Precise determination of the phase boundary between coesite and stishovite in SiO 2

The phase boundary between coesite and stishovite in SiO 2 was determined over the range of 1200–1700 K using a multi-anvil high-pressure apparatus combined with the synchrotron X-ray diffraction technique. The stability of each phase was determined by observing the powdered X-ray diffraction data. The transition boundary between the coesite and stishovite phases was found to occur at P (GPa) = 4.7 + 0.0031 × T (K). The transition pressure determined in this study was in general agreement with what was observed in previous high-pressure experiments and heat capacity measurements. However, the slope of the transition, dP / dT , determined in our study was higher than that estimated in a previous study utilized with the in situ X-ray method. In contrast, the slope calculated by the heat capacity data was in good agreement with that determined in our study. The phase transition determined in our study occurs at around 10 GPa at the normal mantle geotherm, coinciding with the seismic discontinuity around 300 km depth known as the X-discontinuity.