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A pegmatite is a holocrystalline, intrusive igneous rock composed of interlocking phaneritic crystals usually larger than 2.5 cm in size (1 inch);such rocks are referred to as pegmatitic.
The word pegmatites derives from Homeric Greek, πεγνυμι (pegnymi), which means “to bind together”, in reference to the intertwined crystals of quartz and feldspar in the texture known as graphic granite.
Most pegmatites are composed of quartz, feldspar and mica, having a similar silicic composition as granite. Rarer intermediate compositionand mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, feldspathoids and other unusual minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.
Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 metres (33 ft) long have been found, and many of the world’s largest crystals were found within pegmatites. These include spodumene, microcline, beryl, and tourmaline.
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