Anorthosite

Anorthosite is a coarse-grained intrusive rock composed predominantly of plagioclase feldspar, commonly labradorite or bytownite. Its origin often involves the accumulation or separation of plagioclase crystals in magma.

Plagioclase: A group of feldspar minerals forming a continuous solid-solution series from albite to anorthite. It is the dominant mineral, and its composition commonly identifies anorthosite varieties.
Crystal fractionation: The separation of crystals from magma as they form, changing the composition of the remaining melt. Removing or concentrating crystals can create plagioclase-rich rocks.
Lunar highlands: The ancient, heavily cratered regions of the Moon, dominated by light-colored crustal rocks. Their plagioclase-rich crust is a planetary-scale example of anorthosite.
Proterozoic anorthosite: Anorthosite formed mainly during the Proterozoic, often in large massifs emplaced in continental crust. Most enormous terrestrial anorthosite massifs belong to this distinctive geological episode.
Labradorite: A plagioclase feldspar with an intermediate calcium content, known for its characteristic iridescent optical effect. This feldspar commonly makes up the plagioclase-rich rock.
Crystal settling: The movement of crystals through magma under gravity because of density differences. Settling can concentrate plagioclase into layers or accumulations.
Lunar magma ocean: A proposed early lunar layer of global or near-global molten rock that crystallized to form the Moon's crust and mantle. Plagioclase flotation during its crystallization is a leading explanation for lunar anorthosite.
Anorthosite massifs: Large, commonly Proterozoic intrusive complexes dominated by anorthosite and associated mafic rocks. They are the characteristic setting for many major terrestrial bodies of the rock.
Bytownite: A calcium-rich plagioclase feldspar intermediate between labradorite and anorthite. It is another common plagioclase composition in anorthosite.
Magma chamber: A subsurface body of molten or partly molten rock where magma can evolve and crystals can accumulate. Many anorthosites are interpreted as products of crystal processes within large magma bodies.

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