French anorthose, a feldspar, from a- + Greek orthos — more at orth-
First Known Use: 1863
Type of igneous rock composed predominantly of calcium-rich feldspar. It is considerably less abundant than either basalt or granite, but the complexes in which it occurs are often immense. All anorthosites found on the earth consist of coarse crystals, but some from the moon are finely crystalline.