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 formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. The crystals are monoclinic and prismatic, and it has two prominent cleavages meeting at angles near 90 degrees. It is a solid solution in the pyroxene group, with diopside and hedenbergite as important endmembers, and can contain significant aluminium, titanium, sodium, and other elements.
- 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 is a solid solution in the pyroxene group, with diopside and hedenbergite as important endmembers. Its calcium content is limited by a miscibility gap between it and pigeonite and orthopyroxene; when occurring with these other pyroxenes, the calcium content is a function of temperature and pressure, mostly temperature, and 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 higher temperatures.
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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