Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present. Anorthosites are of enormous geologic interest, because it is still not fully understood how they form. Most models involve separating plagioclase crystals based on their density. Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there. Anorthosite on Earth can be divided into five types: Archean-age anorthosites Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth Layers within Layered Intrusions (e.g., Bushveld and Stillwater intrusions) Mid-ocean ridge and transform fault anorthosites Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts) Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins. Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research. Proterozoic anorthosite massifs Age Proterozoic anorthosites were emplaced during the Proterozoic Eon (ca. 2,500–542 Ma), though most were emplaced between 1,800 and 1,000 Ma. Occurrence Proterozoic anorthosites typically occur as extensive stocks or batholiths. The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds of square kilometers) to nearly , in the instance of the Nain Plutonic Suite or Mistastin crater in northern Labrador, Canada. Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the Appalachian Mountains (e.g., the Honeybrook Upland of eastern Pennsylvania), eastern Canada (e.g., the Grenville Province), across southern Scandinavia and eastern Europe. Mapped onto the Pangaean continental configuration of that eon, these occurrences are all contained in a single straight belt, and must all have been emplaced intracratonally. The conditions and constraints of this pattern of origin and distribution are not clear. However, see the Origins section below. Related rocks Many Proterozoic anorthosites occur in spatial association with other highly distinctive, contemporaneous rock types: the so-called 'anorthosite suite' or 'anorthosite-mangerite-charnockite-granite (AMCG) complex'. These rock types can include: Mangerite: a pyroxene-bearing monzonite intrusive igneous rock Charnockite: an orthopyroxene-bearing quartz-feldspar rock, once thought to be intrusive igneous, now recognized as metamorphic Iron-rich felsic rocks, including monzonite and rapakivi granite Iron-rich diorite,