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Augite is a common rock-forming pyroxene 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.

Augite
Augite Rwanda.jpg
Augite – Muhavura volcano
General
CategoryInosilicates
Formula
(repeating unit)
(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
Strunz classification9.DA.15
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Unit cella = 9.699, b = 8.844
c = 5.272 [Å]
β = 106.97°; Z = 4
Identification
ColorBlack, brown, greenish, violet-brown; in thin section, colorless to gray with zoning common
Crystal habitCommonly as stubby prismatic crystals, also acicular, skeletal, dendritic
TwinningSimple or multiple on {100} and {001}
Cleavage{110} good with 87° between {110} and {110}; parting on {100} and {010}
Fractureuneven to conchoidal
Tenacitybrittle
Mohs scale hardness5.5 to 6
LusterVitreous, resinous to dull
StreakGreenish-white
DiaphaneityTransparent to opaque
Specific gravity3.19–3.56
Optical propertiesBiaxial (+)
Refractive indexnα = 1.680–1.735, nβ = 1.684–1.741, nγ = 1.706–1.774
Birefringenceδ = 0.026–0.039
PleochroismX = pale green, pale brown, green, greenish yellow; Y = pale brown, pale yellow-green, violet; Z = pale green, grayish green, violet
References[1][2][3]

CharacteristicsEdit

 
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.

LocationsEdit

Augite is 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]

See alsoEdit

ReferencesEdit

  • 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