Difference between revisions of "Pegmatite" - New World Encyclopedia

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[[Image:Alkaline pegmatite.jpg|right|thumb|250 px|Pegmatite, with blue [[crystal]]s of [[corundum]].]]
 
[[Image:Alkaline pegmatite.jpg|right|thumb|250 px|Pegmatite, with blue [[crystal]]s of [[corundum]].]]
  
'''Pegmatite''' is a very coarse-grained [[igneous rock]], with a grain size of 20 millimeters (mm) or more. Most pegmatites are composed of [[quartz]], [[feldspar]], and [[mica]]; in essence, a "[[granite]]."  Rarer, "intermediate" and "[[Mafic|mafic]]" pegmatites containing [[amphibole]], calcium-[[plagioclase]] feldspar, [[pyroxene]], and other minerals are also known, found in recrystallized zones and apophyses associated with large [[ultramafic to mafic layered intrusions|layered intrusions]].
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'''Pegmatite''' is a very coarse-grained [[igneous rock]], with a grain size of 20 millimeters (mm) or more. Most pegmatites are composed of [[quartz]], [[feldspar]], and [[mica]]; in essence, a "[[granite]]."  Rarer, "intermediate" and "[[Mafic|mafic]]" pegmatites containing [[amphibole]], calcium-[[plagioclase]] feldspar, [[pyroxene]], and other minerals are also known. Crystal size is the most striking feature of pegmatite, with crystals usually over 50 mm in size. However, individual crystals over 10 meters across have also been found, and the world's largest crystal was found within a pegmatite.
  
Crystal size is the most striking feature of pegmatite, with crystals usually over 50 mm in size. However, individual crystals over 10 meters across have also been found, and the world's largest crystal was found within a pegmatite.  
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Pegmatites are considered valuable because they often contain [[rare earth]] minerals and [[gemstone]]s, including [[aquamarine]], [[tourmaline]], [[topaz]], [[fluorite]], and [[apatite]]. They often contain minerals of [[metal]]s such as [[tin]] and [[tungsten]]. They are the primary source of [[lithium]] and [[beryllium]].
  
 
== Classification ==
 
== Classification ==
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Pegmatites are important because they often contain [[rare earth]] minerals and [[gemstone]]s, such as [[aquamarine]], [[tourmaline]], [[topaz]], [[fluorite]], and [[apatite]], often along with [[tin]] and [[tungsten]] minerals, among others. For example, beautiful crystals of [[aquamarines]] and [[topaz]] can be found in pegmatites in the mountains of [[Colorado]] and [[Idaho]].
 
Pegmatites are important because they often contain [[rare earth]] minerals and [[gemstone]]s, such as [[aquamarine]], [[tourmaline]], [[topaz]], [[fluorite]], and [[apatite]], often along with [[tin]] and [[tungsten]] minerals, among others. For example, beautiful crystals of [[aquamarines]] and [[topaz]] can be found in pegmatites in the mountains of [[Colorado]] and [[Idaho]].
  
Pegmatites are the primary source of [[lithium]], either as spodumene, lithiophyllite, or lepidolite (Li-mica). The majority of the world's beryllium is obtained from non-gem quality beryl within pegmatite. Tantalum, niobium, rare-earth elements are sourced from a few pegmatites worldwide, notably the Greenbushes Pegmatite. Bismuth, molybdenum and tin have been won from pegmatite, but this is not yet an important source of these metals.  
+
Pegmatites are the primary source of [[lithium]], either as spodumene, lithiophyllite, or lepidolite (Li-mica). The majority of the world's beryllium is obtained from non-gem quality beryl within pegmatite. Tantalum, niobium, rare-earth elements are sourced from a few pegmatites worldwide, notably the Greenbushes Pegmatite. Bismuth, molybdenum and tin have been won from pegmatite, but this is not yet an important source of these metals.
  
 
== See also ==
 
== See also ==

Revision as of 00:00, 19 November 2007

Pegmatite, with blue crystals of corundum.

Pegmatite is a very coarse-grained igneous rock, with a grain size of 20 millimeters (mm) or more. Most pegmatites are composed of quartz, feldspar, and mica; in essence, a "granite." Rarer, "intermediate" and "mafic" pegmatites containing amphibole, calcium-plagioclase feldspar, pyroxene, and other minerals are also known. Crystal size is the most striking feature of pegmatite, with crystals usually over 50 mm in size. However, individual crystals over 10 meters across have also been found, and the world's largest crystal was found within a pegmatite.

Pegmatites are considered valuable because they often contain rare earth minerals and gemstones, including aquamarine, tourmaline, topaz, fluorite, and apatite. They often contain minerals of metals such as tin and tungsten. They are the primary source of lithium and beryllium.

Classification

Pegmatites can be classified according to their content of particular elements or minerals. For example, "lithian pegmatite" describes a lithium-bearing or lithium-mineral bearing pegmatite, "boron pegmatite" is one that contains tourmaline, and so forth.

There is often no meaningful way to distinguish pegmatites according to chemistry due to the difficulty of obtaining a representative sample. Often, however, groups of pegmatites can be distinguished by contact textures, orientation, accessory minerals and timing. These may be named formally or informally as a class of intrusive rock or within a larger igneous association.

The derivation of a pegmatite is difficult to determine with certainty. Nonetheless, a pegmatite may be referred to as "metamorphic," "granitic," or "metasomatic," based on the researcher's inference about the processes by which it was formed. This is an informal classification method, as the origin and nature of pegmatite formation are seriously debated.

Occurrence

Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. However, pegmatite localities are well recorded only when mineralization of economic importance is found.

Within the metamorphic belts, pegmatite tends to concentrate around granitic bodies within zones of low mean strain and within zones of extension, such as within the strain shadow of a large rigid granite body.

Properties of crystals

Crystal texture and form within pegmatite may be taken to extreme size and perfection. Feldspar within pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallisation, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within pegmatite often shows gigantic perthitic texture visible to the naked eye.

Petrology

Crystal growth rates in pegmatite must be incredibly fast to allow gigantic crystals to grow within the confines and pressures of the Earth's crust. For this reason, the consensus on pegmatitic growth mechanisms involves a combination of the following processes:

  • Low rates of nucleation of crystals coupled with high diffusivity to force growth of a few large crystals instead of many smaller crystals
  • High vapor and water pressure, to assist in the enhancement of conditions of diffusivity
  • High concentrations of fluxing elements such as boron and lithium which lower the temperature of solidification within the magma or vapor
  • Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to occur only within greenschist metmorphic terranes

Despite this consensus on likely chemical, thermal and compositional conditions required to promote pegmatite growth, there are three main theories behind pegmatite formation. They are:

  1. Metamorphic; pegmatite fluids are created by devolatilization (dewatering) of metamorphic rocks, particularly felsic gneiss, to liberate the right constituents and water, at the right temperature.
  2. Magmatic; pegmatites tend to occur in the aureoles of granites in most cases, and are usually granitic in character, often closely matching the compositions of nearby granites. Pegmatites thus represent ex-solved granitic material that crystallizes in the country rocks.
  3. Metasomatic; Pegmatite, in a few cases, could be explained by the action of hot alteration fluids upon a rock mass, with bulk chemical and textural change.

Metasomatism is currently not well favored as a mechanism for pegmatite formation, but it is likely that metamorphism and magmatism both contribute toward the conditions necessary for pegmatite genesis.

Geochemistry

Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Often, bulk samples of rock weighing some 50-60 kg must be crushed to obtain a meaningful and repeatable result. Hence, pegmatite is often characterized by sampling the individual minerals that comprise the pegmatite, and comparisons are made according to mineral chemistry.

Geochemically, pegmatites typically have major element compositions approximating "granite," however, when found in association with granitic plutons it is likely that a pegmatite dyke will have a different trace element composition with greater enrichment in large-ion lithophile (incompatible) elements, including boron, beryllium, aluminum, potassium and lithium, uranium, thorium, and cesium.

Occasionally, enrichment in the unusual trace elements will result in crystallization of equally unusual and rare minerals such as beryl, tourmaline, columbite, tantalite, zinnwaldite and so forth.

Economic importance

Pegmatites are important because they often contain rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, and apatite, often along with tin and tungsten minerals, among others. For example, beautiful crystals of aquamarines and topaz can be found in pegmatites in the mountains of Colorado and Idaho.

Pegmatites are the primary source of lithium, either as spodumene, lithiophyllite, or lepidolite (Li-mica). The majority of the world's beryllium is obtained from non-gem quality beryl within pegmatite. Tantalum, niobium, rare-earth elements are sourced from a few pegmatites worldwide, notably the Greenbushes Pegmatite. Bismuth, molybdenum and tin have been won from pegmatite, but this is not yet an important source of these metals.

See also

References
ISBN links support NWE through referral fees

  • Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
  • Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
  • Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.

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