A medium- to coarse-grained igneous rock, grey to white in colour. It is composed almost entirely of calcium-rich plagioclase feldspar (95%), one of the most common rock-forming minerals, together with minor amounts of pyroxene (4%), olivine, and iron oxides. Anorthosite is thought to comprise a significant fraction of the lunar crust, and is present at all the Apollo and Luna landing sites. Samples of anorthosites 4.4–4.5 billion years old were among the oldest lunar rocks returned by spacecraft.