Pegmatite

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.

Most pegmatite are composed of quartz, feldspar and mica, having a similar silicic composition as granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids and other unusual minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.

Pegmatite contains very large crystals that may often be mined from the rock for various uses. For example, beryl and topaz are common components of this rock that may be used as gemstones, whereas minerals such as flourite may be used as a source of flouride. The crystal composition of the specific pegmatite can have a huge impact on its use.

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. 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.

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 that compose 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).

The number of crystal nuclei in pegmatites must be low and the ability of the necessary chemical components needed for crystal growth to migrate to the crystal surfaces must be enhanced to allow gigantic crystals to grow in pegmatites. 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; 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.

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.