Pegmatite

What is pegmatite?

Pegmatite are intrusive rocks with extreme coarse grained texture that are developed at the final stages of magma crystallization. As huge amount of time has been granted to pegmatite rocks so it has extremely large crystals and sometimes rare minerals are associated with it which are not found in other rocks. Pegmatite contains crystals that are atleast one centimetre long in diameter.

Pegmatite have composition similar to that of granite with abundant quartz, feldspar and mica. These are sometimes also called as granite pegmatites.

Pegmatites sometimes serve as source rock for valuable minerals such as spodumene and beryl which are rarely economical in other type of rocks. Gemstones are also associated such as topaz, tourmaline and aquamarine.

Pegmatite crystals are large

Large crystal growth in pegmatites are not attributed to slow rate of crystallisation but indeed the low-viscosity fluids are the reason. In the early stages of magma crystallisation, the melt contains the water dissolved with other volatile like chlorine, fluorine and carbon dioxide. The water in the early crystallisation is remained in the melt as crystal start to form, water then separates and escapes the melt. The pockets of water are super heated that contains huge amount of dissolved ions. This water moves freely and are more mobile than that of the melt which when crystallises produce large crystal which are the pegmatites.

Mineralogy of pegmatite

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 centimetres 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 almandine garnet.

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 of pegmatite

Pegmatite sample representative is difficult 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).

Pegmatite dykes or pegmatite dike

Pegmatite form from the water separated from the melt so it will crystallise at the margins and in fractures of the country rock. The water enters the fractures and then form crystals which in turn produces pegmatite dykes or pegmatite dikes. These are mined for associated gemstones and minerals which cannot be produced with other rock types.

Rare minerals associated with pegmatites

As the crystallisation occurs, the ions that have no role in common rock formation are dissolved in the water which separate from the melt and when these ions crystallises produces rare Earth minerals such as lithium, beryllium in small ions and large ions have tantalum, columbium and niobium forming minerals tantalite, columbite and niobite.

Occurrence of pegmatite

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.

Multicoloured tourmaline crystal in the Himalaya pegmatite.

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.

 Uses of pegmatite

Pegmatite uses are the country rock used for architectural purposes as slabs, dimension stones etc. Other are to be mined for rare minerals and gemstones such as aquamarine, topaz and tourmaline that are associated with pegmatite and are not formed with other rocks.

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