undepleted supracrustral rock. Economic importance Pegmatites are important because they often contain rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, apatite and corundum, often along with tin, rare earth, and tungsten minerals, among others. Pegmatites have been mined for both quartz and feldspar. For quartz mining, pegmatites with central quartz masses have been of particular interest. Pegmatites are the primary source of lithium either as spodumene, lithiophyllite or usually from lepidolite. The primary source for caesium is pollucite, a mineral from a zoned pegmatite. The majority of the world's beryllium is sourced from non-gem quality beryl within pegmatite. Tantalum, niobium, and rare-earth elements are sourced from a few pegmatites worldwide, such as the Greenbushes Pegmatite, the Kibara Belt of Rwanda and Democratic Republic of the Congo, the Kenticha mine of Ethiopia the Alto Ligonha Province of Mozambique, and the Mibra (Volta) mine of Minas Gerais, Brazil. Occurrence Notable pegmatite occurrences are found worldwide within the major cratons, and within greenschist-facies metamorphic belts. However, pegmatite localities are only well recorded when economic mineralisation is found. Pegmatites are found as irregular dikes, sills, or veins, and are most common at the margins of batholiths (great masses of intrusive igneous rock). Most are closely related spatially and genetically to large intrusions. They may take the form of veins or dikes in the intrusion itself, but more commonly, they extend into the surrounding country rock, especially above the intrusion. Some pegmatites surrounded by metamorphic rock have no obvious connection to a larger intrusion. Pegmatites in low-grade metamorphic rock tend to be dominated by quartz and carbonate minerals. Pegmatites in metamorphic rock of higher grade are dominted by alkali feldspar. Gabbroic pegmatites typically occur as lenses within bodies of gabbro or diabase. Nepheline syenite pegmatites are common in alkaline igneous complexes. References Further reading London, D. (2008): Pegmatites. Canadian Mineralogist Special Publication 10, 347 pp. Tan, Li-ping, 1966, Major Pegmatite Deposits of New York State, New York State Museum Bulletin No. 408 Pegmatopia: David London, School of Geology & Geophysics, University of Oklahoma External links Igneous rocks Economic geology Phaneritic rocks