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. Norbergite remains stable up to the maximum temperature investigated in the present study. The Raman frequencies of all the bands of norbergite were observed to increase linearly with increasing pressure, but decrease linearly or nonlinearly with increasing temperature within the experimental uncertainties and the range investigated. Unlike other dense hydrous magnesium silicates 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 (both internal and lattice modes) vibrations. A mechanism combining the H-H repulsion with hydrogen bonds elongating towards an empty polyhedron is proposed to explain this phenomenon.

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