COMPOSITIONAL VARIATION OF PEROVSKITES IN EARTH'S INTERIOR

Perovskites (bridgmanite and davemaoite) are the most abundant mineral group in the Earth and major constituents of the lower mantle. The crystal structure is dependent on the composition-temperature-pressure and can range from orthorhombic, tetragonal to cubic. Bridgmanite (MgSiO₃) is orthorhombic and davemaoite (CaSiO₃) is cubic at high temperatures. Bridgmanite and davemaoite range in composition, incorporating additional Mg, Ca, minor, and trace elements. Deviations from the ideal formula may result in solid solutions, different crystal structures and thermodynamic parameters. Tetragonal perovskite may form for the MnSiO₃ perovskite or perovskites enriched in Mg and Ca. The stability and crystal chemistry of different perovskite structures is key to determining the composition and origin of heterogeneous regions in the mantle.

In this study we investigate the stability and thermal equation of state of the Mn-Fe-Ca-Mg perovskites from natural and synthetic samples. In the Ca-rich samples, we observe the formation of a cubic perovskite. In the Mn-rich samples, we observe similarities between the a and b lattice parameters such that a=b. Where a does not equal c. This relationship occurs at low temperatures and persist at high temperatures, indicating the formation of a tetragonal perovskite at lower mantle conditions.