anorthosite
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Lunar anorthosite, Apollo 16
Image credit: NASA
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A coarse-grained basic igneous rock made
largely of plagioclase feldspar (95%),
with small amounts of pyroxene (4%), olivine,
and iron oxides. Usually light in color, it may have bands of darker minerals.
Anorthosite forms deep underground, and in dikes
and intrusions; it makes up about 60% of the Earth's crust. Anorthosite
was found in all the rocks returned from the Moon,
including the oldest (dating back 4.4 to 4.5 billion years), and is believed
to make up a significant fraction of the lunar crust.
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