Augite
A common pyroxene mineral with monoclinic crystals and near-90-degree cleavage.
Augite, also known as Augurite, is a common rock-forming pyroxene mineral with the formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. Its crystals are monoclinic and prismatic, and it exhibits two prominent cleavages that intersect at angles close to 90 degrees. As a solid solution within the pyroxene group, augite has diopside and hedenbergite as important endmembers, but it can also incorporate significant amounts of aluminium, titanium, sodium, and other elements. Its calcium content is limited by a miscibility gap with pigeonite and orthopyroxene; when these minerals occur together, the calcium content of augite is primarily a function of temperature, making it useful for reconstructing the thermal history of rocks. As temperature decreases, augite may exsolve lamellae of pigeonite or orthopyroxene. A similar miscibility gap exists with omphacite, though at higher temperatures. Augite has no industrial or economic uses.
The mineral is essential in mafic igneous rocks such as gabbro and basalt, and is common in ultramafic rocks. It also appears in high-temperature metamorphic rocks like mafic granulite and metamorphosed iron formations. It frequently occurs alongside orthoclase, sanidine, labradorite, olivine, leucite, amphiboles, and other pyroxenes. Notable specimens are found in Tillamook, Oregon, on the west coast of the United States, where an augite formation occurs in an ash tuff inclusion high in the Tillamook Forest near the Cedar Butte trailhead. This area yields notable augite crystals, both large and small, displaying a dark, black, shiny luster and clean monoclinic facets. These crystals can be found on the ground or embedded in clusters of broken ash tuff that have eroded from the sloped cliff above the trail. The mineral’s name, from the Greek *augites* meaning “brightness,” was given by Abraham Gottlob Werner in 1792, though ordinary specimens typically have a dull dark green, brown, or black luster.
- formula
- (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
- crystal_system
- Monoclinic
- cleavage
- Two prominent cleavages meeting at angles near 90 degrees
- luster
- Dull (dark green, brown or black); occasional specimens have a shiny appearance
- named_by
- Abraham Gottlob Werner
- common_occurrence
- Mafic igneous rocks (e.g., gabbro, basalt) and high-temperature metamorphic rocks
Lore & Background
Augite, whose name derives from the Greek *augites* meaning "brightness," was named by Abraham Gottlob Werner in 1792, though most common specimens exhibit a dull luster in dark green, brown, or black. Occasional specimens, however, display a shiny appearance that inspired the name. The mineral forms monoclinic, prismatic crystals and possesses two prominent cleavages that intersect at angles near 90 degrees. As a solid solution within the pyroxene group, augite incorporates diopside and hedenbergite as key endmembers, but it can also contain significant amounts of aluminum, titanium, sodium, and other elements. Its calcium content is constrained by a miscibility gap with pigeonite and orthopyroxene; when these minerals coexist, the calcium level in augite depends primarily on temperature, making it valuable for reconstructing the thermal history of rocks. As temperature decreases, augite may exsolve lamellae of pigeonite or orthopyroxene. A separate miscibility gap exists between augite and omphacite, though this occurs at higher temperatures. Augite is an essential mineral in mafic igneous rocks such as gabbro and basalt, and it is common in ultramafic rocks. It also appears in high-temperature metamorphic environments like mafic granulite and metamorphosed iron formations, often alongside orthoclase, sanidine, labradorite, olivine, leucite, amphiboles, and other pyroxenes. Notable specimens, both large and small, with a dark black shiny luster and clean monoclinic facets, are found in an ash tuff inclusion high in the Tillamook Forest near Cedar Butte trailhead in Tillamook, Oregon. These crystals occur on the ground and embedded in broken ash tuff pieces eroded from the cliff above. Augite has no industrial or economic uses.
Reader's Guide
Augite is an essential mineral in mafic igneous rocks such as gabbro and basalt, and is common in ultramafic rocks. It also occurs in relatively high-temperature metamorphic rocks like mafic granulite and metamorphosed iron formations. It commonly occurs with orthoclase, sanidine, labradorite, olivine, leucite, amphiboles, and other pyroxenes. On the west coast of the United States, Tillamook, Oregon has an Augite formation in an ash tuff inclusion high in the Tillamook Forest near the Cedar Butte trailhead, yielding notable crystal specimens with a dark black shiny luster and clean monoclinic facets. The mineral has no industrial or economic uses.
Did You Know?
- Augite is also known as Augurite.
- The calcium content of augite can be used to reconstruct temperature histories of rocks.
- Augite has no industrial or economic uses.
- The name 'augite' comes from the Greek word 'augites', meaning 'brightness'.
Frequently Asked Questions
What is Augite?
Augite (also called Augurite) is a very common rock-forming mineral in the pyroxene family with the formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. It is a solid solution, meaning its exact chemistry can shift across a range of elements rather than being a single fixed composition.
What does Augite look like?
Most specimens show up as dark green, brown, or black prismatic crystals with a generally dull surface, though a few rare pieces do display a shinier luster. The crystals grow in a monoclinic system, giving them a characteristic wedge-like shape.
How can I tell Augite apart from other pyroxenes in the field?
Its two prominent cleavage planes that intersect at angles close to 90 degrees are the most reliable field clue. Pair that with the dark color and prismatic monoclinic habit, and you can usually separate it from the more glassy-looking orthopyroxenes.
How does Augite relate to diopside and hedenbergite?
Augite is essentially a solid solution spanning between the calcium-rich diopside end and the iron-rich hedenbergite end of the pyroxene family. A given specimen's position along that spectrum is determined by how much Mg, Fe, Al, Ti, and Na it incorporates.
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