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Anorthite crystals in a basalt vug from Vesuvius (size:6.9 × 4.1 × 3.8 cm)
(repeating unit)
Strunz classification9.FA.35
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 8.1768, b = 12.8768
c = 14.169 [Å]; α = 93.17°
Formula mass278.203 g·mol−1
ColorWhite, grayish, reddish
Crystal habitAnhedral to subhedral granular
CleavagePerfect [001] good [010] poor [110]
FractureUneven to concoidal
Mohs scale hardness6
DiaphaneityTransparent to translucent
Specific gravity2.72–2.75
Optical propertiesBiaxial (-)
Refractive indexnα = 1.573–1.577 nβ = 1.580–1.585 nγ = 1.585–1.590
Birefringenceδ = 0.012–0.013
2V angle78° to 83°
Melting point1553 °C[1]

Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl2Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth[5] but abundant on the Moon.[6]


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 1 atmosphere, anorthite melts at 1553 °C.[1]


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 Valle di Fassa, 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.


See also[edit]


  1. ^ a b J.R. Goldsmith (1980): The melting and breakdown reactions of anorthite at high pressures and temperatures. Am. Mineralogist. 65, 272-284,
  2. ^ a b Handbook of Mineralogy
  3. ^ Mindat
  4. ^ a b Webmineral
  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)
  6. ^ "Significant Lunar Minerals" (PDF). In Situ Resource Utilization (ISRU). NASA. Retrieved 23 August 2018.