Anorthite

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
Formula mass	278.203 g·mol−1
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°
Melting point	1553 °C
References

Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al₂Si₂O₈. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth^[5] but abundant on the Moon.^[6]

Mineralogy[]

Anorthite crystals (white) 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. At standard pressure, anorthite melts at 1,553 °C (2,827 °F).^[1]

Occurrence[]

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.^[2] Its type localities are Monte Somma and , Italy. It was first described in 1823.^[4] 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, a rock 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.

References[]

^ Jump up to: ^a ^b J.R. Goldsmith (1980): The melting and breakdown reactions of anorthite at high pressures and temperatures. Am. Mineralogist. 65, 272-284, http://www.minsocam.org/ammin/AM65/AM65_272.pdf
^ Jump up to: ^a ^b Handbook of Mineralogy
^ Mindat
^ Jump up to: ^a ^b Webmineral
^ Deer, W.A., Howie, R.A. and Zussman, J. (1966). An Introduction to the Rock Forming Minerals. London: Longman. p. 336. ISBN 0-582-44210-9.CS1 maint: multiple names: authors list (link)
^ "Significant Lunar Minerals" (PDF). In Situ Resource Utilization (ISRU). NASA. Archived from the original (PDF) on 27 May 2010. Retrieved 23 August 2018.

[minsocam.org-1] Jump up to: ^a ^b J.R. Goldsmith (1980): The melting and breakdown reactions of anorthite at high pressures and temperatures. Am. Mineralogist. 65, 272-284, http://www.minsocam.org/ammin/AM65/AM65_272.pdf

[Handbook-2] Jump up to: ^a ^b Handbook of Mineralogy

[3] Mindat

[Webmineral-4] Jump up to: ^a ^b Webmineral

[Deer_etal_1966-5] Deer, W.A., Howie, R.A. and Zussman, J. (1966). An Introduction to the Rock Forming Minerals. London: Longman. p. 336. ISBN 0-582-44210-9.CS1 maint: multiple names: authors list (link)

[NASA1-6] "Significant Lunar Minerals" (PDF). In Situ Resource Utilization (ISRU). NASA. Archived from the original (PDF) on 27 May 2010. Retrieved 23 August 2018.

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Anorthite

Contents

Mineralogy[]

Occurrence[]

See also[]

References[]