New stishovite-like phase of silica formed by hydrostatic compression of cristobalite

In order to clarify the effect of non-hydrostaticity of pressure on the crystal lattice of minerals, various pressures with different hydrostaticity were applied to silica (cristobalite) at room temperature and high pressure in situ x-ray observations were made. Under strongly non-hydrostatic compression, cristobalite transformed to an ''X-I phase'' reported in the previous study, while under quasi-hydrostatic compression, a new polymorph was synthesized. Although the new polymorph gives very similar diffraction profile to stishovite, its unit cell dimension is meaningfully different from that of stishovite. Present study clarified that in cristobalite the crystal structure of high pressure polymorph changes completely depending on the hydrostaticity of applied pressure.