Abstract
We have carried out high-pressure and high-temperature in-situ XRD experiments of quartz and cristobalite using a Kawai-type apparatus (SPEED-1500) at BL04B1 of SPring-8. When using cristobalite, we observed formation of seifertite before appearance of stishovite when heating. The metastable field of seifertite can be kinetically defined, which expands to much lower pressures than previously thought.