Pegmatiter

Pegmatites are exceptionally coarse-grained igneous rocks. Their defining characteristic is the unusually large size of their mineral crystals, often exceeding one centimeter in diameter and sometimes reaching several meters. This coarse grain size is a result of slow cooling of a highly evolved, water-rich magma. The water content in the magma acts as a flux, lowering the melting point and allowing for extended crystal growth before complete solidification.

These rocks are typically found as dikes, sills, and irregular bodies, often intruding into pre-existing rock formations. While pegmatites can be composed of common rock-forming minerals like quartz, feldspar, and mica, their slow cooling also allows for the concentration of rare elements and incompatible elements. This makes pegmatites significant sources for a variety of valuable minerals, including lithium (in minerals like spodumene), tantalum (in tantalite), niobium (in columbite), beryllium (in beryl), and rare-earth elements. Gemstones such as tourmaline, aquamarine, topaz, and garnet are also frequently found in pegmatites, due to the favorable conditions for large crystal formation.

The mineralogy of pegmatites can be quite diverse, but they are often characterized by a simple mineral assemblage, with a few dominant minerals and a scattering of rarer accessory minerals. The texture is typically graphic, where quartz and feldspar are intergrown in a way that resembles cuneiform writing. Pegmatites are genetically linked to larger igneous intrusions, usually granitic or syenitic in composition, representing the final residual melt. Their formation is a testament to the complex processes occurring within Earth's crust.