X-ray diffraction study of phase stability in SiO2 at deep mantle conditions

The stability of SiO2 phases were examined by X-ray diffraction at pressure up to 131 GPa and temperature > 1300 K, corresponding to Earth's deep lower mantle conditions. Three materials, stishovite, cristobalite, and silica glass were used as starting materials, and an argon pressure-transmitting medium was loaded with the sample for five out of six sets of laser-heated diamond cell experiments. X-ray fluorescent crystals were loaded with the sample to ensure alignment of X-ray beam and heating laser. Our results from three different staring materials indicate that the CaCl2-type phase is the stable phase throughout our studied P-T range. We observe the α-PbO2-type phase for cristobalite starting material at pressures greater than 48 GPa at room temperature. However, this transforms to the CaCl2-type structure upon heating between 72-113 GPa. Transition kinetics and metastability may be important in explaining differences in results reported above 75 GPa, and key experimental variables are discussed.