HR: 11:50h
AN: MR22A-07    [Abstracts]
TI: Crystal Chemistry and High P,T Behavior of the Post-Perovskite Phase of MgSiO$_{3}$
AU: * Ross, N L
EM: nross@vt.edu
AF: Virginia Tech, Dept. of Geosciences 4044 Derring Hall, Blacksburg, VA 24061 United States
AU: Tsuchiya, J
EM: junt@cems.umn.edu
AF: University of Minnesota, Dept. Chemical Engineering and Materials Science, Minnesota Supercomputing Institute, Minneapolis, MN 55455 United States
AU: Tsuchiya, T
EM: takut@cems.umn.edu
AF: University of Minnesota, Dept. Chemical Engineering and Materials Science, Minnesota Supercomputing Institute, Minneapolis, MN 55455 United States
AU: Wentzcovitch, R M
EM: wentzcov@cems.umn.edu
AF: University of Minnesota, Dept. Chemical Engineering and Materials Science, Minnesota Supercomputing Institute, Minneapolis, MN 55455 United States
AB: A new polymorph of MgSiO$_{3}$ more stable than the {\it Pbnm} perovskite phase has recently been identified at pressures and temperatures corresponding to the lowermost part of the Earth's mantle by both experimental and theoretical techniques [1,2]. The post-perovskite phase of MgSiO$_{3}$ has been identified as being isostructural with CaIrO$_{3}$. The post perovskite structure belongs to space group {\it Cmcm} and consists of layers of SiO$_{6}$ octahedra alternating with layers of Mg atoms in approximate 8-fold coordination. The SiO$_{6}$ octahedra share edges that form chains running parallel to the {\it a}-axis that are interconnected by corner-sharing apical oxygen atoms along the {\it c}-axis. Magnesium atoms are located between the SiO$_{6}$ layers at the centers of bicapped trigonal bipyramids sharing faces that run parallel to the {\it a}-axis. In this presentation we compare the crystal chemistry of this new phase with that of MgSiO$_{3}$ perovskite. In particular, we will discuss how the differences in the structure may influence trace element partitioning and transport properties. We will also present results from first principles calculations of the post perovskite phase carried out to high pressures (up to 180 GPa) and temperatures (up to 4000K) using a similar technique recently applied to high P,T calculations of MgSiO$_{3}$ perovskite [3]. Compression of the post perovskite structure is anisotropic with the {\it b}-axis (perpendicular to the layers) about 30% more compressible than {\it a} and {\it c}, which have very similar compressibilities. The major structural change with increasing pressure is the decrease of the Mg-O and Si-O bond lengths; the Si-O1-Si angle that connects the SiO$_{6}$ octahedra along [001] shows a very slight increase from 136.8$^{o}$ at 30 GPa (and 300K) to 138.8$^{o}$ at 180 GPa (and 300K). The effect of temperature on the structure appears to be inversely related to that of pressure. References: [1] Murakami M., Hirose K., Kawamura K., Sata N. Ohishi Y. (2004) Science, 855;[2] Oganov, A.R., Ono S. (2004) Nature, 445; [3] Wentzcovitch R.M., Karki B.B., Cococcioni M., de Gironcoli S. (2004) Phys.Rev. Lett. 92 018501.
DE: 3620 Crystal chemistry
DE: 3924 High-pressure behavior
DE: 3210 Modeling
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting