Anorthite
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 but abundant on the Moon.
| Anorthite | |
|---|---|
Anorthite crystals in a basalt vug from Vesuvius (size:6.9 × 4.1 × 3.8 cm) | |
| General | |
| Category | Feldspar |
| Formula (repeating unit) | CaAl2Si2O8 |
| IMA symbol | An |
| 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 |
| References | |
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