Anorthite is one of the plagioclase feldspars, an important group of minerals abundant in the Earth's crust. Anorthite's chemical formula is CaAl2Si2O8.
Anorthite is the calcium-rich endmember of the plagioclase system, the other endmember being albite (sodium-rich). The range of intermediate compositions have varying ratios of calcium and sodium ions.
Anorthite occurs in igneous and metamorphic rocks. Its type localities are Monte Somma and Valle di Fassa, Italy. It also makes up much of the lunar highlands, and was discovered in samples from comet Wild 2.