These minerals can be commercial sources of: beryllium, bismuth, boron, cesium, lithium, molybdenum, niobium, tantalum, tin, titanium, tungsten, and many other elements. of water separate from the melt. In most cases the mining operations are very small, employing less than a dozen people. Most pegmatites are found in sheets of rock (dikes and veins) near large masses of igneous rocks called batholiths. Pegmatites are extreme igneous rocks that form during the final stage of a magmas crystallization. If the pegmatite is sound and attractive, it might be cut into slabs and polished for building facing, countertops, tile or other decorative stone products and sold commercially as a "granite.". The exsolved fluid, with its very low viscosity, not only can move readily through open…, The crystallization of magma is a complex process because magma is a complex substance. The specimen shown here is about two inches (five centimeters) across. igneous rock: Pegmatites and late-stage mineralization. Pegmatites are little different from the common igneous rocks in major elements of bulk composition, and they range from felsic to mafic (silica-rich to silica-poor); granitic and syenitic types are most abundant. Specimen is about 12.7 x 7.7 x 7.5 centimeters. The view seen here is about 12 inches across. Quartz and alkali feldspars are the essential constituents; the most common varietal and accessory minerals are muscovite, biotite, apatite, garnet, and tourmaline. Specimen is about 4.5 x 3.5 x 3.5 centimeters. Ore minerals, chiefly sulfides and oxides, are widespread in pegmatites but rarely are abundant. This is how "pegmatite dikes" are formed. Many granitic pegmatites contain unusual concentrations of the less-abundant elements. Note miner at right center for scale. Corrections? Economic lode concentrations generally occur in zoned pegmatite bodies (i.e., those within which two or more different rock types are systematically disposed). large crystals are attributed to low-viscosity fluids that allow ions to be very mobile. The ions in the water are much more mobile than They also can be a source of gemstones. So its concentration in the melt grows as crystallization progresses. Let us know if you have suggestions to improve this article (requires login). Some are segregations within much larger bodies of intrusive igneous rocks, others are distributed in the rocks that surround such bodies, and still others are not recognizably associated with igneous rocks. Eventually there is an overabundance of water, and pockets Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Igneous rocks formed large crystals are usually ttributed to a slow rate of crystallization but with pegmatites’ large crystals are attributed to low-viscosity fluids that allow ions to be very mobile.During the crystallization stage, the melt usually contains a significant amount of dissolved water and other volatiles such as chlorine, fluorine, and carbon dioxide. Pegmatites are not rare rocks, but their overall volume is small. Water is not removed from the melt during the early crystallization process, Examples are small ions such as lithium and beryllium that form spodumene and beryl; or large ions such as tantalum and niobium that form minerals such as tantalite and niobite. Finally there is an overabundance of wate… Large sheets of mica are mined from pegmatite. Jahns & Burnham (1969) and Jahns & Tuttle (1963) cited experimental evidence that pegmatites formed by equilibrium crystallization of coexisting granitic melt and hydrous fluid at or slightly below the hydrous granite liquidus. However, with pegmatites, Feldspar is another mineral frequently mined from pegmatite. Himalaya pegmatite: A specimen of the Himalaya Pegmatite of San Diego County, California, that is famous for yielding gem- and mineral-specimen-quality tourmaline and other fine crystals. Geologically, the pegmatites can be defined as very coarse- to gigantic-sized (tens of meters) igneous rocks of commonly granitic (high silica, low magnesium) with minor gabbroic or even ultramafic(low silica, high magnesium) composition. For example: a large crystal of spodumene at the Etta Mine in South Dakota was 42 feet long, 5 feet in diameter and yielded 90 tons of spodumene! Specimen and photo by Arkenstone / www.iRocks.com. Most pegmatites have a composition that is similar to granite with abundant quartz, feldspar, and mica. Occasionally it is encountered in a dimension stone quarry that produces granite for architectural use. Large crystals of feldspar, quartz, and hornblende are visible. ions in the melt. This is a pocket piece with feldspar, smoky quartz, cleavelandite, and a fantastic multicolor tourmaline crystal. These are sometimes called "granite pegmatites" to indicate their mineralogical composition. The high vapor pressure and … These ions can form the rare minerals that are often found in pegmatites. They form small marginal parts of large magma intrusions known as batholiths. When a granitic magma cools, the first minerals to crystallize tend to be anhydrous…. Large crystals in igneous rocks are usually attributed to a slow rate of crystallization. It can also be done at an outcrop where the pegmatite is easily discovered by people. Because these dikes and pockets are small in size, the mining operations that exploit them are also small. Pegmatites form from waters that separate from a magma in the late stages of crystallization; this activity often occurs in small pockets along the margins of a batholith. Pegmatite rock has very few uses. Pegmatite is a coarse-grained intrusive igneous rock formed from crystallized magma below the Earth’s crust. Pegmatite, 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 and in which extreme textural variations, especially in grain size, are characteristic. Crabtree pegmatite: One of the most interesting pegmatites is the Crabtree Pegmatite of western North Carolina. This article was most recently revised and updated by, https://www.britannica.com/science/pegmatite, Victoria and Albert Museum - Classical Ballet, College of Natural Resources - Pegmatites. Large crystals of feldspar, smoky quartz, and hornblende are visible. Pegmatite is the host rock for many rare mineral deposits. These are used to make components for electronic devices, retardation plates, circuit boards, optical filters, detector windows, and many other products. Pegmatite lithium deposits, also … The view seen here is about six inches across. To be called a "pegmatite," a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. Polished pegmatite countertop: A portion of a countertop made from polished pegmatite. Giant spodumene crystals: Molds of giant spodumene crystals at the Etta Mines, Black Hills, Pennington County, South Dakota. Pegmatites form from waters that separate from a magma in the late stages of crystallization; this activity often occurs in small pockets along the margins of a batholith. Most bodies of pegmatite are tabular, podlike (cigar-shaped), or irregular in form and range in size from single crystals of feldspar to dikes (tabular bodies injected in fissures) many tens of metres thick and more than a kilometre long; many are intimately associated with masses of fine-grained aplite. Gemstones found in pegmatite include: amazonite, apatite, aquamarine, beryl, chrysoberyl, emerald, garnet, goshenite, heliodor, kunzite, lepidolite, morganite, spodumene, topaz, tourmaline, zircon, and many others. Pegmatites are extreme igneous rocks that form during the final stage of a magma’s crystallization. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Alkaline pegmatite, with blue corundum crystals, Rio de Janeiro, Brazil. This specimen is about 7 x 7 x 7 centimeters in size and contains numerous small emerald crystals that are several millimeters in length. It is used as a primary ingredient for making glass and ceramics. has nothing to do with the mineral composition of the rock. Rare elements concentrated in large crystals make pegmatite a potential source of valuable ore. Polished pegmatite countertop: A portion of a countertop made from polished pegmatite. While pegmatite occurrences are not uncommon, those bearing gem-quality minerals in mineable quantities are exceedingly r… Certain magmas, such as those which form granites, contain several percent water dissolved in them.