A new, post-stishovite highpressure polymorph of silica

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THE possibility of ‘post-stishovite’ phases of silica (SiO2) has been the subject of extensive study during the past two decades, not only because of the intrinsic crystallographic interest but also because of its relevance to the geochemistry of the lower mantle. Here in situ X-ray diffraction observations of SiO2 compressed to pressures of up to 124 GPa using a laser-heated diamond-anvil cell. Stishovite was formed by heating α-quartz or amorphous silica at pressures of up to 92 GPa; on heating at 108 and 124 GPa, however, the materials crystallized into a CaCl2-type structure, which results from a distortion of the Stishovite structure and is slightly denser. The quenched and recovered sample was always Stishovite; thus the transformation from Stishovite to the CaCl2 structure is reversible on release of pressure.