Samples of phase D were synthesized in high pressure and high temperature experiments with a multi-anvil press. Phase D may have a chemical range of Mg/Si=0.65¿0.05 and H2O in the range 6~14 wt% on the basis of electron microprobe analyses. The nature of phase D was characterized by both powder X-ray diffraction and Raman spectroscopy. The resolution of the Raman spectra of phase D is in general very poor, though the X-ray diffraction data suggest that it is well crystalline. No clear OH Raman band of phase D was demonstrated, however. Variations of Raman spectra of phase D were investigated up to 18 GPa at room temperature and in the range 81--373 K at atmospheric pressure. Lower temperatures and/or high hydrostatic pressures did not improve the quality of the spectra. Phase D is extremely unstable when it was irradiated by laser and/or heated above 373 K at atmospheric pressure. Among the silicate vibrations, the pressure variation has been studied for only two bands and the temperature variation for four bands. The Raman frequencies for these bands of phase D increase with increasing pressure but decrease with increasing temperature, which are the same as the other dense hydrous magnesium silicates so far studied. ¿ 1998 American Geophysical Union