Enstatite and ferrosilite form almost complete solid solution (Mg 2+ substitutes for Fe 2+ up to about 90 mol%). Solid solutions are dominated by Ca 2+, Fe 2+ and Mg 2+. Since pyroxenes found in nature or planetary settings rarely exceed 50 mol% Ca, the most useful part of the diagram is the “pyroxene quadrilateral” where compositions are displayed in the lower half of the Mg 2Si 2O 6 Fe 2Si 2O 6 CaSiO 3 ternary. Note that wollastonite is considered a pyroxenoid and is the most common of the pyroxenoids. Most important group of rock-forming ferromagnesian silicates whose major end members are enstatite (MgSiO 3), ferrosilite (FeSiO 3), and, compositionally, wollastonite (CaSiO 3). Image Source: SkyFall Meteorites (Mendy Ouzillou) Ternary Composition Diagram for Pyroxenes.
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