augite

n.
A dark-green to black pyroxene mineral, (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆, that contains large amounts of aluminum, iron, and magnesium.

[Latin augītis, a precious stone, from Greek augītēs, from augē, brightness.]

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A group of monoclinic calcic pyroxenes which have the general chemical formula (Ca,Mg,Fe)(Mg,Fe)Si₂O₆, in which calcium is the dominant cation in the first cation position. Monoclinic pyroxene with substantial iron or magnesium in place of calcium is called pigeonite, and has a different crystal structure from augite. Augite is generally considered a combination of the four end members diopside (CaMgSi₂O₆), hedenbergite (CaFe²⁺Si₂O₆), enstatite (Mg₂Si₂O₆), and ferrosilite (Fe²⁺₂Si₂O₆), but it almost always has substantial aluminum and minor to substantial amounts of sodium, ferric iron, chromium, and titanium.

Augite occurs in both igneous and metamorphic rocks. It is nearly universal in basalts and gabbros, and occurs somewhat less frequently in less mafic igneous rocks. Magnesium-rich augite is a characteristic mineral in many ultramafic rocks and in rocks of the Earth's mantle. Augite and pigeonite are also rather common constituents of lunar basalts and basaltic meteorites. See also Diopside; Eclogite; Enstatite; Pigeonite; Pyroxene.

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(Ca,Na)(Mg,Fe,Al)(Si,Al)
Monoclinic -- prismatic

Environment

A rock-making mineral, the most abundant pyroxene and an essential dark component of many of the darker plutonic rocks. A major constituent grain of basalt and traprock, and common as larger porphyritic crystals in fine-grained igneous rocks and in volcanic ejecta.

Crystal description

The best crystals, loose or weakly cemented in andesite and basalt porphyries, usually are not over an inch (2 cm or so) in size. They are commonly very perfect, however. Granular, massive augite is frequent in other rocks -- including pyroxenites, which are almost wholly augite.

Physical properties

Black, yellow-brown in microcrystalline forms. Luster glassy; hardness 5-6; specific gravity 3.2-3.4; fracture uneven; cleavage perfect prismatic at 87°-93°. Black crystals translucent only on the thinnest splinter edges.

Composition

Actually a whole series of minerals with different names, from aluminum substitutions for both magnesium and silicon. The Al ₂ O ₃ ranges from 3% to 9%. Magnesium and iron percentages also vary, as do calcium and sodium.

Tests

Fusible with some difficulty; insoluble in hydrochloric acid.

Distinguishing characteristics

Free crystals are very typical and easily recognized. Massive material is distinguished from amphibole by its acute cleavage angle, and from tourmaline by prismatic cleavage. Acmite-aegirine crystals are more elongated with a very different (alkali) paragenesis; diopside-hedenbergite is lighter in color.

Occurrence

Common as a rock constituent in the U.S., but well-formed crystals freed from volcanic rocks are not common. At Vesuvius, when the crater had a floor, Ɖ-inch (1-2 cm) crystals were abundant in the rotted lava that filled it. Similar crystals are found in the ancient lavas of the German Eifel, Bohemia, and near Barcelona, Spain. Loose, perfect crystals of Vesuvian size, exploded, sharp, and free of adhering lava, all formed during the recurrent paroxysms of the Stromboli volcano in Italy. Tiny crystals, with olivine, form a sandy layer on the summit rim of Haliakala, Maui.

Large crystals are found in the marble of St. Lawrence Co., New York, and in calcite-rich deposits in Renfrew Co., Ontario. A black pyroxene is also one of the once-abundant minerals from Franklin, New Jersey.

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