Pegmatite
(c) 2007 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Pegmatite is a rock type based purely on grain size. Generally pegmatite is defined as a rock bearing abundant interlocking crystals 3 centimeters and larger. However, most pegmatite bodies consist largely of quartz and feldspar and are associated with granitic rocks.
Pegmatite bodies are thought to form predominantly in granites during their final stage of solidification. The final fraction of mineral material is high in water and often also in elements such as fluorine or lithium. This fluid is forced to the edge of the granite pluton and forms thick veins or pods. The fluid apparently solidifies rapidly at relatively high temperatures, under conditions that favor a few very large crystals rather than many small ones. The largest crystal ever found was in a pegmatite, a spodumene grain some 14 meters long.
Pegmatites are sought out by mineral collectors and gemstone miners not only for their large crystals, but for their examples of rare minerals. The pegmatite in this ornamental boulder near Denver, Colorado, features large books of biotite and blocks of alkali feldspar.
Learn more about pegmatites at pegmatology.com, maintained by Skip Williams and Karen Webber of the University of New Orleans.
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