Jahns, Richard H. Department of Earth Sciences, Stanford University, Stanford, California.
Last reviewed:August 2019
- Compositional types
- Shape and size
- Mineral composition
- Internal structure
- Related Primary Literature
- Additional Reading
Exceptionally coarse-grained and relatively light-colored crystalline rock composed chiefly of minerals found in ordinary igneous rocks. Extreme variations in grain size also are characteristic (Fig. 1), and close associations with dominantly fine-grained aplites are common. Pegmatites are widespread and very abundant where they occur, especially in host rocks of Precambrian age, but their aggregate volume in the Earth's crust is small. Individual bodies, representing wide ranges in shape, size, and bulk composition, typically occur in much larger bodies of intrusive igneous rocks or in terranes of metamorphic rocks, from most of which they are readily distinguished by their unusual textures (Fig. 2) and often by concentrations of relatively rare minerals. Many pegmatites have been economically valuable as sources of clays, feldspars, gem materials, industrial crystals, micas, silica, and special fluxes, as well as beryllium, bismuth, lithium, molybdenum, rare-earth, tantalum-niobium, thorium, tin, tungsten, and uranium minerals. See also: Aplite; Clay mineral; Feldspar; Gem; Mica
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