Raman spectra of phase C (superhydrous phase B) at various pressures and temperatures

Variations in the Raman spectra of the magnesium silicate phase C, or the superhydrous phase B, were studied at < or = 420 kbar at room T and in the range 80-873 K at 1 atm. With the exception of the 3355 cm (super -1) OH band, the frequency of all Raman bands of phase C increases with increasing P, but the frequency of the two well-defined OH bands at P > approximately 100 kbar depends on the presence of shear stress. With the exception of the three OH bands and the ambient 325 cm (super -1) band, the frequency of all other Raman bands of phase C decreases with increasing T. Phase C appears to remain intact up to the highest T attainable in this study. However, Raman spectra obtained at various conditions seem to suggest that amorphization may start to occur in phase C at above 773 K, but dehydration is incomplete even at higher T. These P and T effects on phase C differ in many respects from those observed earlier in phases A and B.