pegmatit

Pegmatit, or pegmatite, is a coarse-grained igneous rock notable for its exceptionally large mineral grains. Crystals in pegmatites are typically larger than 2.5 centimeters, and some bodies contain crystals that reach decimeter to meter scale. The rock texture is granular, with a matrix of quartz, alkali feldspar (such as microcline or orthoclase), and mica (usually muscovite). Accessory minerals are common and may include tourmaline, spodumene, beryl, cassiterite, lepidolite, apatite, and rare element minerals.

Formation and mineralogy: Pegmatites form during the late stages of crystallization of granitic magmas. They are enriched in volatile components (water, fluorine) and incompatible elements, which lowers the solidification temperature and promotes rapid growth of unusually large crystals. This process often concentrates rare minerals and gem minerals within the pegmatite veins or chambers, producing enriched pockets or sheet-like bodies that cut through surrounding rocks.

Geologic setting and occurrence: Pegmatites are typically associated with granitic intrusions and can occur as dikes, veins, or more extensive sheetlike bodies. They are found in continental crust worldwide and vary in size from small greenstone-like veins to extensive pegmatite belts. Because of their volatile-rich origin, pegmatites often host deposits of lithium, tantalum, niobium, tin, cesium, and a variety of gemstones such as tourmaline, beryl, and aquamarine.

Economic significance: The coarse crystals and concentration of rare minerals make pegmatites important economically. Lithium-bearing pegmatites, in particular, are a major source of lithium ore, while other pegmatites provide tantalum, niobium, tin, and gem minerals.