Lunar Sample 64536 from Apollo 16, a mix of cataclastic ferroan anorthosite with impact melt. Image Credit:

A phaneritic, intrusive igneous rock made with a modal composition (i.e. volume%) > 90% plagioclase feldspar of undefined composition (anorthitic to albitic, or combination thereof), and a small mafic component between 0 – 10% such as pyroxene, ilmenite, magnetite, and olivine 1.

The name anorthosite is derived from the calcium-rich plagioclase feldspar end-member, anorthite. As such, the plagioclase feldspar component is generally (but not necessarily) anorthitic (An90-100), with a small albitic component (Ab0-10) and an even smaller orthoclase content (Or0-5) in a solid solution (see feldspar ternary diagram below). This general composition holds true for lunar anorthosite (a major component of the lunar highlands). However, terrestrial Archean anorthosites contain more calcic plagioclase (An80-90), while Proterozoic anorthosites contain intermediate plagioclase (An40-60). Based on phase equilibria, anorthosites with albitic plagioclase (An<40) are unlikely.