Proterozoic pegmatite swarm in the headwall of the cirque of a small mountain glacier, northeastern Baffin Island, Nunavut.
1 General description 2 Petrology 3 Mineralogy 4 Geochemistry 5 Economic importance 6 Nomenclature 7 Occurrence 8 References 9 Further reading 10 External links
The single feature that is diagnostic to all pegmatites is their large
size crystal components.
Pegmatite bodies are usually of minor size compared to typical intrusive rock bodies. Pegmatite
Pegmatite body size is on the order of magnitude of one to a few hundred meters. Compared to typical igneous rocks they are rather inhomogeneous and may show zones with different mineral assemblages. Crystal size and mineral assemblages are usually oriented parallel to the wall rock or even concentric for pegmatite lenses. Petrology Crystal growth rates in pegmatite must be very slow to allow gigantic crystals to grow within the confines and pressures of the Earth's crust. Thus, the possible growth mechanisms in a wide variety of known pegmatites may likely involve 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 metamorphic terranes
Despite this hypothesis on likely chemical, thermal and compositional conditions required to promote pegmatite growth there are three main theories behind pegmatite formation;
Theory name Theory
Metamorphic pegmatite fluids are created by removal of volatiles from 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. Mineralogy
Pegmatitic granite, Rock Creek Canyon, eastern Sierra Nevada, California. Note pink potassium feldspars and cumulate-filled chamber.
The mineralogy of a pegmatite is in most 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.
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 from
centimeters to meters across.
Garnet, commonly almandine or spessartine, 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
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. Syenite pegmatites are quartz depleted and contain large feldspathoid crystals instead. Geochemistry
Elbaite tourmaline (olive-green) and lepidolite mica (violet), from a lithium-enriched pegmatite in Brazil
Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Often, bulk samples of some 50–60 kg 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, 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 dikes and tourmaline-bearing pegmatites within the area of influence of a composite granite intrusion (Mount Isa Inlier, Queensland, Australia). Economic importance Pegmatites are important because they often contain rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, apatite and corundum, often along with tin and tungsten minerals, among others. Pegmatites are the primary source of lithium either as spodumene, lithiophyllite or usually from lepidolite. The primary source for caesium is pollucite, a mineral from a zoned pegmatite. 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 or mineral of interest, for instance "lithian pegmatite" to describe a Li-bearing or Li-mineral bearing pegmatite, or "boron pegmatite" for those containing tourmaline. There is often no meaningful way to distinguish pegmatites 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. While difficult to be certain of derivation of pegmatite in the strictest sense, often pegmatites are referred to as "metamorphic", "granitic" or "metasomatic", based on the interpretations of the investigating geologist. Rocks with similar texture to pegmatites are called pegmatitic. Occurrence
Pegmatite (light colored) in dark mica schist, Île de Noirmoutier, France.
Pegmatite (pink), Isle of Skye, Scotland.
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
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.
Aplite and porphyry dikes and veins may intrude pegmatites and wall rocks adjacent to intrusions, creating a confused sequence of felsic intrusive apophyses (thin branches or offshoots of igneous bodies) within the aureole of some granites. References
^ USGS pegmatite definition, retrieved 2009-08-28
^ London, David; Morgan, George B. (2012-08-01). "The Pegmatite
Puzzle". Elements. 8 (4): 263–268. doi:10.2113/gselements.8.4.263.
^ Schwartz, G. (1928). "The
Black Hills Mineral Region". American Mineralogist. 13: 56–63. ^ London, D.; Kontak, D. J. (3 September 2012). "Granitic Pegmatites: Scientific Wonders and Economic Bonanzas". Elements. 8 (4): 257–261. doi:10.2113/gselements.8.4.257. ^ Simmons, W. B.; Pezzotta, F.; Shigley, J. E.; Beurlen, H. (2012-08-01). "Granitic Pegmatites as Sources of Colored Gemstones". Elements. 8 (4): 281–287. doi:10.2113/gselements.8.4.281. ISSN 1811-5209. ^ Linnen, R. L.; Lichtervelde, M. Van; Cerny, P. (2012-08-01). "Granitic Pegmatites as Sources of Strategic Metals". Elements. 8 (4): 275–280. doi:10.2113/gselements.8.4.275. ISSN 1811-5209.
London, D. (2008): Pegmatites. Canadian Mineralogist Special
Publication 10, 347 pp.
Tan, Li-ping, 1966, Major
Pegmatite Deposits of New York State, New York State Museum Bulletin No. 408 Pegmatopia: David London, School of Geology & Geophysics, University of Oklahoma
Media related to
Pegmatite at Wikimedia Commons
v t e
Common igneous rocks classified by silicon dioxide content
Type Ultramafic <45% SiO2 Mafic 45–52% SiO2 Intermediate 52–63% SiO2 Intermediate–felsic 63–69% SiO2 Felsic >69% SiO2
Volcanic rocks: Subvolcanic rocks: Plutonic rocks:
Komatiite, Picrite basalt Kimberlite, Lamproite Peridotite
Diabase (Dolerite) Gabbro
Andesite Microdiorite Diorite
Dacite Microgranodiorite Granodiorite
Rhyolite Microgranite, Aplite Granite
GND: 4173606-0 N