Structural properties and phase transitions in Earth minerals: A first principles study.

摘要

Density functional calculations within the framework of LDA and GGA are used to investigate high pressure and high temperature (i.e., high P-T) thermodynamic properties and phase transitions in Earth mantle-forming magnesium silicates minerals. Following a short review of mantle structure and mineralogy, we describe fundamental methods used in this study---density functional theory (DFT), pseudopotential method (PP), density functional perturbation theory (DFPT), molecular dynamics (MD), and the quasiharmonic approximation (QHA), which allows for determination of thermodynamic properties of rocks at mantle conditions well below their melting points. Our calculations widely reproduce very well high P-T experimental studies on various minerals, including Raman and infrared (IR) frequencies, thermodynamic properties, and phase transitions. This indicates QHA is a good approach to investigate thermodynamics properties and phase equilibrium of minerals. This approach is then used to investigate the nature of the major seismic discontinuities defining the Earth transition zone. We conclude that the 410- and 660-km density discontinuities can be well accounted for by phase transitions in the olivine system within the pyrolite compositional model (about 60 vol.% olivine in the upper mantle) and that the 520-km density discontinuity can not, which supports indirectly the view that calcium-perovskite exsolution from majorite garnet may play a role in explaining the 520-km density discontinuity.