Augite

Augite is a common member of the clinopyroxene group, a monoclinic pyroxene mineral that forms in igneous and metamorphic rocks. Its general chemical formula is (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6, reflecting a complex solid solution among diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6) with possible sodium and aluminum substitutions. In practice, augite typically contains calcium, magnesium, iron, and aluminum as major components, with silicon and oxygen in the pyroxene framework.

Augite crystals are usually dark green to black with a vitreous luster. It has a Mohs hardness of about 5.5–6 and a specific gravity around 3.2–3.5. Cleavage is two directions at nearly 90 degrees, producing prismatic crystals that often show fine striations. The mineral exhibits weak pleochroism and tends to darken in color in lighter light, reflecting its mixed iron-magnesium composition.

Geological occurrence: augite is a defining mineral in many mafic and ultramafic rocks, including basalt, andesite, diorite, gabbro, and peridotite-derived rocks. It also appears in high-temperature metamorphic rocks such as granulites and hornfelses. It crystallizes from silicate melts at relatively high temperatures and remains stable over a range of crustal conditions.

Significance: as a major rock-forming mineral, augite aids in classifying volcanic and plutonic rocks and informs crystallization histories and tectonic settings. Its presence helps distinguish pyroxene-rich suites from amphibole or feldspathic minerals in petrological studies.