Pegmatite with blue corundum crystals

Pegmatite containing lepidolite, tourmaline, and quartz from the White Elephant Mine in the Black Hills, South Dakota. Corundum (from Tamil kurundam குருந்தம் or kuruvindam குருவிந்தம் is a Crystalline form of The Black Hills ( Pahá Sápa in Lakota, Moˀȯhta-voˀhonáaeva in Cheyenne) are a small isolated Mountain range rising from the South Dakota ( is a state located in the Midwestern region of the United States of America.

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. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock

Most pegmatites are composed of quartz, feldspar and mica; in essence a "granite". Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Feldspar is the name of a group of rock-forming Minerals which make up as much as 60% of the Earth 's crust. The word "mica" is thought to be derived from the Latin word la micare, "glitteren" in reference to the brilliant appearance of this mineral (especially Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. 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. Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric" Ultramafic to mafic layered intrusions are found in typically ancient Cratons and are rare but worldwide in distribution

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. A solid solution is a Solid - state Solution of one or more solutes in a Solvent. Rock microstructure includes the texture of a rock and the small scale rock structures Perthite feldspar within pegmatite often shows gigantic perthitic texture visible to the naked eye. Perthite is used to describe an intergrowth of two Feldspars a host grain of Potassium -rich Alkali feldspar (near K-feldspar KAlSi3O8

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;

1. Metamorphic; pegmatite fluids are created by devolatilisation (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. Nucleation is the onset of a Phase transition in a small region Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet 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 the chemical alteration of a rock by Hydrothermal and other fluids

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.

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. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock. Hydrothermal circulation in its most general sense is the circulation of hot water 'hydros' in the Greek meaning water and 'thermos' meaning heat Greisen is a highly altered granitic rock or Pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn. Skarn is a 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. Almandine, also known incorrectly as almandite, is a species of mineral belonging to the Garnet Group Grossular, also incorrectly called grossularite, is a Calcium - Aluminium mineral species of the Garnet group with the Formula Pegmatites associated with granitic domes within the Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red, orange and brown almandine garnet. The Yilgarn Craton is a large 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, niobite-tantalite, columbate and columbite ] Mn)( Nb, Ta)2 Tantalite, ] Mn) Ta 2 O 6] is a Mineral that is close to Columbite. Ferrocolumbite, also called niobite, niobite-tantalite, columbate and columbite ] Mn)( Nb, Ta)2 "Kunzite" redirects here For the Sailor Moon character see Shitennou. Lepidolite (KLi2Al(AlSi3O10(FOH2 is a lilac or rose-violet colored phyllosilicate Mineral of the Mica group Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium Cassiterite is a Tin Oxide Mineral, SnO2. It is generally opaque but is translucent in thin crystals

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 dike 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, et cetera. Granite (ˈɡrænɪt is a common and widely occurring type of intrusive, Felsic, igneous rock.

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. The Mineral beryl is a Beryllium Aluminium cyclosilicate with the Chemical formula Be3Al2(SiO36 Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium Ferrocolumbite, also called niobite, niobite-tantalite, columbate and columbite ] Mn)( Nb, Ta)2 Tantalite, ] Mn) Ta 2 O 6] is a Mineral that is close to Columbite. Zinnwaldite, K[[lithium Li]] Fe[[aluminum Al]](Al Si 3 O 10(O H, F)2 is a potassium lithium iron 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 dikes and tourmaline-bearing pegmatites within the area of influence of a composite granite intrusion (Mt Isa Inlier, Queensland, Australia).

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

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. Rare earth elements and rare earth metals are according to IUPAC, the collection of seventeen Chemical elements in the Periodic table, namely A gemstone or gem, also called a precious or semi-precious stone, is a piece of attractive Mineral, which &mdash when cut and polished &mdash Aquamarine (Lat aqua marinā, "water of the sea" is a Gemstone -quality transparent variety of Beryl, having a delicate blue or Turquoise Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium Topaz is a Silicate mineral of Aluminium and Fluorine with the Chemical formula Al 2 Si[[oxygen O]]4( Fluorite (also called fluorspar) is a Mineral composed of Calcium fluoride, Ca[[Fluorine F2]] Tin is a Chemical element with the symbol Sn (stannum and Atomic number 50 Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 For example, beautiful crystals of aquamarines and topaz can be found in pegmatites in the mountains of Colorado and Idaho. Aquamarine (Lat aqua marinā, "water of the sea" is a Gemstone -quality transparent variety of Beryl, having a delicate blue or Turquoise Topaz is a Silicate mineral of Aluminium and Fluorine with the Chemical formula Al 2 Si[[oxygen O]]4( The State of Colorado ( or chiefly by nonresidents) is a state located in the Rocky Mountain region of the United States of America. The State of Idaho ( is a state in the Pacific Northwest region of the United States of America.

Pegmatites are the primary source of lithium either as spodumene, lithiophyllite or usually from lepidolite (Li-mica). Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 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.

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 distinguish 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 believes a particular pegmatite was formed. This is an informal classification method, as the origin and nature of pegmatite formation and genesis is still heavily debated.

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. Greenschist - also known as greenstone - is a general field petrologic term applied to metamorphic and/or altered Mafic Volcanic rock

Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. A craton ( Greek kratos / κρἀτος ( neut. "strength" is an old and stable part of the Continental crust that has survived However, pegmatite localities are only well recorded when economic mineralisation is found.

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. Greisen is a highly altered granitic rock or Pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn. Some skarns associated with granites also tend to host pegmatites. Skarn is a 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.

References

Major Pegmatite Deposits of New York State, New York State Museum Bulletin No. 408 Tan, Li-ping, 1966