Pegmatite |
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Pegmatite |
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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 are composed of quartz, feldspar and mica; in essence a "granite". Rarer "intermediate" and "mafic" pegmatite containing amphibole, Ca-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions. Crystal size is the most striking feature of pegmatite, with crystals usually over 50mm in size. However, individual crystals over 10 meters across have been found, and the world's largest crystal was found within a pegmatite. Similarly, crystal texture and form within pegmatite may be taken to extreme size and perfection. Feldspar within pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallisation, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within pegmatite often shows gigantic perthitic texture visible to the naked eye. |
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