POST-STISHOVITE PHASE BOUNDARY IN SIO2 DETERMINED BY IN SITU X-RAY OBSERVATIONS

Abstract

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)xT (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.