Pegmatite is a very coarse-grained
igneous rockthat has a grain size of 20 mm or more; such rocks are referred to as "pegmatitic".
Most pegmatites are composed of
quartz, feldsparand 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.
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,
exsolutionlamellae, and when affected by hydrous crystallisation, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthitefeldspar within pegmatite often shows gigantic perthitic texture visible to the naked eye.
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
nucleationof 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
boronand lithiumwhich lower the temperature of solidification within the magmaor 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;
#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
#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
#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.
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
graniteand granite-associated hydrothermalsystems, granite-associated mineralisation styles, for example greisens, and somewhat with skarnassociated mineralisation.
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.
almandineor 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.
Tantalum and niobium minerals (
columbite, tantalite, niobite) are found in association with spodumene, lepidolite, tourmaline, cassiteritein the massive Greenbushes Pegmatite in the Yilgarn Craton of Western Australia, considered a typical metamorphic pegmatite unassociated with granite.
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.
Occasionally, enrichment in the unusual trace elements will result in crystallisation of equally unusual and rare minerals such as
beryl, tourmaline, columbite, tantalite, zinnwalditeand 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 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.
Pegmatites are important because they often contain rare earth minerals and
gemstones, such as aquamarine, tourmaline, topaz, fluorite, and apatite, often along with tinand tungstenminerals, among others. For example, beautiful crystals of aquamarinesand topazcan be found in pegmatites in the mountains of Coloradoand Idaho.
Pegmatites are the primary source of
lithiumeither 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.
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 is essentially restricted to Barrovian Facies Sequence
metamorphicrocks of at least middle greenschistfacies, and often also intimately associated with granites intruding into such terranes.
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.
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. Aplitedykes 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.
Major Pegmatite Deposits of New York State,New York State Museum Bulletin No. 408Tan, Li-ping, 1966
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