Abstract

b-cristobalite (SiO2) and b-carnegieite (NaAlSiO4) have been investigated by Raman spectroscopy up to their melting points at 1999 and 1799 K, respectively. In both cases, the major spectral changes are observed at the a-b transitions, near 540 and 970 K, especially at low frequencies where 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 important 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 point as for other silicate minerals.