Pegmatites are intrusive igneous rocks which are made up of various grain sizes which often have crystal growth within them. It could be a segregation that is found in a plutonic rock or a dike, seams of rocks that run across country rock, or a lens, pods of coarse-grained rock which are included in other rocks. Pegmatites are formed around areas that contain magma.
Pegmatites have a similar composition to other intrusive igneous rocks. Their modifier in their names refer to these rocks. For example granite pegmatite or gabbo pegmatite. They are, however, most commonly found in granites.
They can have simple or exotic mineral make-up. A simple one could have quartz, feldspar and mica. Where complex pegmatites can contain minerals such as tourmaline, topaz, fluorite, and apatite.
When intrusive rocks cool and crystallize, pegmatites form. A sequential process of crystallization happens when the parent body cools. Volatile components such as boron, fluorine or chlorine form a residual magma. At times, residual water allows the magma to cool at a slow rate so that coarse crystals can form. When there are many volatiles that become rare minerals, this become a more complicated process.
Pegmatites have unusually large crystal sizes of the minerals. Some may be less than an inch where others can measure up to several feet. Pegmatites do create large individual crystals from different minerals but generally when there is a very large crystal, it would have intergrown with the pegmatite.
Another value of pegmatite is how rare elements become concentrated in the remaining magma. Some of the rare elements include tin, tantalum and niobium. Due to the fact that pegmatites are formed in areas with magma, it allows how some areas may be more concentrated than others. Some of the minerals may push their way to the surface or they may blend together under the surface. Each of these processes would produce rocks of different sizes and complexities.
Most formations of pegmatite are pod-like, or irregular. They can occur all around the world are are ample in supply in rocks that are geologically aged. They car be found in large bodies of intrusive igneous rocks or in the rocks that are around these formations or even in areas that are not recognizably igneous.
Feldspar, sheet mica, beryllium, tantalum-niobium and lithium minerals are are created from granitic and syenititic pegmatite. They are also the source of mica, molybdenite, cassiterite, tungsten and types of kaolin. Pegmatite also contains, in low amounts, sulfides and oxides.