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Plagioclase

Any of a common rock-forming series of triclinic feldspars, consisting of mixtures of sodium and calcium aluminum silicates. Plagioclase is a form of feldspar that has a chemical make up of NaAlSi₃O₈. Plagioclase is usually white but can also be grey and greenish white. This mineral was abundant in the Moon rock samples. Also called oligoclase.

Any member of the series of abundant feldspar minerals that usually occur as light- to medium-grey-coloured, transparent to translucent grains or crystals. Plagioclase ranges in composition from albite to anorthite. It is used in the manufacture of glass and ceramics; iridescent varieties are valued as gemstones. The primary importance of plagioclase, however, derives from its role as a rock-forming mineral.

Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal's refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab).

Plagioclase is an important rock-forming mineral and occurs widely, primarily in igneous rocks such as basalts. It can sometimes be confused with alkali feldspars (such as orthoclase) but can be distinguished by striations on the cleavage surfaces caused by the intersection of lamellar twins with the cleavage surface.

The plagioclase feldspar family is the most common mineral family in the earth's crust. It is almost ubiquitous in igneous rocks. Gabbros, basalts and anorthosites tend to have more calcium-rich plagioclase, generally labradorite. Andesites and diorite tend to have moderately sodium-rich plagioclase, usually in the andesine range. Granitic or syenite rocks tend to have more sodium-rich plagioclase. Nearly pure albite occurs in granitic pegmatites, some times as platy variety called "clevelandite".

Plagioclase does not survive extensive weathering, generally breaking down to clay minerals. Some plagioclase, however, may occur in greywacke or arkoses formed by rapid erosion and burial of material. Metamorphic rocks may be rich in plagioclase, particular gneisses and granulate and amphibolites. Nearly pure calcium-rich plagioclase may occur in some marbles.

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