High-pressure Raman spectroscopic study of chondrodite

Variation of Raman spectra of both natural (F-bearing) and synthetic (F-fre e) chondrodite samples were studied up to 400 kbar at room temperature. Amb ient Raman frequencies for the synthetic sample are in general lower than t hose for the natural one. This is correlated with a slight expansion of the volume of the synthetic sample due to substitution of OH for F. The freque ncies of all Raman bands for both samples increase monotonically with incre asing pressure. The positive pressure dependences in the O-H stretch freque ncies for both F-free and F-bearing samples are contrary to those for other dense hydrous magnesium silicates. A mechanism involving both the hydrogen -hydrogen repulsion and hydrogen bondings is proposed to explain the abnorm al behavior. The effects of substitution of F for OH on both the ambient an d high-pressure Raman spectra of chondrodite are also discussed.