Anorthite

Not to be confused with Anorthosite.

Anorthite
Anorthite crystals in a basalt vug from Vesuvius (size:6.9 × 4.1 × 3.8 cm)
General
Category	Tectosilicate minerals, feldspar group, plagioclase series
Formula	CaAl2Si2O8
IMA symbol	An [1]
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 conchoidal
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous
Streak	White
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	1550 ± 2°C [2]
References	[3] [2] [4] [5]

Anorthite (an = not, ortho = straight) is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl ₂ Si₂O₈. Anorthite is found in igneous rocks.

Mineralogy

Anorthite crystals (white) in lava from Miyake Island, Japan (size: 2.4 × 1.7 × 1.7 cm)

Anorthite is the calcium-rich endmember of the plagioclase solid solution series, the other endmember being albite (NaAlSi₃O₈). Pure anorthite, containing no sodium, is rare on Earth. ^[6] Anorthite also refers, however, to plagioclase compositions with more than 90 molecular percent of the anorthite endmember (and up to 10 molecular percent of the albite endmember). The composition of plagioclases is often expressed as a molar percentage of An%, or (for a specific quantity) An_n, where n = Ca/(Ca + Na) × 100. ^[7] This equation predominantly works in a terrestrial context; exotic locales and in particular Lunar rocks may need to account for other cations, such as Fe²⁺, to explain differences between optically and structurally derived An% data observed in Lunar anorthites. ^[8]

At standard pressure, pure anorthite (An₁₀₀) melts at 1550 ± 2 °C (2822 °F). ^[2]

Occurrence

Anorthite is a 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. ^[3] Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823. ^[5] 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, collected during the 1971 Apollo 15 mission, 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.

See also

• List of minerals
• Lunar highlands
• Lunar regolith

References

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. Rankin, George; Wright, Fred (1915). "The ternary system CaO-Al2O3-SiO2, with optical study by F.E. Wright". American Journal of Science (229): 1–79.
3. Handbook of Mineralogy
4. Mindat
5. Webmineral
6. Ndimofor, A.N. (2018). The Fundamentals of Crystallography & Mineralogy. Denver: Spears Media Press. p. 68. ISBN   978-1-942876-24-3.
7. Bennett, Emma N.; Lissenberg, C. Johan; Cashman, Katharine V. (21 May 2019). "The significance of plagioclase textures in mid-ocean ridge basalt (Gakkel Ridge, Arctic Ocean)". Contributions to Mineralogy and Petrology. 174 (6) 49. doi: 10.1007/s00410-019-1587-1 . PMC   6530810 .
8. Wenk, H. -R.; Wilde, W. R. (1 August 1973). "Chemical anomalies of Lunar plagioclase, described by substitution vectors and their relation to optical and structural properties". Contributions to Mineralogy and Petrology. 41 (2): 89–104. doi:10.1007/BF00375035.