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Pegmatite

A coarse-grained granite, sometimes rich in rare elements such as uranium, tungsten, and tantalum.

Almost any wholly crystalline igneous rock that is at least in part very coarse-grained, the major constituents of which include minerals typically found in ordinary igneous rocks (such as granites) and in which extreme textural variations, especially in grain size, are characteristic. Usually found as irregular dikes, lenses, or veins, pegmatite deposits occur in all parts of the world and are the chief source of commercial feldspar, sheet mica, and beryllium, tantalum-niobium, and lithium minerals.

A very coarse plutonic rock, generally granitic in composition. Usually forming dikes that cut granite or the gneisses and schists that border granite masses. They represent the last liquid portion of the crystallizing magma. They are coarse because the liquid residue at the time of their crystallization contained a high percentage of water and other volatile elements that did not go into the makeup of the common minerals of granite, and which were for that reason concentrated in the residue. They are interesting mineralogically because minerals of the rarer elements are found with the coarse quartz, feldspar, and mica that principally compose them.

Pegmatites are bodies of unusually coarse-grained rock formed deep in the crust of the Earth. They precipitated from a fluid phase separated from the residual granitic melt in the final stages of the solidification of magma.

Generally, pegmatites have a granitic composition and occur as dikes, lenses or veins. Most of their minerals are more than 1 cm in diameter, but individual crystals can reach tens of meters in length. Simple pegmatites contain albite, quartz, microcline and possible minor muscovite.

Complex pegmatites carry rare mberylinerals such as columbite, beryl, zircon, monazite, polycrase and uraninite. The major pegmatites of New York occur primarily in Westchester, Saratoga, Warren and Essex Counties. They were an important industrial source of feldspar for ceramics and mica for insulators and also provided rare elements such as niobium, beryllium, and uranium for high tech industries.

See also: Pegmatite International, a site devoted to the mineralogy, geochemistry, petrogenesis, and economic occurrences of pegmatites and related rocks.

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