Raman spectra of norbergite at various pressures and temperatures

The Raman spectra of natural samples of norbergite were investigated up to 320 kbar at room temperature and between 82-873 K at atmospheric pressure. Two to four well-separated OH Raman bands of norbergite were observed. Norb ergite remains stable up to the maximum temperature investigated in the pre sent study. The Raman frequencies of ail the bands of norbergite were obser ved to increase linearly with increasing pressure, but decrease linearly or nonlinearly with increasing temperature within the experimental uncertaint ies and the range investigated. Unlike other dense hydrous magnesium silica tes so far studied, both the temperature and pressure variations of the OH Raman bands of norbergite follow the same trend as those of the silicate (b oth internal and lattice modes) vibrations. A mechanism combining the H-H r epulsion with hydrogen bonds elongating towards an empty polyhedron is prop osed to explain this phenomenon.