The Probert Encyclopaedia of Rocks & Minerals

PEGMATITE

Pegmatite is a variety of extremely coarse-grained igneous rock chemically similar to and closely associated with granite. The mineral constituents of
pegmatite are largely those typical of acidic intrusive rocks: orthoclase feldspar, quartz, and mica. Individual mineral crystals may be up to several meters in length. The growth of crystals this large indicates that the parent magma, from which the rock solidified, was able to cool very slowly, after injection into fissures extending outward from a central magma chamber.
Pegmatite is widely distributed in the crust of the earth but is found especially in older mountain chains, where it is restricted essentially to those surface areas in which igneous and metamorphic rocks are prevalent. Minerals containing the elements boron, beryllium, and lithium are abundant in some pegmatites. Other pegmatites contain commercial quantities of feldspar, mica, and gem-grade tourmaline.
Research Pegmatite