Most common pyroxene mineral, occurring chiefly as blocky crystals in basalts, gabbros, andesites, and various other dark igneous rocks. It also is a common constituent of lunar basalts and meteorites and may be found in certain metamorphic rocks, such as pyroxenites. Because the diopside-hedenbergite series and augite are nearly indistinguishable, the term augite is sometimes used to designate any dark green to black pyroxene with monoclinic symmetry (three unequal crystallographic axes with one oblique intersection).
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Augite is a mineral described chemically as (Ca,Mg,Fe)SiO3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic. Augite has two prominent prismatic cleavages, meeting at angles near 90°.
Augite is a solid solution in the pyroxene group. Diopside and hedenbergite are important endmembers in augite, but augite can also contain significant aluminium, titanium, and sodium and other elements. The calcium content of augite is limited by a miscibility gap between it and pigeonite and orthopyroxene: when occurring with either of these other pyroxenes, the calcium content of augite is a function of temperature and pressure, but mostly of temperature, and so can be useful in reconstructing temperature histories of rocks. With declining temperature, augite may exsolve lamellae of pigeonite and/or orthopyroxene. There is also a miscibility gap between augite and omphacite, but this gap occurs at lower temperature and is not well understood.
Occasional specimens have a shiny appearance that give rise to the mineral's name, which is from the Greek augites, meaning "brightness", although ordinary specimens have a dull (dark green, brown or black) finish.