Anorthosite is an scarce, coarsely-crystalline, intrusive igneous rock that is exclusively or near-exclusively composed of Ca-rich plagioclase feldspar. Minor amounts of blackish-colored pyroxene are usually present. Anorthosites are typically mottled light to medium to dark gray in color. Some have iridescent flashes of blue if the plagioclase feldspar mineral labradorite is present.
Most anorthosites on Earth are Mesoproterozoic in age. Some famous anorthosite localities include the Adirondack Mountains of New York State, the southern Laramie Range of Wyoming, Labrador, and the Moon.