augite

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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.

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.

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.

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.

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.

Fusible with some difficulty; insoluble in hydrochloric acid.

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.

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.

• Mineral compounds

Augite
Augite - Muhavura volcano
General
Category	Silicate mineral
Chemical formula	(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
Strunz classification	9.DA.15
Crystal symmetry	Monoclinic prismatic H-M symbol: (2/m) Space group: C 2/c
Unit cell	a = 9.699 Å, b = 8.844 Å, c = 5.272 Å; β = 106.97°; Z=4
Identification
Color	Black, brown, greenish, violet-brown; in thin section, colorless to gray with zoning common
Crystal habit	Commonly as stubby prismatic crystals, also acicular, skeletal, dendritic
Crystal system	Monoclinic
Twinning	Simple or multiple on {100} and {001}
Cleavage	{110} good with 87° between {110} and {110}; parting on {100} and {010}
Fracture	uneven to conchoidal
Tenacity	brittle
Mohs scale hardness	5.5 to 6
Luster	Vitreous, resinous to dull
Streak	Greenish-white
Diaphaneity	Transparent to opaque
Specific gravity	3.19 - 3.56
Optical properties	Biaxial (+)
Refractive index	nα = 1.680 - 1.735, nβ = 1.684 - 1.741, nγ = 1.706 - 1.774
Birefringence	δ = 0.026 - 0.039
Pleochroism	X = pale green, pale brown, green, greenish yellow; Y = pale brown, pale yellow-green, violet; Z = pale green, grayish green, violet
References	[1][2][3]

Augite is a common rock forming single chain inosilicate mineral with formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.

Contents 1 Characteristics 2 Locations 3 See also 4 References

Characteristics

Euhedral crystal of augite from Teide (4.4 x 3.0 x 2.3 cm)

Augite is a solid solution in the pyroxene group. Diopside and hedenbergite are important endmembers in augite, but augite can also contain significant aluminium, titanium, and sodium and other elements. The calcium content of augite is limited by a miscibility gap between it and pigeonite and orthopyroxene: when occurring with either of these other pyroxenes, the calcium content of augite is a function of temperature and pressure, but mostly of temperature, and so 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 lower temperature and is not well understood.

Locations

It's an essential mineral in mafic igneous rocks; for example, gabbro and basalt and common in ultramafic rocks. It also occurs in relatively high-temperature metamorphic rocks such as mafic granulite and metamorphosed iron formations. It commonly occurs in association with orthoclase, sanidine, labradorite, olivine, leucite, amphiboles and other pyroxenes.^[1]

Occasional specimens have a shiny appearance that give rise to the mineral's name, which is from the Greek augites, meaning "brightness", although ordinary specimens have a dull (dark green, brown or black) luster. It was named by Abraham Gottlob Werner in 1792.^[2]

Transparent augites containing dendritic patterns are used as gems and ornamental stones known as shajar in parts of India. It is found near the Ken River.^[4] Local jewelers export raw shajar stone and items to different parts of India. Banda is one city noted for trade of shazar stone.^[5]

See also

• Fassaite

References

• Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0

1. ^ ^{a b} Handbook of Mineralogy
2. ^ ^{a b} Augite on Mindat.org
3. ^ Webmineral data for Augite
4. ^ [1] Handmade in India: A Geographic Encyclopedia of India Handicrafts
5. ^ [2] The journal of the Numismatic Society of India, Volume 59

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