Augiitti

Augite, known in Finnish as augiitti, is a common member of the pyroxene group of inosilicate minerals. It belongs to the clinopyroxenes and forms part of the diopside–augite–hedenbergite solid-solution series. The general composition is (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6, with wide variation in calcium, sodium, iron, magnesium, aluminum, and titanium contents. Augite crystallizes in the monoclinic system and commonly occurs as prismatic to blocky crystals.

Physical properties are typical of mafic pyroxenes: color ranges from dark green to black; luster is vitreous; it has two good cleavages at about 87 and 93 degrees and a hardness of 5–6 on the Mohs scale. The specific gravity generally lies around 3.2–3.6. In hand specimen, augite is often opaque or translucent with a subdued to strong pleochroism in thin sections.

Occurrence and formation: augite is a major constituent in many igneous rocks, especially basalt, gabbro, and diorite, and appears in some high-temperature metamorphic rocks such as hornfels. It forms early in the crystallization of magnesium- and iron-rich magmas and remains stable over a range of pressures and temperatures, commonly recording the cooling history of the rock.

Related minerals: due to solid solution, augite ranges toward diopside (CaMgSi2O6) and hedenbergite (CaFeSi2O6); ferric varieties and alkali-rich members occur in specific rock types.

Uses and significance: augite is a primary rock-forming mineral in mafic rocks and serves as an indicator of high-temperature igneous processes in petrology; it has no significant ore value but is important for mineral identification and geological interpretation.