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Pegmatite is a very coarse-grained igneous rock that has a grain size of 20 mm or more; such rocks are referred to as pegmatitic.
Most pegmatites consist of granite that contains quartz, feldspar and mica. Pegmatites are important because they often contain other rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, apatite, tin, and tungsten, among others. Crystals over 10 meters across have been found.
Granitic pegmatites, boron-granitic pegmatites, lithia pegmatites, and boron-lithia pegmatites exist.
Pegmatites are formed from magma that has cooled very slowly, usually deep underground. When the magma cools slowly underground, and there is an opportunity for the various chemical components of the magma to precipitate out and form crystals. A good example of a rock formed this way is common granite. If you look at granite, you will see that it is made up of small crystals of feldspar, quartz and mica. When magma comes to the surface, such as with a volcano or lava vent, the magma cools very rapidly and produces glassy or very fine-grained rocks like obsidian, pumice or rhyolite.
As result of their slow cooling, pegmatites can have very large crystals, sometimes many feet in length. The long cooling time can also concentrate rarer elements in the pegmatites. Consequently, it is not uncommon to find rare minerals and even gem stones in pegmatites. For example, beautiful crystals of aquamarines and topaz can be found in pegmatites in the mountains of Colorado and Idaho. Pegmatites are also a source for rare earth minerals such as columbite and tantalite.
Pegmatites tend to form veins and thick bands in granites. They can also form pockets that can contain beautifully formed crystals. This is because the crystals are free to grow in the empty space of the pocket without crowding or distortion.
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