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 278.20 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[1]
References [2][3][4]

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]

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

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

References

  1. 1 2 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
  2. 1 2 Handbook of Mineralogy
  3. ↑ Mindat
  4. 1 2 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.
  6. ↑ "Significant Lunar Minerals" (PDF). In Situ Resource Utilization (ISRU). NASA. Retrieved 23 August 2018.
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