High-pressure Raman spectroscopy of Ca(Mg,Co)Si2O6 and Ca(Mg,Co)Ge2O6 clinopyroxenes

In situ high-pressure Raman spectra were collected on four pyroxenes, with composition CaCoSi2O6, CaMgSi2O6, CaCoGe2O6 and CaMgGe2O6, up to P = 7.6 and 8.3 GPa for silicates and germanates, respectively. The peak wavenumbers υi increase almost linearly with pressure; the slope dυi/dP is more pronounced for the modes at higher wavenumbers, and higher in germanates than in silicates. No phase transition or change in the compressional behaviour was observed within the investigated P-range. The strong dependence of the peak position with pressure of the high energy stretching modes is due to the high sensitivity of the vibrational frequencies probed by Raman spectroscopy to subtle changes in the tetrahedral deformation, which are overlooked by single-crystal X-ray diffraction.