Anorthite
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Template:Infobox mineral 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 igneous rocks.
MineralogyEdit
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.<ref name="Ndimofor2018">Template:Cite book</ref> 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
At standard pressure, pure anorthite (An100) melts at 1550 ± 2 °C (2822 °F).<ref name="Rakin & Wright (1915)">Template:Cite journal</ref>
OccurrenceEdit
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.<ref name=Handbook/> Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823.<ref name=Webmineral/> 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.