byDr. Nitish Priyadarshi
Pegmatite, meaning “joined together,” was first applied to graphic granite sometime before 1822. Subsequently, it has been extended to refer to any abnormally coarse grained rock of overall igneous character. The term “pegmatite” was first used by a French mineralogist René Haüy but he used this term as a synonym of graphic granite. Contemporary meaning was given to the rock type in 1845 by an Austrian mineralogist Wilhelm Heidinger.
Most of pegmatites are more than 1 centimeter across; grains up to a meter or two across are relatively common; and individual crystals up to several meters in greatest dimension have been reported. In any case, no matter how large the grains, most pegmatites have typical igneous rock textures.
Pegmatites are associated with plutonic or intrusive rocks and were evidently formed by slow crystallization at considerable depths below the surface.
Individual pegmatite masses may be classified as simple or complex. The simple ones have an overall homogeneity and consist almost wholly of microcline perthite and quartz plus or minus minor amounts of biotite and /or black tourmaline.
The complex pegmatites tend to be compositionally zoned and to contain, along with quartz and microcline, large quantities of clevelandite, noteworthy amounts of muscovite, and well formed crystals of such minerals as apatite, beryl, topaz, colored tourmaline, and spodumene, plus a number of less common minerals that contain elements such as lithium, niobinum, tantalum, cesium, uranium, and the rare earths.
Pegmatites consist of minerals which are found also in the rocks from which they are derived, e.g. granite-pegmatites contain principally quartz and feldspar while gabbro-pegmatites consist of diallage and plagioclase. Rare minerals, however, often occur in these veins in exceptional amount and as very perfect crystals. The minerals of the pegmatites are always those which were last to separate out from the parent rock. As the basic minerals are the first formed the pegmatites contain a larger proportion of the acid or more siliceous components which were of later origin.
Most simple pegmatites occur as dikes within large igneous masses or their surrounding country rocks. Most complex pegmatites occur as lenticular pods or irregularly shaped masses within the country rocks surrounding large masses or as apparently isolated masses within metamorphic rock terranes.
Pegmatites are not rare rocks but their overall volume is small. They form small marginal parts of large magma intrusions known as batholiths. They form as a late-stage magmatic fluid starts to crystallize. This fluid is rich in water, other volatiles, and chemical elements incompatible in main magmatic minerals.
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.
Reference:
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.
http://www.sandatlas.org/2012/09/pegmatite/