Augite

Augite is a fundamental, widespread, and geologically indispensable mineral—the most common member of the clinopyroxene subgroup and one of the most abundant silicate minerals in Earth’s crust and upper mantle. Whenever you examine a dark basalt lava flow, a gabbro intrusion, or the dark mineral grains in a slice of andesite under a petrographic microscope, the dominant dark mineral is almost certainly augite. Without augite and the pyroxene group, our understanding of igneous petrology, planetary geology, and mantle processes would be vastly impoverished.

The name was coined in 1792 by German mineralogist Abraham Gottlob Werner from the Greek auge (shine, luster)—a reference to the bright vitreous luster on freshly broken cleavage faces that Werner observed in well-formed volcanic crystals from sites like Mount Vesuvius.

Mineralogy and Crystal Chemistry

Augite is a clinopyroxene (monoclinic pyroxene) with the general formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆. This complex formula reflects the extensive solid solution that characterizes augite: calcium, sodium, magnesium, iron, aluminum, and titanium can all substitute for each other within specific structural sites. This chemical flexibility makes augite one of the most compositionally variable common minerals—essentially a catch-all for the calcium-iron-magnesium pyroxene field that doesn’t fit more specific end-member designations like diopside, hedenbergite, or enstatite.

Augite belongs to the single-chain inosilicate family: the crystal structure consists of infinite chains of (Si,Al)O₄ tetrahedra running parallel to the c-axis, with Ca, Mg, Fe, and Al ions linking adjacent chains. The bonding within the chains is much stronger than between chains, which produces the characteristic two-direction pyroxene cleavage.

Formation and Geological Occurrence

Augite is a high-temperature mineral that crystallizes early in the cooling sequence of mafic and intermediate magmas:

Basalt and Gabbro: Augite is the primary dark mineral in basalt (fine-grained, extrusive) and gabbro (coarse-grained, intrusive)—the rocks that form oceanic crust and large igneous intrusions. In basalt, augite appears as black phenocrysts or as part of the fine-grained groundmass.

Andesite and Diorite: Augite is common in intermediate rocks, though hornblende often replaces it as conditions become more hydrous.

Pyroxenite: Coarse-grained rocks composed almost entirely of pyroxene; may form as cumulate layers in large layered intrusions.

Lunar and Meteoritic Rocks: Augite has been identified in lunar basalts returned by the Apollo missions and in stony meteorites (chondrites and achondrites). The Moon’s dark mare regions are largely basaltic flows rich in augite and plagioclase. Studying augite compositions in meteorites allows planetary scientists to infer the thermal histories of parent bodies in the early solar system.

High-Grade Metamorphic Rocks: In granulite-facies metamorphic rocks formed at very high temperatures (>700°C), augite is a stable pyroxene phase.

Mantle Xenoliths: Augite occurs in peridotite xenoliths—fragments of Earth’s upper mantle carried to the surface by deep-sourced volcanic eruptions—providing direct samples of mantle material.

Crystal Form and Physical Properties

Augite crystallizes in the monoclinic system, forming short, stubby, 8-sided prismatic crystals with characteristic squared cross-sections. Well-formed augite phenocrysts in volcanic rocks (particularly in alkaline basalts associated with ocean islands and rift zones) can be striking, with smooth, lustrous black crystal faces.

Hardness: 5.5 to 6 on the Mohs scale.

Cleavage: Two directions of good cleavage intersecting at approximately 87° and 93°—nearly but not perfectly perpendicular. This near-90° intersection is the key diagnostic feature distinguishing pyroxenes (87–93°) from amphiboles (56–124°). In hand specimen, the cleavage produces blocky, roughly square or rectangular fragments.

Color: Dark green to very dark brownish-green to black, reflecting iron content. High-Ti augite in alkaline basalts can appear purple-tinged in transmitted light.

Luster: Vitreous on cleavage faces; somewhat dull on irregular fractures.

Specific Gravity: 3.19–3.56, varying with iron content (higher Fe = higher SG).

Transparency: Opaque in hand specimen; very thin sections appear greenish-brown in transmitted light under a petrographic microscope.

Augite vs. Hornblende: The Key Field Distinction

One of the most practically important mineralogical identifications in petrography is distinguishing augite from hornblende—two minerals that are frequently dark, opaque, and superficially similar in hand specimen. The key distinctions:

Feature	Augite (Pyroxene)	Hornblende (Amphibole)
Cleavage angle	87–93° (near-perpendicular)	56–124° (diamond shape)
Crystal shape	Short, stubby 8-sided prisms	Long, slender 6-sided prisms
Luster	Slightly duller	Often slightly more metallic
Specific gravity	3.2–3.6	3.0–3.5

In thin section under polarized light, augite and hornblende have very different optical properties (extinction angles, birefringence, absorption patterns) that make them immediately distinguishable to a trained petrographer.

Scientific Applications

Augite’s complex chemistry makes it invaluable for geochemical and petrological research:

Geothermobarometry: The temperature and pressure at which augite crystallized can be calculated from the partitioning of elements between augite and coexisting minerals (particularly olivine, plagioclase, and orthopyroxene). This provides quantitative information about magma conditions and crustal depths.

Planetary Science: Augite compositions in lunar basalts and achondrite meteorites constrain the differentiation and thermal histories of the Moon and asteroid parent bodies. The discovery of augite in Mars rocks by Mars rovers helped confirm that Martian basalts are broadly similar in mineralogy to terrestrial oceanic basalts.

Ocean Floor Spreading: Augite-bearing basalts of the mid-ocean ridge system record the geochemical signature of the mantle source region, allowing reconstruction of mantle compositions and flow patterns over geological time.

Identification

In the field, augite is identified by: dark green to black color; stubby 8-sided crystal habit; near-perpendicular cleavage; occurrence in basalt or gabbro; hardness ~6. Under the microscope, the characteristic augite optical properties (high birefringence, inclined extinction, typical pleochroism from colorless to pale green to light brown) make identification straightforward.

Collecting Augite

Large, well-formed augite phenocrysts from volcanic localities—particularly alkaline basalts from ocean islands and continental rift settings—can be spectacular mineral specimens. Notable localities for collector-quality augite include Vesuvius and Etna in Italy; the Laacher See volcanic field in Germany; and various rift valley basalts in East Africa. Augite within the matrix of polished basalt slabs can be appreciated for its dark, crystalline beauty in architectural stone applications.

Metaphysical Properties

In crystal healing traditions, augite is considered a deeply grounding stone. Its dark color, volcanic origin, and connection to Earth’s deep mantle associate it strongly with root chakra energy. Practitioners believe it anchors the user to the physical plane, promotes endurance, stability, and practical focus during chaotic times. Its origins in the deepest, hottest parts of the earth make it a stone associated with primal strength, transformation under pressure, and the patient, unstoppable power of geological forces over time.