pegmatites

Pegmatites are extremely coarse-grained igneous rocks characterized by crystals that are often tens of centimeters to meters across. They are commonly granitic in composition and occur as dikes, veins, or irregular bodies that cut through surrounding rocks. The crystals in pegmatites are much larger than those in typical granite.

They form during the final stages of cooling of granitic magmas. The residual melt becomes enriched in volatile components such as water, boron, and fluorine, and in highly incompatible elements. This promotes rapid growth of unusually large crystals and the concentration of rare minerals, often with a distinct vertical or lateral zoning from the core to the margins.

Mineralogy: The essential minerals are quartz, feldspar (orthoclase or microcline), and mica (muscovite and/or biotite). Accessory minerals are diverse and may include tourmaline, beryl, spodumene, petalite, lepidolite, cassiterite, columbite-tantalite, zircon, and various gemstones. Lithium-bearing pegmatites are important sources of spodumene and other Li-bearing minerals; tantalum- and niobium-rich varieties occur as well.

Distribution and significance: Pegmatites occur in many tectonic settings worldwide, particularly in association with granitic intrusions. They are economically important as sources of lithium, beryllium, tantalum, niobium, tin, and gem-quality minerals such as tourmaline, emerald-like beryl, and topaz. In addition to mining value, pegmatites provide valuable records of late-stage crustal differentiation and mineralization.