Anorthite

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Anorthite from Japan

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.

1 Mineralogy
2 Occurrence
3 Details
4 See also
5 References

[edit] Mineralogy

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.

[edit] Occurrence

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.

[edit] Details

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