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Pegmatite en.wikipedia.org 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. http://www.gc.maricopa.edu A PEGMATITE is an igneous rock distinguished by its abnormally large crystals. The crystals are normally larger that a few centimeters and can often be dozens of centimeters long or much longer (meters long). Unlike other igneous rocks that develop from the molten state, pegmatites grow from aqueous solutions. The solutions allow for ease of movement of the nutrients to the site of crystal growth. Thus pegmatites can produce large crystals in a short (geologically) period of time. http://www.gemandmineral.com A pegmatite is a coarse grained igneous rock (formed deep within the earth) having a grain size of 3cm or more. Mostly pegmatites are granitic in origin, that is they are composed of granite and its constituents like quartz, feldspar and mica. In addition to these basic minerals there are also generally rare earth elements. Pegmatites are known to contain aquamarine, tourmaline, beryl, topaz, cassiterite, fluorite, apatite, tin and tungsten plus a host of other minerals. Minerals formed in pegmatites literally had the time to enjoy the benefits of a slow crystallization process from a rich chemical stew. One of the most common indicators of a potential pegmatite is the presence of an aplite dike. The pegmatite bodies occur as lenses within the aplite dike structures. Where other minerals such as garnet occur they are the result of a secondary metamorphic process, i.e. one where temperature and pressure have altered the original deposit. Pegmatites are most famous for their large, high quality mineral specimens. The first gem tourmaline discovered in California goes back to 1872 when Henry Hamilton found and recognized the mineral during a collecting trip to the southeast slope of Thomas Mountain in Riverside County. This fact was unrecorded for several years since the occurrence was not made public even to the California State Mining Bureau. The major pegmatite districts of California are the Mesa Grande District, Pala District, Ramona District, Coahuilla District, Rincon District, Ramona District, and Jacumba District. |
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