Phase Diagram of MgSiO3 at Pressures up to 24 GPa and Temperatures up to 2200 °C: Phase Stability and Properties of Tetragonal Garnet
- 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
Sawamoto, H. (1987) Phase Diagram of MgSiO3 at Pressures up to 24 GPa and Temperatures up to 2200 °C: Phase Stability and Properties of Tetragonal Garnet, 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/GM039p0209
- 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
The phase diagram for pure MgSiO3 has been obtained at pressures up to 24 GPa and at temperatures up to 2200°C using a MAS type of high-pressure apparatus. It is found that MgSiO3 has many transformation modes at different temperatures. At temperatures above 1800°C, MgSiO3 has a polymorphic sequence with five phases in order of increasing pressure: orthopyroxene, clinopyroxene, tetragonal garnet, ilmenite, and perovskite. At lower temperature, it exhibites seven phases or phase-assemblages: orthopyroxene, clinopyroxene, tetragonal garnet, modified spinel plus stishovite, spinel plus stishovite, ilmenite, and perovskite, in order of increasing density. All phase boundaries are determined by the present experiments, with consideration to thermo dynamic constraints. The pressure-temperature slope of the phase boundary between tetragonal garnet and perovskite is determined to be positive by using parameters representing well-constrained phase boundaries and the volume differences between the related phases.
The powder X-ray diffraction analysis of MgSiO3 with tetragonal symmetry shows that it is isostructural with CaGeO3, CdGeO3, and MnSiO3; these compounds possess an ordered cationic distribution on the noequivalent sites. The unit cell dimensions of the tetragonal garnet are a=11.470(1) Å, c=11.398(2) Å and c/a=0.994, and the molar volume is about 1% smaller than that estimated for a hypothetical cubic garnet. Therefore tetragonal garnet is a high-pressure and low-entropy phase. The elastic properties of MgSiO3 with tetragonal symmetry are predicted from elasticity systematics: bulk modulus K = 177 GPa and rigidity μ=101 GPa, their pressure derivatives dK s /dP=4.5 and dμ/dP=1.44, and seismic wave velocities Vp =9.37 km/s and Vs =5.32 km/s.
The tectonic significance of the garnet phase with a higher entropy than perovskite is also discussed.