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Definition: anorthosite from The Chambers Dictionary

a coarse-grained rock consisting almost entirely of plagioclase feldspar. [Fr anorthose (from Gr an- (privative), and orthos right) and - ite (3)]

Summary Article: Anorthosite
From Rock and Gem

AN INTRUSIVE IGNEOUS ROCK, anorthosite is composed of at least 90 per cent calcium-rich plagioclase feldspar – principally labradorite and bytownite. Olivine, pyroxene, garnet, and iron oxides make up the remaining 10 per cent. Anorthosite is not a common rock on Earth, but where it does occur, it is found as immense masses, or as layers between mafic and ultramafic rocks such as gabbro and peridotite. There are large anorthosite-bearing rock bodies in New York State and Montana, USA; eastern Canada; and South Africa. Many anorthosites have an interesting “cumulate” texture, where well-formed crystals appear to have settled out of the liquid magma, in a similar way to a sediment. Anorthosite is extremely common on the surface of the Moon, especially the far side. The ancient, rough, light-colored highlands of the Moon are made of anorthosite and similar rocks.

  • Properties
  • Rock type Ultramafic, plutonic, igneous
  • Major minerals Calcium plagioclase
  • Minor minerals Olivine, pyroxene, garnet
  • Color Light gray to white
  • Texture Medium to coarse

Anorthosite from the Moon

Fine-grained anorthosite is thought to be the first rock that crystallized from the “magma ocean” that covered the Moon’s surface early in its history.

© 2008 Dorling Kindersley Limited

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