THEORETICAL INVESTIGATION OF MINERALS IN THE D'' LAYER

The Earth's lower mantle is made of Fe- and Al- bearing MgSiO₃ perovskite, (Mg,Fe)O magnesiowüstite and CaSiO₃ perovskite. Recently a phase transition in MgSiO₃ perovskite offered mineralogical explanations for the appearance of the D'' layer, the lowermost hundreds of kilometers of the mantle, lying at the contact with the core. Due to the extreme thermodynamic conditions ranging at these depths, which are hardly attainable experimentally, theoretical and computational mineral physics are excellent alternative tools of investigation.

We follow this approach and exemplify the use of first-principle calculations based on the density-functional theory to explore properties of Fe- and Al- bearing perovskite and post-perovskite at conditions characteristic for the Earth's lower mantle and beyond.

We discuss the MgSiO₃ phase diagram and show that the addition of Fe²⁺ reduces and the addition of Al increases the perovskite-to-post-perovskite transition pressure. We analyze in detail the effect of Fe²⁺ on the crystal structure of MgSiO₃ post-perovskite, and discuss issues related to the spin state of iron. We show that the structure expands with the addition of iron. This increase is constant with pressure for the a (the direction of edge-sharing of octahedra) and b (the direction perpendicular to the octahedral layers) lattice parameters and increases with increasing pressure for the c (the direction of corner-sharing of octahedra) lattice parameter.

We calculate the elastic constants and their evolution with respect to pressure for both perovskite and post-perovskite. On this basis we compute the seismic properties of different assemblages and show that we may correlate the preferred grain orientation and the chemical variation with some seismological observations, like anomalies in shear velocity and seismic anisotropy.

We also calculate the thermodynamic properties and thermal equation of state of perovskite and post-perovskite and compute the powder Raman spectra. We obtain an excellent agreement between the theoretical and experimental Raman spectra for perovskite and predict the peak position and intensity of the spectra for post-perovskite.