Pegmatite is a common, intrusive igneous rock bearing a crystalline structure that is made up of interlocking crystals of larger than 2.5cm in size. Pegmatite rocks are characterized by their exceptionally large and varying grain size. Pegmatite is a plutonic rock with a variable texture and coarseness, and abnormally large crystal sizes. Some pegmatite rocks are known to have small, irregular patches that are less than 1cm in size. The name pegmatite was first coined by a French mineralogist called Rene Hual.
Pegmatites are known to be composed of a variety of minerals; some of them contain unusual and valuable minerals. Pegmatites may contain large crystals of mica and beryl tourmaline. Complex pegmatites may contain tourmaline, topaz, cassiterite, lepidolite, etc. Common elements that are derived from pegmatite rocks are tantalum, tungsten, boron, bismuth, tin, radium, sheet mica, and sulfide minerals of various metals.
Pegmatites often form in the veins that open up at the end of the crystallization of large, intrusive masses. Most of the large igneous minerals are derived from pegmatites. Pegmatites are found as small, marginal deposits in parts of large magma intrusions; these deposits are called batholiths. Pegmatites are formed as a late-stage magmatic fluid that crystallizes. This fluid is rich in water, chemical elements, and other volatiles. The high volatile content of pegmatites reduces the viscosity of magma and promotes mineral growth.