Theoretical Study of the Structural Properties and Equations of State of MgSiO3 and CaSiO3 Perovskites: Implications for Lower Mantle Composition
- Murli H. Manghnani and
- Yasuhiko Syono
Published Online: 21 MAR 2013
Copyright © 1987 by Terra Scientific Publishing Company (TERRAPUB), Tokyo.
High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto
How to Cite
Wolf, G. H. and Bukowinski, M. S. T. (1987) Theoretical Study of the Structural Properties and Equations of State of MgSiO3 and CaSiO3 Perovskites: Implications for Lower Mantle Composition, in High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto (eds M. H. Manghnani and Y. Syono), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM039p0313
- Published Online: 21 MAR 2013
- Published Print: 1 JAN 1987
Print ISBN: 9780875900667
Online ISBN: 9781118664124
- Mineralogy and Crystal Chemistry;
- Phase transformations;
- High Pressure-High Temperature Research
We present a detailed theoretical study on the stability and equations of state of MgSiO3 and CaSiO3 perovskites. Results are obtained as a function of temperature and pressure through a minimization of the free energy with respect to the structural parameters, as given by self-consistent quasiharmonic lattice dynamics. Bonding forces are derived from the parameter-free modified electron-gas theory. A cubic structure for CaSiO3 and an orthorhombic structure for MgSiO3 are predicted at zero pressure in accord with observation. The calculated compressibility and thermal expansivity of MgSiO3 are in very good agreement with available data. The model predicts that the degree of distortion in MgSiO3 increases weakly with pressure but this increase in distortion has only a minor effect on its compressibility. At high temperatures, model MgSiO3 perovskite exhibits critical soft-mode behavior and undergoes successive second-order transitions to tetragonal and cubic phases. The transition temperatures increase with pressure such that the orthorhombic phase is stable throughout most of lower mantle, although an adiabatic extrapolation of the lower mantle to zero pressure may approach or even cross these phase boundaries. We also find that an adiabatic extrapolation of the lower mantle seismic properties will overestimate its inferred zero-pressure density and bulk modulus. Hence, compositional constraints on the lower mantle that are based on comparisons of decompressed seismological properties with zero-pressure mineralogic data will overestimate the relative proportion of perovskite to magnesiowüstite. Supplementing available data with theoretical estimates for the high-pressure and high-temperature properties of relevant mantle phases, we find that both a pyrolitic composition, at relatively low geotherm temperatures appropriate to whole-mantle convection, and a pyroxene composition, at the higher geotherm temperatures appropriate to layered-mantle convection, are compatible with the seismic data.