anorthite: see feldspar.

Feldspar mineral, calcium aluminosilicate (CaAl₂Si₂O₈), that occurs as white or grayish, brittle, glassy crystals. Primarily a rock-forming mineral, it is used in the manufacture of glass and ceramics. Anorthite occurs in basic igneous rocks (see acid and basic rocks).

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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.

Mineralogy

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

Occurrence

Anorthite is a rare compositional variety of plagioclase. It occurs in mafic igneous. It also occurs in granulite facies metamorphic rocks, in metamorphosed carbonate rocks and corundum deposits. Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823.

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.

See also

• Feldspars
• List of minerals
• In-Situ Resource Utilization

References