orthopyroxene

Orthopyroxene is a major subgroup of the pyroxene mineral family, a group of inosilicate minerals that crystallize in the orthorhombic system. It forms a solid-solution series with enstatite (MgSiO3) and ferrosilite (FeSiO3) and is commonly represented by the general formula (Mg,Fe)SiO3. Orthopyroxenes share the single-chain SiO4 tetrahedra of pyroxenes and are distinguished from clinopyroxenes by their orthorhombic crystal structure and related symmetry.

Orthopyroxene is abundant in mafic and ultramafic rocks such as peridotite, pyroxenite, and basalts, where it commonly coexists with olivine and clinopyroxene. It forms by crystallization of magmas at high temperatures and low water activity and by high-temperature metamorphism in granulite facies; it can also appear as grains in some metamorphic rocks. The Mg-rich end-member enstatite is typical of mantle-derived rocks, while Fe-rich ferrosilite appears in higher iron-content and high-temperature settings.

In hand specimen, orthopyroxene crystals are usually elongated and prismatic, with two good cleavages intersecting at near 90 degrees. They are typically colorless to pale green in hand specimen and have a vitreous luster. Hardness is about 5–6 on the Mohs scale. In thin section they display characteristic optical properties and are pleochroic to a weak extent.

As a major rock-forming mineral in many igneous and metamorphic rocks, orthopyroxene provides important clues to crystallization temperature, pressure, and the textural history of rocks. Its Mg–Fe ratio is used in geothermobarometry and petrogenetic modeling, aiding interpretations of mantle-derived versus crustal processes.