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.

This entry comes from Encyclopædia Britannica Concise.
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