Augite

Augite
Augite - Muhavura volcano
General
Category	Inosilicates
Formula (repeating unit)	(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
Strunz classification	09.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

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.

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 higher temperatures. There are no industrial or economic uses for this mineral.

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.

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.

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. Local jewelers export raw shajar stone and items to different parts of India. Banda is one city noted for trade of shazar stone.

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