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| Anorthite | |
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| 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 | 123 |
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.
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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.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.
See also
References
- ^ a b http://www.handbookofmineralogy.org/pdfs/anorthite.pdf Handbook of Mineralogy
- ^ http://www.mindat.org/min-246.html Mindat
- ^ a b http://webmineral.com/data/Anorthite.shtml Webmineral
