Anorthite
Calcium endmember of the plagioclase feldspar series
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Through Calcium, Aluminium and Igneous rock
Mineralogy
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. 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. 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.
At standard pressure, pure anorthite (An100) melts at 1550 ± 2 °C (2822 °F).
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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. Its type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823. 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.