Anorthite (an = not, ortho = straight) is the calcium endmember of the plagioclase feldspar mineral series.

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.

[8] 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.

[5] It is more rare in surficial rocks than it normally would be due to its high weathering potential in the Goldich dissolution series.

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.