The experimental Raman spectra of monoclinic tridymite and its higher temperature orthorhombic modification were successfully simulated with valence force fields applied to the X-ray determined structural parameters. The normal modes contributing to the corresponding Raman peaks in both the phases share essentially the same types of atomic displacements or bond deformations. In the modes with the wave numbers between 100 and 600 cm⁻¹, predominant are the librations of Si-O-Si planes around Si-Si lines, being followed by bending of Si-O-Si or O-Si-O angles. The librational motions of SiO₄ units around Si appear in the modes below 100 cm⁻¹. Significant bond stretching and compression appear in the modes with wave numbers higher than 700 cm⁻¹: the deformations are classified into 10 types, 7 for Si-O-Si and 3 for SiO₄. Some modes in the highest frequency group between 1150 and 1250 cm⁻¹ show symmetric bond stretching and compression of SiO₄ units, while those between 1000 and 1150 cm⁻¹ antisymmetric stretching and compression of Si-O-Si.
      A low-frequency normal mode is identified to have the polarization vectors of O atoms, which are nearly parallel with the largest principal axes of their thermal ellipsoids. A disorder model is presented to account for the extremely large mean square displacements of O atoms, and a possibility is suggested for the normal mode to boost the hopping of the SiO₄ units between the two orientations.