Post-stishovite phase boundary in SiO2 determined by in situ X-ray observations

A laser heating diamond anvil cell experiment, with an angle-dispersive X-ray diffraction using synchrotron radiation source at the SPring-8, has been developed to observe the phase transition in silica (SiO2) between the P42/mnm (rutile-type) and Pnnm (CaCl2-type) up to pressures of 100 GPa and at temperatures up to 2200 K. The transition was observed in the vicinity of 55 GPa at room temperature, and showed a positive temperature dependence of the transition pressure. The phase boundary was determined to follow the equation P (GPa)=(51±2)+(0.012±0.005)×T (K). Our result gives a transition pressure of near 80 GPa and a depth of 1900 km at an expected lower mantle temperature of 2000–2500 K. Therefore, this SiO2 transition is not the cause of recent observations of seismic anomalies between 800 and 1600 km depth in the mid–lower mantle.