Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>. Anorthite is found in igneous rocks.
Mineralogy
thumb|left|Anorthite crystals (white) in lava from Miyake Island, Japan (size: 2.4 × 1.7 × 1.7 cm)
Anorthite is the calcium-rich endmember of the plagioclase solid solution series, the other endmember being albite (NaAlSi<sub>3</sub>O<sub>8</sub>). 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) An<sub>n</sub>, 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 Fe<sup>2+</sup>, to explain differences between optically and structurally derived An% data observed in Lunar anorthites.
At standard pressure, pure anorthite (An<sub>100</sub>) melts at 1550 ± 2 °C (2822 °F).
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.