Transformation of stishovite to a denser phase at lower-mantle pressures

Abstract

WHETHER stishovite, the highest-pressure polymorph of SiO2 known from natural samples, transforms to a denser structure at higher pressures has long been of interest. A suggested transition from rutile to the CaCl2 structure driven by a vibrational-mode instability1 was supported by the observation of a frequency decrease (softening) of a Raman mode with increasing pressure2. Subsequent X-ray diffraction measurements provided evidence3 for stability of the CaCl2 phase near 100 GPa. Electronic-structure calculations predict, however, that the transition occurs at much lower pressure, where a shear modulus vanishes and before the Raman mode softens completely4. Here we use in situ Raman spectroscopy and a new theoretical model to investigate the high-pressure behaviour of stishovite. At 50 GPa, the pressure dependence of the soft B1g mode abruptly changes and the Eg mode splits, as predicted for transformation to the CaCl2 structure. Our results demonstrate that any free silica in the, deep mantle (below 1,200-1,500 km) will exist in the CaCl2 structure at considerably lower pressures than previously thought3.