Pegmatite

A term originally suggested by Haüy as a synonym for graphic granite, but now mostly used as primarily a textural term for the very coarse-grained facies of any type of igneous rock. The term describes both a coarse-grained igneous texture and a structural body consisting of such rock.

An unusually coarse-grained igneous rock, with interlocking, equigranular crystals, usually found as irregular dikes, lenses, or veins, esp. at the margins of batholiths. Most grains are 1 cm or more in diameter. The dykes and veins typically have symmetrical zonations, with crystals growing inwards from the walls. Although pegmatites having gross compositions similar to various other rock types are known, their composition is generally that of granite. The mineralogy may be simple or complex and may include rare minerals rich in such elements as lithium, beryllium, caesium, boron, fluorine, niobium, tantalum, uranium, and rare earths, often in miaroles. Pegmatites represent the last and most hydrous portion of a magma to crystallize, rich in volatiles like F, B, P, etc. and hence contain high concentrations of minerals present only in trace amounts in normal granitic rocks. Arguably, these rocks can be considered to be as much hydrothermal as igneous in origin.

The most up-to-date classification scheme for pegmatites was presented by Müller et al. (2022) and Wise et al. (2022).

The history of pegmatite research until the early 1960s is carefully summarised and critical discussed by Schneiderhöhn (1961) in much detail.


See also pegmatoid: a term commonly used for pegmatite-like rocks that don’t quite fit the definition. Most of them are metamorphogenic.
See also aplite, a fine-grained "analogue".