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Encyclopedia > Pegmatite
Pegmatite with blue corundum crystals
Pegmatite with blue corundum crystals

Pegmatite is a very coarse-grained igneous rock that has a grain size of 20 mm or more; such rocks are referred to as pegmatitic. Image File history File linksMetadata Download high-resolution version (2407x1490, 1238 KB) [edit] Summary Alkaline pegmatite Description: wheathered feldspar and blue corundum crystals. ... Image File history File linksMetadata Download high-resolution version (2407x1490, 1238 KB) [edit] Summary Alkaline pegmatite Description: wheathered feldspar and blue corundum crystals. ... Corundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. ... Volcanic rock on North America Plutonic rock on North America Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ...


Most pegmatites are composed of quartz, feldspar and mica; in essence a "granite". Rarer "intermediate" and "mafic" pegmatite containing amphibole, Ca-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions. Quartz is amongst one of the most common minerals in the Earths continental crust. ... Feldspar is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earths crust. ... rock with mica Mica sheet mica flakes The mica group of minerals includes several closely related materials having highly perfect basal cleavage. ... Quarrying granite for the Mormon Temple, Utah Territory. ... It has been suggested that this article or section be merged with Ultramafic intrusion. ...


Crystal size is the most striking feature of pegmatite, with crystals usually over 50mm in size. However, individual crystals over 10 meters across have been found, and the world's largest crystal was found within a pegmatite.


Similarly, 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. A binary phase diagram displaying solid solutions over the full range of relative concentrations. ... Rock microstructure includes the texture of a rock and the small scale rock structures. ... A perthite is a structural (perthitic structure) variety of the potassium feldspars microcline and orthoclase (KAlSi3O8). ...

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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; Bubbles in a soft drink each nucleate independently, responding to a decrease in pressure. ... This article or section does not cite its references or sources. ... It has been suggested that Boriding be merged into this article or section. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/gray Atomic mass 6. ... Magma is molten rock located beneath the surface of the Earth (or any other rocky planet), and which often collects in a magma chamber. ...

  1. Metamorphic; pegmatite fluids are created by devolatilisation (dewatering) of metamorphic rocks, particularly felsic gneiss, to liberate the right consituents 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 exsolved granitic material which crystallises 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 and it is likely that metamorphism and magmatism are both contributors toward the conditions necessary for pegmatite genesis. Metasomatism is a geologic process where metamorphism causes an alteration in a mineral or rock mass that involves a chemical change of the substance with the addition of material, as when chrysolite (olivine) is converted to serpentine basically by the addition of water. ...

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Mineralogy

The mineralogy of a pegmatite is in all cases dominated by some form of feldspar, often with mica and usually with quartz, being altogether "granitic" in character. Beyond that, pegmatite may include most minerals associated with granite and granite-associated hydrothermal systems, granite-associated mineralisation styles, for example greisens, and somewhat with skarn associated mineralisation. Hydrothermal circulation in the oceans is the passage of the water through mid-ocean Ridge (MOR) systems. ... Greisen is a highly altered granitic rock or pegmatite. ... Skarn is a fine grained metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white. ...


It is however impossible to quantify the mineralogy of pegmatite in simple terms because of their varied mineralogy and difficulty in estimating the modal abundance of mineral species which are of only a trace amount. This is because of the difficulty in counting and sampling mineral grains in a rock which may have crystals centimetres, decimetres or even metres acoss.


Garnet, commonly almandine or grossular, is a common mineral within pegmatites intruding mafic and carbonate-bearing sequences. Pegmatites associated with granitic domes within the Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red, orange and brown almandine garnet. Almandine, or almandite, is a name applied to certain kinds of precious garnet, being apparently a corruption of alabandicus, which is the name applied by Pliny to a stone found or worked at Alabanda, a town in Caria in Asia Minor. ... A pale green, pink, brown, or black garnet, Ca3Al2(SiO4)3, occurring alone or as a constituent of the common garnet. ... The Yilgarn Craton is a craton which constitutes the bulk of the Western Australian land mass. ...


Tantalum and niobium minerals (columbite, tantalite, niobite) are found in association with spodumene, lepidolite, tourmaline, cassiterite in the massive Greenbushes Pegmatite in the Yilgarn Craton of Western Australia, considered a typical metamorphic pegmatite unassociated with granite. Ferrocolumbite, also called niobite, columbate and columbite [(Fe, Mn)(Nb, Ta)2O6] is a black mineral that is an ore of niobium and tantalum. ... Tantalite is a mineral that is close to columbite. ... ... Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate - LiAl(SiO3)2 - and is a source of lithium. ... Lepidolite (KLi2Al(Al,Si)3O10(F,OH)2) is a lilac or rose-violet colored phyllosilicate mineral of the mica group that is a secondary source of lithium. ... The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. ... Cassiterite is a tin oxide mineral, SnO2. ...

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Geochemistry

Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Often, bulk samples of some 50-60kg of rock must be crushed to obtain a meaningful and repeatable result. Hence, pegmatite is often characterised by sampling the individual minerals which 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, boron, beryllium, aluminium, potassium and lithium, uranium, thorium, cesium, etcetera. Quarrying granite for the Mormon Temple, Utah Territory. ...


Occasionally, enrichment in the unusual trace elements will result in crystallisation of equally unusual and rare minerals such as beryl, tourmaline, columbite, tantalite, zinnwaldite and so forth. In most cases, there is no particular genetic significance to the presence of rare mineralogy within a pegmatite, however it is possible to see some causative and genetic links between, say, tourmaline-bearing granite dykes and tourmaline-bearing pegmatites within the area of influence of a composite granite intrusion (Mt Isa Inlier, Queensland, Australia). The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6. ... The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. ... Ferrocolumbite, also called niobite, columbate and columbite [(Fe, Mn)(Nb, Ta)2O6] is a black mineral that is an ore of niobium and tantalum. ... Tantalite is a mineral that is close to columbite. ... Zinnwaldite (K Li Fe Al (AlSi3 ) O10 (OH, F)2; Potassium lithium iron aluminum silicate hydroxide fluoride) is a mineral in the mica group. ...


However, it is not always easy to prove causative or associative links between granite and pegmatite.

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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. A rare earth is an oxide of a rare earth element. ... A selection of gemstone pebbles made by tumbling rough rock with abrasive grit, in a rotating drum. ... Aquamarine Aquamarine (Lat. ... The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals. ... Topaz Colorless topaz, Minas Gerais, Brazil Topaz is a mineral, a nesosilicate of aluminium and fluorine with the chemical formula Al2SiO4(F,OH)2. ... Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF2. ... ske| Fracture|| Conchoidal to even Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, and chlorapatite, named for high concentrations of OH-, F-, or Cl- ions, respectively, in the crystal. ... General Name, Symbol, Number tin, Sn, 50 Chemical series poor metals Group, Period, Block 14, 5, p Appearance silvery lustrous gray Atomic mass 118. ... General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Atomic mass 183. ... Aquamarine is gemstone-quality blue beryl, closely related to the emerald. ... Topaz Colorless topaz, Minas Gerais, Brazil Topaz is a mineral, a nesosilicate of aluminium and fluorine with the chemical formula Al2SiO4(F,OH)2. ... Official language(s) English Capital Denver Largest city Denver Area  Ranked 8th  - Total 104,185 sq mi (269,837 km²)  - Width 280 miles (451 km)  - Length 380 miles (612 km)  - % water 0. ... Official language(s) None Capital Boise Largest city Boise Area  Ranked 14th  - Total 83,642 sq. ...


Pegmatites are the primary source of lithium either as spodumene, lithiophyllite or usually from lepidolite (Li-mica). The majority of the world's beryllium is sourced 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. General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/gray Atomic mass 6. ...

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Nomenclature

Pegmatites can be classified according to the elements of interest or mineral of interest, for instance "lithian pegmatite" to describe a Li-bearing or Li-mineral bearing pegmatite, "Boron pegmatite" for those containing tourmaline, etcetera.


There is often no meaningful way to distnguish pegmatite according to chemistry due to the difficulty of obtaining a representative sample, but often groups of pegmatites can be distinguished on 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 (Suite, Super Suite, etcetera)


While difficult to be certain of derivation of pegmatite in the strictest sense, often pegmatites are referred to as "metamorphic", "granitic" or "metasomatic", an inference as to the processes by which the author beleves a particular pegmatite was formed. This is an informal classification mehod, as the origin and nature of pegmatite formation and genesis is still heavily debated.

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Occurrence

Pegmatite is essentially restricted to Barrovian Facies Sequence metamorphic rocks of at least middle greenschist facies, and often also intimately associated with granites intruding into such terranes. The term Metamorphic can be associated with a number of meanings:- Metamorphic rock The term for rocks that have been transformed by extreme heat and pressure. ... Greenschist is a general field petrologic term applied to metamorphically altered mafic volcanic rock. ...


Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. However, pegmatite localities are only well recorded when economic mineralisation is found. World geologic provinces. ...


Within the metamorphic belts, pegmatite tends to concentrate around granitic bodies within zones of low mean strain and within zones of extension, for example within the strain shadow of a large rigid granite body. Similarly, pegmatite is often found within the contact zone of granite, transitional with some greisens, as a late-stage magmatic-hydrothermal effect of syn-metamorphic granitic magmatism. Some skarns associated with granites also tend to host pegmatites. Greisen is a highly altered granitic rock or pegmatite. ... Skarn is a fine grained metamorphic rock that is usually variably colored green or red, occasionally grey, black, brown or white. ...


Aplite dykes and porphyry dykes may exploit pegmatite within wall rocks to intrusions and vice versa, creating a confused sequence of felsic intrusive apophyses within the aureole of some granites. Aplite is a fine to very fine-grained sugary-textured rock of granite composition. ...


  Results from FactBites:
 
Earlham College - Geology 211 - Template (800 words)
Pegmatites are coarse grained, generally igneous mineral deposits which result from disparate rates of cooling within a trapped pocket of magma, possibly due to volitiles which remain after the rest of the pluton has crystallized.
Unzoned pegmatites are generally formed through replacement or recrystallization of the wall rock rather than crystallization out of a fluid and therefore lack the diversity of minerals or the complexity of structure seen in zoned pegmatites.
Unzoned pegmatites are generally less influenced by hydrothermal factors than the zoned are, which explains in part the lesser mobility of ions which inhibits their ability to crystallize into distinct zones.
Pegmatite: World of Earth Science (382 words)
A pegmatite may be a segregation within an associated plutonic rock or a dike or vein that intrudes the surrounding country rock.
The composition range of pegmatites is similar to that of other intrusive igneous rocks and is indicated by using modifier, e.g., granite pegmatite or gabbro pegmatite.
Pegmatites are also valued for the suite of rare elements that tend to be concentrated in the residual magmas.
  More results at FactBites »

 
 

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