anorthite

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The calcium-rich plagioclase feldspar with composition Ab₀An₁₀₀ to Ab₁₀An₉₀ (Ab = NaAlSi₂O₈; An = CaAl₂Si₂O₈), occurring in olivine-rich igneous rocks and rare volcanic ejecta (for example, at Mount Vesuvius, Italy, and Miyakejima, Japan). Hardness is 6 on Mohs scale, specific gravity 2.76, melting point 1550°C (2822°F). The crystal structure of Ab₀An₁₀₀ consists of an infinite three-dimensional array of corner-sharing [AlO₄] and [SiO₄] tetrahedra, alternately linked together in a framework of [Al₂Si₂O₈]_∞²⁻ composition in which charge-balancing calcium (Ca²⁺) cations occupy four distinct, irregular cavities. Natural anorthite has no commercial uses, but the synthetic material [CaO · Al₂O₃ · 2SiO₂ (known as CAS₂)] is important in the ceramic industry and in certain composite materials with high-temperature applications. See also Feldspar.

Anorthite
Anorthite crystals in a basalt vug from Vesuvius (size:6.9 x 4.1 x 3.8 cm)
General
Category	Feldspar mineral
Chemical formula	CaAl2Si2O8
Crystal symmetry	Triclinic 1 pinacoidal
Unit cell	a = 8.1768 Å, b = 12.8768 Å, 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
Crystal system	Triclinic
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	[1][2][3]

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.

Contents 1 Mineralogy 2 Occurrence 3 See also 4 References

Mineralogy

Anorthite crystal in lava from Miyake Island, Japan (size: 2.4 x 1.7 x 1.7 cm)

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 rock. 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 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.

See also

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

References

Wikimedia Commons has media related to: Anorthite

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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