A rock of mafic composition, the total silica content is less than 55 percent, and quartz is virtually absent. Anorthosite comprises at least 90 percent plagioclase feldspar (labradorite-bytownite). Other minerals in the rock include olivine, pyroxene, and iron oxides. Garnet sometimes forms in reaction rims around pyroxene.

TEXTURE Generally coarse-grained, granular, and light in color, these rocks may have a parallel alignment of dark minerals.

ORIGIN Forms in plutonic environments in stocks, dykes, and sheet-shaped intrusions. It is often associated with gabbros in layered sequences and makes up the light-colored regions on the Moon’s surface.

Group: IGNEOUS

Origin: Intrusive

Grain size: Coarse

Crystal shape: Anhedral, Euhedral

Classification: Mafic

Occurrence: Pluton

Color: Light