Variations of Raman spectra of phase C, or the superhydrous phase B, were investigated up to about 420 kbar at room temperature and in the range 80–873 K at atmospheric pressure. With the exception of the 3355 cm-1 OH band, the frequency of all observable Raman bands of phase C increases monotonously with increasing pressure. However, the frequency for the two well-defined OH bands at pressures greater than about 100 kbar depends strongly on the presence of shear stress. With the exceptions of the three OH bands and the ambient 325 cm-1 band, the frequency of all other Raman bands of phase C decreases with increasing temperature within the experimental uncertainties and the range investigated. Phase C appears to be intact up to the highest temperature attainable in the present study. However, Raman spectra obtained at various conditions seem to suggest that amorphization may start to occur in phase C above 773 K, but dehydration is not completed at the highest temperature studied. It is interesting to find that amorphization occurs at lower temperature than dehydration in phase C. The pressure and temperature effects on the Raman frequencies of phase C are different in many aspects from those observed earlier in phases A and B.