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Anorthosite is a phaneritic, plutonic, igneous rock characterized by the predominance of plagioclase feldspar (90-100%), and a minimal mafic component (0-10%).

It is closely related to the diorites and gabbros, which are all plagioclase feldspars. It differs from the others of this group mainly by having little or no mafic minerals. The feldspar is usually labradorite or bytownite: the former may be highly irridescent in well-developed examples, and is a collector and lapidary item.

Anorthosite is less common and less widespread than other related igneous rocks, but is found in very large deposits in certain locations. Examples of major occurrences are in the American Southwest, Appalachian Mountain chain, southeast Canada, across southern Scandinavia and into Eastern Europe.

The occurrences are all of a similar ancient age, 1-2x109 years old, clustering around 1.3x109. Mapped onto the Pangean continental configuration of that era, they are all contained in a single straight belt. The conditions and constrains of this pattern of origin and distribution are not clear.

The main economic values of anorthosite bodies is the titanium-bearing mafic minerals. The mafics are also a good source of iron. Workable titanium minerals are easily obtained from placer deposits, thought, and more commercially developed iron ores generally displace anorthosites, making them secondary sources.

Anorthosite was prominently represented in rock samples brought back from the Moon, and is important in investigations of Mars, Venus and meteorites.

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