1989 Edition


  • A. E. Wright
Reference work entry

Anorthosite is a coarse-grained rock composed essentially of plagioclase feldspar in the range andesine to anorthite. The term was coined by T. S. Hunt in 1863 to apply to rocks composed of “anorthose” feldspars, an obsolete term for triclinic feldspars. Anorthosite is now universally applied to plagioclase-rich rocks of whatever origin, although no single quantitative mineralogical definition is in general use. Anorthosites are polygenetic and there are many different origins, igneous, metamorphic and metasomatic, and all present problems in petrogenesis. For igneous anorthosites there are three variables to consider in the definition of the rock, viz. (1) plagioclase %, (2) composition of the plagioclase, and (3) quartz % The minimum % of plagioclase is variously recommended as 85, 90 or 95%, 90% being the preferred usage (Buddington 1939; Streckeisen, 1967). Generally with a fall in plagioclase % the rock grades into gabbroic anorthosite or norite. In differentiates of basic rocks,...

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© Van Nostrand Reinhold 1989

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