Anorthite | |
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General | |
Category | Feldspar |
Formula (repeating unit) | 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 | 1553 °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]
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