pegmatite

Pegmatite is a coarse-grained igneous rock that forms when crystals grow very large, giving the rock a distinctive, chunky texture. It is typically granitic in composition, consisting mainly of quartz, alkali feldspar, and mica, but it is distinguished by crystals that can reach centimeters to meters in size.

The texture and mineralogy of pegmatite reflect its origin in late-stage crystallization of granitic magmas or from highly volatile-rich fluids. Crystallization from a residual, water- and flux-rich melt allows the growth of unusually large crystals and the concentration of incompatible elements into accessory minerals. Many pegmatites host unusual or rare minerals.

Mineral assemblages are variable, but common constituents include quartz, feldspar (microcline or orthoclase), and muscovite. Accessory and economically important minerals include tourmaline, beryl, spodumene (lithium-bearing), lepidolite (lithium-rich mica), cassiterite (tin), wolframite or tantalite (tantalum and niobium), zircon, and other rare-earth minerals. Some pegmatites are classified as rare-element pegmatites because they concentrate lithium, beryllium, tantalum, niobium, cesium, rubidium, and other critical elements.

Occurrence and geology: pegmatites occur most often in granitic or granodioritic terranes and are commonly emplaced as dikes, pods, or sheet-like bodies that cut through surrounding rocks. They may show sharp internal zoning from a Li- or Be-rich core to more common granite-related minerals toward the margins.

Economic importance: pegmatites are important sources of several industrial minerals, especially lithium and tantalum, as well as beryllium, cesium, and rubidium. Their accessibility and the size of crystals have also made them notable targets for mineral collectors.