Anorthosite

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: 

1. Archean-age anorthosites

2. Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth 

3. Layers within Layered Intrusions (e.g., Bushveld and Stillwater intrusions)

4. Mid-ocean ridge and transform fault anorthosites

5 . 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.