Anorthite

Anorthite
Anorthite crystals in a basalt vug from Vesuvius (size:6.9 × 4.1 × 3.8 cm)
General
Category	Feldspar
Formula (repeating unit)	CaAl2Si2O8
Strunz classification	9.FA.35
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 8.1768, b = 12.8768 c = 14.169 [Å]; α = 93.17° β = 115.85°, γ = 92.22°; Z = 8
Identification
Color	White, grayish, reddish
Crystal habit	Anhedral to subhedral granular
Twinning	Common
Cleavage	Perfect [001] good [010] poor [110]
Fracture	Uneven to concoidal
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	2.72 – 2.75
Optical properties	Biaxial (-)
Refractive index	nα = 1.573 – 1.577 nβ = 1.580 – 1.585 nγ = 1.585 – 1.590
Birefringence	δ = 0.012 – 0.013
2V angle	78° to 83°
References

Anorthite is the calcium endmember 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 series, 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 mafic igneous rock. It also occurs in metamorphic rocks of granulite facies, 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 is more rare in surficial rocks than it normally would be due to its high weathering potential in the Goldich dissolution series.

It also makes up much of the lunar highlands; the Genesis Rock is made of anorthosite, which is composed largely of anorthite. 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.