High-temperature dynamics in cristobalite (SiO2) and carnegieite (NaAlSiO4): a Raman spectroscopy study

beta-cristobalite (SiO2) and beta-carnegieite (NaAlSiO4) have been investig ated by Raman spectroscopy up to their melting points at 1999 and 1799 K, r espectively. In both cases, the major spectral changes are observed at the alpha-beta transitions, near 540 and 970 K, especially at low frequencies w here the modes, involving mainly intertetrahedral motion, merge to a single , broad envelope. These observations and comparisons made between the Raman spectra of the crystalline and liquid phases are consistent with the rigid -unit model recently proposed for describing the dynamics of SiO4 and AlO4 tetrahedra in framework silicates. From a structural standpoint, an importa nt consequence is that the atomic mobility associated with premelting sets in at the low temperatures of these transitions, not in the vicinity of the melting Feint as for other silicate minerals.