Normal coordinate analysis of the optically active vibrations (k=0) of crystalline magnesium orthosilicate Mg2SiO4 (forsterite)

A normal coordinate analysis for the vibrations of the olivine type crystal Mg2SiO4 (forsterite) in the limit k=0 has been carried out to investigate the group behavior of the SiO4 ion and also to identify the ’’external’’ modes. The crystal FG matrix method of Shimanouchi et al. is used with a potential function which combines general valence and short range force constants. Calculations have also been done for the 26Mg isotope. A dipole–dipole interaction model has been introduced through stretch–stretch interaction to account for the correlation field splitting of ν3b of the SiO4 group. Conclusions are also drawn about the nature of different vibrational modes.