Quartz and cristobalite: High-temperature cell parameters and volumes of fusion

The cell parameters of quartz and cristobalite have been measured up to their melting points from X-ray diffraction experiments with synchrotron radiation. After a smooth increase up to 1300 K, the volume of beta-cristobalite decreases continuously up to the melting temperature of 2000 K where it is back to the 750 K.value of 27.4 cm(3)/mol. A slightly negative thermal expansion coefficient is also observed for quartz above 1400 K, resulting in a volume of 23.45 cm(3)/mol at 1673 K. From these results and the molar volume of SiO2 liquid, the volumes of fusion of beta-cristobalite and quartz are -0.1 and 3.85 cm(3)/mol, respectively. With these values and the known entropies of fusion, only a slight adjustment to 1673 K of the melting temperature of quartz has been found necessary to ensure consistency with available phase equilibria data.