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Plagioclase in Anorthosite

Gallery of Feldspars

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Plagioclase ranges in composition from Na[AlSi3O8] to Ca[Al2Si2O8]—sodium to calcium aluminosilicate—including every mixture in between. (more below)
A gravestone specimen
Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Plagioclase tends to be more transparent than alkali feldspar; it also very commonly shows striations on its cleavage faces that are caused by multiple crystal twinning within grains. These appear as the lines in this polished specimen.

Large grains of plagioclase like this specimen display two good cleavages that are off square at 94° (plagioclase means "slanted breakage" in scientific Latin). The play of light in these large grains is also distinctive, resulting from optical interference inside the mineral. Both oligoclase and labradorite show it.

The igneous rocks basalt (extrusive) and gabbro (intrusive) contain feldspar that is almost exclusively plagioclase. True granite contains both alkali and plagioclase feldspars. A rock consisting of only plagioclase is called anorthosite. A noteworthy occurrence of this unusual rock type makes up the heart of New York's Adirondack Mountains (see the next page of this gallery); another one is the Moon. This specimen, a gravestone, is an example of anorthosite with less than 10 percent dark minerals.

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