DEFINITION OF PEGMATITE

A very coarse-grained igneous rock that typically

forms dikes, irregular lenses, or veins along the margins of

batholiths and plutons. Most crystals in pegmatites are larger

than 0.4 inches (1 cm) in diameter and form an interlocking

network of grains. The majority of pegmatites have approximately

granitic compositions, containing quartz, feldspars,

and mica, but many pegmatites also contain rare minerals

including gems and semiprecious minerals such as emeralds,

rubies, sapphires, and tourmalines. They may also be enriched

in rare elements including rare earth elements, lithium, uranium,

boron, fluorine, niobium, tantalum, and beryllium. Pegmatites

form in the late stages of crystallization of granitic

plutons when the residual fluids are enriched in volatiles and

there is significant interaction between the fluids from the pluton

and hydrothermal fluids from the country rock.

See also GRANITE; PLUTON.

Pennsylvanian See CARBONIFEROUS.

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