Anorthosite is a magmatic intrusive rock. It is light colored (leucocratic) and has a medium to coarse grain size (phaneritic). It is mainly composed of plagioclase (andesine, labradorite, bytownite) and minor pyroxene, olivine, and iron-titanium oxides (ilmenite, magnetite). Proterozoic anorthosite forms large massifs associated with granitoids (North America, Scandinavia). Archean coarse-grained (megacrystic) anorthosite occurs in intrusions (dikes and sills) and flows of basaltic composition.
Anorthosite is a common constituent of the lighter surfaces of the Moon called lunar highlands or terrae. Formation of anorthosite requires the concentration of plagioclase from mafic magma by flotation in a magma ocean (as is proposed to have occurred on the Moon), ascent of plagioclase-rich mushes, or low-pressure crystallization in magma chambers.