This is the reason why pegmatites are so coarse-grained and why they contain so much unusual minerals. They are coarse-grained because of high volatile content which makes the magma less viscous and therefore enhances mineral growth (chemical elements are free to move to look for and join a suitable and already existing crystal). Unusual minerals form because the fluid is enriched in exotic chemical elements like lithium, boron, beryllium, rare earth elements, etc. These elements are forced to form their own mineral phases because they are rejected by major rock-forming minerals like quartz, feldspar, and others.
Pegmatites are very irregular not only in distribution, width and persistence, but also in composition. The relative abundance of the constituent minerals may differ rapidly and much from point to point. Sometimes they are rich in mica, in enormous crystals for which the rock is mined or quarried (India). Other pegmatites are nearly pure feldspar, while others are locally (especially near their terminations) very full of quartz. They may in fact pass into quartz veins (alaskites) some of which are auriferous.
This wealth of minerals makes pegmatites often valuable as a mineral resource. Pegmatites may be mined because of their high content of feldspars, clay (if weathered), mica, or many metal-bearing minerals. Pegmatite is also a source of gems like beryl, tourmaline, zircon, etc.
Most pegmatites are granites with or without exotic minerals but mafic pegmatites (gabbro, diorite) are known as well. Silica undersaturated (without quartz) magmatic rocks may be also pegmatitic.
Pegmatite intrusions in the rocks in Ranchi city of India.
Above pictures shows the pegmatite intrusions in the rocks of Ranchi city.
The Pegmatite veins are common traversing almost all the rock type of the area. Their width varies from few meters to tens of meters. Coarse pegmatitic intrusive are quite abundant, though they tend to cluster at places and result in irregular distribution. They are mainly composed of feldspar (with potash feldspars being much more abundant than the plagioclase) and quartz. The other minerals which are present are tourmaline, muscovite and rarely garnet. At places, the pegmatite contains streaks and specks of pyrite.
Rifle range- This is the highest point of Ranchi, situated in Bariatu, east of Bharamdih hills. The major rock type is granite gneiss with augen shaped crystals of felsic minerals. Large number of intrusions of pegmatites veins is very common in this part . These intrusions are very thick are in the form of sills and dykes, mainly dykes. Pegmatite contain large crystals of tourmaline and garnet. Muscovite is also present in some proportion.
The other hills where the intrusion of pegmatite is seen are Jagnnathpur hill, Tagore hill, Bharamdih hill etc.
Dietrich, R.V. and Skinner, B.J. 1979. Rocks and rock minerals. John Wiley & Sons, U.S.A.
Priyadarshi, N. 1998. A handbook geology of Chotanagpur. Aoyushi Publication, Ranchi, India.