A NEW SiO₂ PHASE AT PHYSICAL CONDITIONS OF EARTH'S LOWER MANTLE: THEORETICAL AND SYNCHROTRON X-RAY EVIDENCES. L.S. Dubrovinsky, S.K. Saxena, A.B. Belonshko (Institute of Earth Sciences, Uppsala University, Sweden).

Numerous studies have suggested the existence of a post-stishovite phase. However, none of the experimental or theoretical studies discovered a phase which could be more stable than stishovite at high pressures, except for the possibility of transition into CaC¹² structure (Kingma et al., 1995), which is likely to be a second order transition. Quasiharmonic lattice dynamic (QLD) and molecular dynamics (MD) study with recently developed interatomic potentials (Tsuneyuki et al, 1989; Belonoshko and Dubrovinsky, 1995) was used for simulation of phase diagram of silica up to 150 GPa. The possibility of a new silica phase with Pnc2 structure was found. This phase is more stable than stishovite at pressures above approximately 75-120 GPa at a wide range of temperature.

We used the Mao-Bell type diamond-anvil cell following the sample preparation method as described elsewhere. A 10 micron thick iron foil was surrounded by dried silica gel. Silica gel was taken as starting material to avoid the well-known kinetic problems in synthesing of silica phases. An area of approximately 60 micron in diameter was heated to 2000+/-50 K with Nd:YAG laser for several minutes. The temperature was measured using spectroradiometric method. The quenched samples, still under pressure 080 GPa, were studied at Brookhaven National Laboratory synchrotron X-ray facility. Due to heating, the silica had recrystallized to a new dense form of SiO² over a large area. Based on the X-ray data taken at 080 GPa after heating at 2000 K, this phase may be assigned to a structure which is intermediate between a-PbO² and ZrO² structure (space group Pnc2) with unit cell parameters a=4.300(1); b=3.899(2); c=4.689(4), V=11.84(2) cm₃/mole. This phase is slightly denser than stishovite. It is likely to be more stable in the lower mantle than stishovite, if there is any free silica.