Raman spectroscopy of CaM2+Ge2O6(M2+= Mg, Mn, Fe, Co, Ni, Zn) clinopyroxenes

The Raman spectra of Ge-clinopyroxenes CaM2+Ge2O6 (M2+12 = Mg, Mn, Fe, Co, Ni, Zn), general formula M2M1T2O6, are reported for the first time. Their spectral features are discussed by comparison with corresponding Si-pyroxenes. The vibrational frequencies of germanates may be roughly obtained by a scale factor of about ~ 0.8 by those of the corresponding silicates, due to the Ge-Si mass difference. The main peaks in the germanate Raman spectra at ~ 850 and ~540 cm-1 may be related to Ge-O tetrahedral stretching and chain bending, respectively; minor peaks between 200 and 400 cm-1 are ascribed to bending and stretching of the non-tetrahedral cations. Within Gepyroxenes, possible correlations between crystallographic parameters and the vibrational frequencies are investigated. The main stretching mode at ~ 850 cm-1 shows wavenumber changes with M2+ substitutions, but no simple correlation can be found with M2+ cation mass or size. On the other hand, the chain bending wavenumber linearly decreases with increasing ionic radius of the M2+ cation: the expansion of the M1 polyhedron reduces the chain kinking angle and the Ge-Ge distances correspondingly increase.