System MgO-SiO₂

Abstract

The two oxides, MgO and SiO₂, account for 85% of the Earth’s mantle. Thus, the phase relations in the system MgO-SiO₂ (MS), shown in Figs. 2.1 and 2.2, are fundamental for understanding the mineral composition of the mantle. There are 5 enstatite polymorphs (Mg₂Si₂O₆): protoenstatite, orthoenstatite, low clinoenstatite, high-T clinoenstatite, and high-P clinoenstatite, which can coexist in silica undersaturated compositions with forsterite, wadsleyite, or ringwoodite (Mg₂SiO₄). At high pressures, high-P clinoenstatite breaks down to majorite at the temperatures above 1600 °C, and to wadsleyite + stishovite or ringwoodite + stishovite at lower temperatures. Akimotoite and MgSiO₃ perovskite appear at progressively still higher pressures. The MgSiO₃ perovskite can coexist with periclase (MgO) in the lower mantle.