in excess of 200 kbars at room temperature. Sample 1 was pure MgSiO₃ majorite, sample 2 was a solid solution consisting of 93.41 mol% MgSiO₃ and 6.59 mol% Mg₃Al₂(SiO₄)₃. Although the X-ray patterns of both samples could be indexed assuming tetragonal symmetry, the possibility that sample 2 may be orthorhombic cannot be completely ruled out. High pressure Raman spectra of sample 1 show a continuous shift of all bands to higher frequency with increasing pressure; no indications of any phase transition were seen. Sample 2, however, undergoes a phase transition between 51 and 85 kbars upon compression (see Fig. 3.4-4); this phase transition is perfectly reversible between 54 and 38 kbars upon decompression. In the frequency range between 300 and 800 cm^-1, several bands that are present in the low-pressure phase disappear from the high-pressure spectra while some of the remaining bands show increases in intensity. The high-pressure spectrum of sample 2 at 189.2 kbar is very similar to the room pressure spectrum of sample 1. The observed changes at pressure could be a due to symmetry change from an orthorhombic to a tetragonal space group.
 

Fig. 3.4-4: High-pressure Raman spectra of a majorite rich garnet (sample 2, Majorite 93.4 mol% - Pyrope 6.6 mol%). Note the distinct changes of the spectrum between 51 and 84 kbars, indicative of a phase transition. (* denote peaks of the pressure medium)