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Augite



Augite

General
CategorySilicate mineral
Chemical formula(Ca,Mg,Fe)SiO3
Identification
ColorPale brown to purplish or greenish brown
Crystal systemMonoclinic
Cleavage{110} good
Fractureuneven
Mohs Scale hardness5 to 6.5
LusterVitreous
Refractive indexα = 1.671 - 1.735,
β = 1.672 - 1.741,
γ = 1.703 - 1.774
Specific gravity3.19 - 3.56

Augite is a single chain inosilicate mineral described chemically as (Ca,Mg,Fe)SiO3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic. Augite has two prominent prismatic cleavages, meeting at angles near 90°.

Additional recommended knowledge

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.  

It is found in igneous rocks such as gabbro and basalt. It also occurs in relatively high-temperature metamorphic rocks such as mafic granulite.

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) finish.

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
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Augite". A list of authors is available in Wikipedia.
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