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  Anorthite is a compositional variety of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl2Si2O8.



Anorthite is the calcium-rich endmember of the plagioclase solid solution, the other endmember being albite, NaAlSi3O8. Anorthite also refers to plagioclase compositions with more than 90 molecular percent of the anorthite endmember.


Anorthite is a rare compositional variety of plagioclase. It occurs in unusual igneous and metamorphic rocks on Earth. Its type localities are Monte Somma and Valle di Fassa, Italy.

It also makes up much of the lunar highlands. Anorthite was discovered in samples from comet Wild 2, and the mineral is an important constituent of Ca-Al-rich inclusions in rare varieties of chondritic meteorites.


Crystallographic system Triclinic
Colours Normally colourless or white, colourless in thin section.
Lustre Vitreous
Hardness 6-6.5
Refractive index 1.51
Cleavage 3: 1 perfect, 1 good, 1 poor
Twinning A variety of types are present, including multiple lamellar and Carlsbad twins
Pleochroism None

See also


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