Anorthite

Anorthite
Anorthite crystals in a basalt vug from Vesuvius (size:6.9 × 4.1 × 3.8 cm)
General
CategoryFeldspar
Formula
(repeating unit)
CaAl2Si2O8
IMA symbolAn[1]
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°
β = 115.85°, γ = 92.22°; Z = 8
Identification
Formula mass278.203 g·mol−1
ColorWhite, grayish, reddish
Crystal habitAnhedral to subhedral granular
TwinningCommon
CleavagePerfect [001] good [010] poor [110]
FractureUneven to conchoidal
TenacityBrittle
Mohs scale hardness6
LusterVitreous
StreakWhite
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[2]
References[3][4][5]

Anorthite (an = not, ortho = straight) 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[6] but abundant on the Moon.[7]

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  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
  3. ^ Handbook of Mineralogy
  4. ^ Mindat
  5. ^ Webmineral
  6. ^ 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.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. ^ "Significant Lunar Minerals" (PDF). In Situ Resource Utilization (ISRU). NASA. Archived from the original (PDF) on 27 May 2010. Retrieved 23 August 2018.