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Pegmatite

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Pegmatite is an igneous rock that form end of the stage a magma’s crystallization. Pegmatites contain exceptionally large crystals and they contain rarely minerals than other types of rocks. They have interlocking crystals usually larger than 2.5 cm in size. Generally most Pegmatites are found in sheets of rock that are dikes and veins Also near large masses of igneous rocks called batholiths.

Pagmatites most attractive feature is their crystal size, average crystal size over than 5 cm. Individual crystals over 10 metres (33 ft) long have been found, and many of the world’s largest crystals were found within pegmatites. These include spodumene, microcline, beryl, and tourmaline.

Pegmatite crystal texture form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye. The product of pegmatite decomposition is euclase.

Name origin: The term of pegmatite derives from Homeric Greek, πήγνυμι (pegnymi), which means “to bind together”

Colour: Pink, grey- white

Group: Intrusive Igneous Rock

Texture: Phaneritic Text, Gigantic perthitic texture, display exaggerated and perfect twinning

Mineralogy: Quartz, feldspar and mica, amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids

Pegmatite Composition

The pegmatite mineralogy is dominated by some form of feldspar, frequently with mica and generally with quartz. Usually seeing that pegmatite include most minerals associated with granite and granite-associated hydrothermal systems. However, It is impossible to quantify the mineralogy. Because of their varied, it is 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.

Pegmatites is intruding mafic and carbonate-bearing sequences include garnet, commonly almandine or spessartine

Syenite pegmatites are quartz depleted and contain large feldspathoid crystals instead.

Formation of the Pegmatite

Igneous rocks formed large crystals are usually ttributed to a slow rate of crystallization but with pegmatites’ large crystals are attributed to low-viscosity fluids that allow ions to be very mobile.During the crystallization stage, the melt usually contains a significant amount of dissolved water and other volatiles such as chlorine, fluorine, and carbon dioxide. Early crystallization process, water is not removed. So its concentration in the melt grows as crystallization progresses. Finally there is an overabundance of water, and pockets of water separate from the melt. These pockets of superheated water are extremely rich in dissolved ions. The ions in the water are much more mobile than ions in the melt. This allows them to move about freely and form crystals rapidly. This is why crystals of a pegmatite grow so large.

The extreme conditions of crystallization sometimes produce crystals that are several meters in length and weigh over one ton. Pegmatites form from waters that separate from a magma in the late stages of crystallization; this activity often occurs in small pockets along the margins of a batholith. Pegmatite can also form in fractures that develop on the margins of the batholith. This is how pegmatite dikes are formed.

Large intrusions of feldspathoid syenites are commonly accompanied by pegmatites, veins, lenses and pods of coarse-crystalline material comprising the same minerals as found in the surrounding rocks (nepheline, sodalite, alkali feldspars, aegirine and sodic amphiboles), as well as copious quantities of rare minerals enriched in water and incompatible trace elements: Sr (lamprophyllite, belovite), Ba (barytolamprophyllite, batisite), REE (rinkolite, belovite), Zr (eudialyte, lovozerite, catapleiite), Nb (labuntsovite, loparite), Li (mangan-neptunite, tainiolite), Be (leucophanite, epididymite, chkalovite) and Th (steenstrupine). There is little doubt that some of these minerals crystallized from super-evolved melts left over after the formation of the surrounding alkaline rock.

However, this early magmatic stage is commonly followed by hydrothermal processes involving aqueous (and, in some cases, F- or CO3-rich) fluids, which leave their distinct mineralogical overprint, like the crystallization of fine-grained (sugary) albite, fibrous or spherulitic aegirine, exotic carbonate minerals, natrolite and various other zeolites.

In some cases, the primary mineral assemblage and texture are completely obliterated by late-stage hydrothermal reworking (metasomatism). These complex associations of minerals are described collectively as pegmatites or, to distinsguish them from their granitic counterparts, alkaline pegmatites.

Where is it found?

Pegmatite is found in all over the world. They are most abundant old rocks.Some are found in large intrusive igneous rocks, while others are scattered over rocks surrounding intrusive magmatic rocks.

Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. 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.

Characteristics and Properties of Rock

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.

Pegmatite Uses

Pegmatite are used as architectural stone. If the pegmatite is sound and attractive, it might be cut into slabs and polished for building facing, countertops, tile or other decorative stone products and sold commercially as a “granite.”

Pegmatite is used in gemstone mining because of it has large crystal minerals. Gemstones found in pegmatite include: amazonite, apatite, aquamarine, beryl, chrysoberyl, emerald, garnet, kunzite, lepidolite, spodumene, topaz, tourmaline, zircon, and many others.

Pegmatite is used for rare mineral mining. These minerals can be commercial sources of: beryllium, bismuth, boron, cesium, lithium, molybdenum, niobium, tantalum, titanium, tungsten, and many other elements.

Pegmatite is frequently mined for industrial minerals. Large sheets of mica are mined from pegmatite. These are used to make components for electronic devices, retardation plates, circuit boards, optical filters, detector windows, and many other products. Feldspar is another mineral frequently mined from pegmatite. It is used as a primary ingredient for making glass and ceramics. It is also used as a filler in many products.

Facts About The Rock

  • Crystals of pegmatite grow large because of the extreme conditions of crystallization. Basically, the presence of residual water has allowed the magma to cool slowly to permit coarse crystal growth.
  • Complex pegmatites result because of the presence exotic volatiles such as chlorine, fluorine, and carbon dioxide.
  • The extreme conditions may produce crystals many feet in length and may weigh more than a ton.
  • An example of a very large crystal of spodumene is at the Etta Mine in South Dakota. It was 42 feet long, 5 feet in diameter, and yielded about 90 tons of spodumene.
  • Pegmatite also forms in small pockets along the margins of a batholith leading to pegmatite dikes. It will form from waters that separate from a magma in the late stages of crystallization.
  • The dikes and pockets are small in size, taking place underground following a dike or a small pocket. Pegmatites usually do not support large mining operations.
  • Rare elements concentrated in large crystals may make pegmatite a potential source of valuable ore. Pegmatite deposits may also contain gemstones, industrial minerals, and rare minerals.
  • Overall pegmatite rock has very few uses.
  • It has limited use as an architectural stone and is sometimes encountered in a dimension stone quarry that is producing granite.
  • Pegmatite, if sound and appealing, might be cut into slabs, polished, and used for building facades, countertops, tile, or other decorative stone products.
  • It is most often sold commercially as a granite product.
  • The gemstones found in pegmatite include apatite, aquamarine, beryl, emerald, garnet, topaz, zircon, kunzite, and many others. Many are excellent-quality and are large crystals.
  • Pegmatite is the host rock for numerous mineral deposits and can be the commercial sources of beryllium, bismuth, tin, titanium, tungsten, niobium, and many other elements.
  • Large sheets of mica are often mined from pegmatite and used to make components electronic devices, circuit boards, optical filters, detector windows, and many other products.
  • Pegmatite is also used as a primary ingredient for making glass and ceramics, and as a filler for many other products.
  • Pegmatites occur in all parts of the world and are the most abundant in rocks of relatively great geologic age.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Wikipedia contributors. (2018, December 9). Pegmatite. In Wikipedia, The Free Encyclopedia. Retrieved 17:30, April 12, 2019, from https://en.wikipedia.org/w/index.php?title=Pegmatite&oldid=872883451
  • Softschools.com. (2019). Pegmatite Facts. Available at: http://www.softschools.com/facts/rocks/pegmatite_facts/2977/ [Accessed 12 Apr. 2019].
Cite this article as: Geology Science. (2019). Pegmatite. [online] Available at: http://geologyscience.com/rocks/pegmatite/ [12th November 2019 ]
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