| Anorthite | |
|---|---|
 | |
| General | |
| Category | Tectosilicateminerals,feldspargroup,plagioclase series |
| Formula | CaAl2Si2O8 |
| IMA symbol | An[1] |
| Strunz classification | 9.FA.35 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (sameH-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 thecalciumendmember of theplagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl2Si2O8. Anorthite is found inigneous rocks.
Anorthite is the calcium-rich endmember of the plagioclasesolid solution series, the other endmember beingalbite (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) Ann, 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 betweenoptically andstructurally derived An% data observed in Lunar anorthites.[8]
Atstandard pressure, pure anorthite (An100) melts at 1550 ± 2 °C (2822 °F).[2]
Anorthite is a compositional variety of plagioclase. It occurs inmaficigneous rock. It also occurs inmetamorphic rocks ofgranulitefacies, in metamorphosedcarbonate rocks, andcorundum deposits.[3] Itstype localities areMonte Somma andValle 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 theGoldich dissolution series.
It also makes up much of thelunar highlands; theGenesis Rock, collected during the 1971Apollo 15 mission, is made ofanorthosite, a rock composed largely of anorthite. Anorthite was discovered in samples fromcometWild 2, and the mineral is an important constituent ofCa-Al-rich inclusions in rare varieties ofchondritic meteorites.
