These are used to make components for electronic devices, retardation plates, circuit boards, optical filters, detector windows, and many other products. Pegmatite can also form in fractures that develop on the margins of the batholith. 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. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Eventually there is an overabundance of water, and pockets
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. Complex pegmatites commonly contain tourmaline, lepidolite, topaz, cassiterite, fluorite, beryl, etc. Finally there is an overabundance of wate… Feldspar is another mineral frequently mined from pegmatite. The view seen here is about six inches across. Many granitic pegmatites contain unusual concentrations of the less-abundant elements. 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. 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. "syenite pegmatite," and any other plutonic rock name combined with "pegmatite" are possible. They form as a late-stage magmatic fluid starts to crystallize. Some of the worldâs best tourmaline, aquamarine, and topaz deposits have been found in pegmatites. They form small marginal parts of large magma intrusions known as batholiths. Pegmatites are not rare rocks, but their overall volume is small. so its concentration in the melt grows as crystallization progresses. 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. The mining of pegmatites might be done in an underground operation that follows a dike or exploits a small pocket. Premium Membership is now 50% off! If the mine contains nice crystals, the minerals are often more valuable as mineral specimens and faceting rough than being sold as an ore. Pegmatite is often mined for industrial minerals. Let us know if you have suggestions to improve this article (requires login). A pegmatite is an igneous rock, formed underground, with interlocking crystals usually larger than 2.5 cm in size (1 in). However, gem deposits refer only to those of granitic composition. 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 … To be called a "pegmatite," a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. However, pegmatite deposits often contain gemstones, industrial minerals, and rare minerals. It can also be done at an outcrop where the pegmatite is easily discovered by people. Topaz on albite: A crystal of imperial topaz on an albite matrix from a pocket in the Katlang Pegmatite of Pakistan. Pegmatite can also form in fractures that develop on the margins of the batholith. This is why crystals of a pegmatite grow so large. In most cases the mining operations are very small, employing less than a dozen people. Pegmatite can also form in fractures that develop on the margins of the batholith. However, with pegmatites,
Early crystallization process, water is not removed. Updates? Large crystals of feldspar, quartz, and hornblende are visible. 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. Specimen and photo by Arkenstone / www.iRocks.com. Because these dikes and pockets are small in size, the mining operations that exploit them are also small. Many fine clear emeralds were produced, and much of the rock was sold as "emerald matrix" for slabbing and cabochon cutting. Large crystals of feldspar, smoky quartz, and hornblende are visible. Pegmatites are sometimes sources of valuable minerals such as spodumene (an ore of lithium) and beryl (an ore of beryllium) that are rarely found in economic amounts in other types of rocks. Pegmatites usually do not support large mining operations that employ dozens of workers and have continuous activity of many years. Specimen and photo by Arkenstone / www.iRocks.com. Rare ions that do not participate in the crystallization of common rock-forming minerals become concentrated in the melt and in the excluded water. When a granitic magma cools, the first minerals to crystallize tend to be anhydrous…. Specimen is about 4.5 x 3.5 x 3.5 centimeters. While pegmatite occurrences are not uncommon, those bearing gem-quality minerals in mineable quantities are exceedingly r… Most pegmatites have a composition that is similar to granite with abundant quartz, feldspar, and mica. These minerals can be commercial sources of: beryllium, bismuth, boron, cesium, lithium, molybdenum, niobium, tantalum, tin, titanium, tungsten, and many other elements. This is how pegmatite dikes are formed. Certain magmas, such as those which form granites, contain several percent water dissolved in them. 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. It is a granitic pegmatite that intrudes the boundary between two rock units in a dike that is up to two meters wide. 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. So its concentration in the melt grows as crystallization progresses. The extreme conditions of crystallization sometimes produce crystals that are several meters in length and weigh over one ton. Specimen is about 12.7 x 7.7 x 7.5 centimeters. This is how "pegmatite dikes" are formed. Coexistence of residual magma and a volatile-rich fluid (generally aqueous) promotes the partitioning and segregation of constituents, as well as the growth of very large crystals. They also can be a source of gemstones. The view seen here is about 12 inches across. 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.". 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. The ions in the water are much more mobile than
It is also used as a filler in many products. Occasionally it is encountered in a dimension stone quarry that produces granite for architectural use. These ions can form the rare minerals that are often found in pegmatites. Pegmatite is the host rock for many rare mineral deposits. 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. During the early states of a magmaâs crystallization, the melt usually contains a significant amount of dissolved water and other
They also yield significant quantities of gem minerals, mica, molybdenite, cassiterite, tungsten minerals, rare-earth minerals, and certain types of kaolin, either directly or as the sources of placer deposits. 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. Pegmatites are extreme igneous rocks that form during the final stage of a magmas crystallization. Ore minerals, chiefly sulfides and oxides, are widespread in pegmatites but rarely are abundant. igneous rock: Pegmatites and late-stage mineralization. Corrections? Pegmatite lithium deposits, also … It was mined for emeralds by a series of owners, which included Tiffany and Company, between 1894 and the 1990s. Water is not removed from the melt during the early crystallization process,
Large sheets of mica are mined from pegmatite. USGS photo. of water separate from the melt. The specimen shown here is about two inches (five centimeters) across. 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.