Anorthite

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Anorthite

Anorthite from Japan
General
Category Feldspar mineral
Chemical formula CaAl2Si2O8
Identification
Color White, grayish, reddish
Crystal habit Anhedral to subhedral granular
Crystal system Triclinic
Twinning Common
Cleavage Perfect [001] good [010]
Fracture Uneven to concoidal
Tenacity Brittle
Mohs Scale hardness 6
Luster Vitreous
Refractive index nα = 1.573 - 1.577 nβ = 1.580 - 1.585 nγ = 1.585 - 1.590
Optical Properties Biaxial (-), 2V 78° to 83°
Birefringence δ = 0.012 - 0.013
Specific gravity 2.72 - 2.75
Diaphaneity Transparent to translucent
References [1][2][3]

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.

Contents

[edit] 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.

[edit] 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.[1] Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823.[3]

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.

[edit] See also

[edit] References

  1. ^ a b http://www.handbookofmineralogy.org/pdfs/anorthite.pdf Handbook of Mineralogy
  2. ^ http://www.mindat.org/min-246.html Mindat
  3. ^ a b http://webmineral.com/data/Anorthite.shtml Webmineral
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