Phase relations of bridgmanite, the most abundant mineral in the Earth's lower mantle.

Abstract

The knowledge of phase relations of constitutive minerals is essential to investigate the structure, dynamics and evolution of the Earth and planetary interiors. This paper reviews the phase relations of bridgmanite, the most abundant mineral in the Earth's lower mantle, with an ideal composition of MgSiO₃. Bridgmanite has an orthorhombic structure with larger dodecahedral A and smaller octahedral B cation sites. The A-sites can incorporate Mg²⁺, Fe²⁺, Fe³⁺, and Al³⁺, while the B-sites accommodate Si⁴⁺, Al³⁺ and Fe³⁺. The incorporation of hydrogen and large cations like Ca is likely limited, although these issues are still debated. Al³⁺ and Fe³⁺, respectively, can form the charge-coupled components, AlAlO₃ and Fe³⁺Fe³⁺O₃ occupying both A- and B-sites. When both Al³⁺ and Fe³⁺ are present, Al³⁺ occupies B-sites, and Fe³⁺ occupies A-sites, forming Fe³⁺AlO₃. In systems with excess MgO, Al and Fe³⁺ also form the oxygen vacancy components MgAl³⁺O_2.5□_0.5 and MgFe³⁺O_2.5□_0.5. The phase relationships of bridgmanite with coexisting phases are discussed as a function of pressure, temperature, and oxygen fugacity from the simple MgSiO₃ system to the complex MgO-Fe²⁺O-Fe³⁺₂O₃-Al₂O₃-SiO₂ system.