Anorthosite

Anorthosite is a coarse-grained, intrusive igneous rock dominated by plagioclase feldspar, typically calcium-rich varieties such as anorthite or labradorite, often comprising more than 90% of the mineralogy. It usually contains little to no quartz and only minor amounts of mafic minerals such as pyroxene or olivine. The rock is pale to medium gray and displays a granular texture with interlocking crystals.

In formation, anorthosites form by fractional crystallization of basaltic magmas or by cumulate processes in magma chambers, resulting in the bulk accumulation of plagioclase crystals. In planetary geology, the term is used for rocks that crystallized from magmas in which plagioclase floated to form crustal layers.

On the Moon, the crust is dominated by anorthosite, formed when plagioclase crystals floated to the top of a global magma ocean about 4.5 billion years ago, creating the bright, highly reflective lunar highlands.

Terrestrial occurrences are widespread in ancient continental crust. The Adirondack Mountains in New York are a notable example where extensive anorthosite exposures occur, as are various Precambrian shield areas around the world in association with layered intrusions.

In addition to its geological significance, anorthosite is used as a decorative stone in some architectural and landscape applications due to its light color and uniform mineralogy. It is studied for insights into early planetary differentiation and crust formation.