Earth scientists classify igneous rocks, those which have cooled from molten magma, based on the minerals present and the size of the mineral crystals. The typical igneous rock is either made of crystals too small to identify with the naked eye, such as basalt; or, like granite, is composed of crystals the size of a pea or slightly bigger. One unusual type of igneous rock, however; contains larger crystals – from thumbnail-sized chunks to crystals the size of cars or telephone poles. Igneous rock deposits with these large (and sometimes gigantic) crystals are called pegmatites.
Pegmatite bodies are generally small compared to granite batholiths and lava flows; often restricted to thin, sheet-like bodies called dikes that fill cracks and fissures. Other pegmatites are found in small, localized pods or lenses at the margins of the batholiths that fill cooled magma chambers beneath volcanic chains. Pegmatities are late-stage igneous rocks that mark the last stage of cooling of the magma.
Typical pegmatites mainly contain quartz, muscovite mica and potassium feldspar; the same group of minerals that comprise granite. Dark ferromagnesian (iron- and magnesium-bearing) minerals such as hornblende, pyroxenes and biotite are absent or rare in typical pegmatites. There are some pegmatites associated with gabbro, the ferromagnesian-rich coarse-grained equivalent of basalt. Those pegmatites rich in minerals containing iron and magnesium are called “gabbroic pegmatites” to distinguish them from typical pegmatites.
Crystal size in all igneous rocks is controlled, in part, by the length of time allowed for the magma to cool. Fast cooling, usually at the surface, produces the fine-grained extrusive rocks like basalt. Slow cooling at depth gives us coarse-grained intrusive rocks such as granite. The larger crystals of pegmatite are not entirely due to slow cooling, however. Pegmatite deposits mark the final stages in the cooling of magma, which leads to the concentration of gaseous elements and water in the melted rock, along with chemical elements that do not combine well with the silicon, potassium and aluminum that remain in the melt.
The high water content of the remaining magma means that the molten rock has low viscosity. This “runny” magma allows the forming mineral grains to gather atoms into their crystal structure for much longer than would happen in thicker, more gooey magma; resulting in large crystals. The time spent during cooling is not as important to the size of crystals, then, as is the water content of the magma.
Throughout the cooling and crystallization of magma; abundant elements such as iron, aluminum, sodium, calcium and potassium readily combine with the silicon and oxygen in the magma to form the most common minerals. In the final stages of cooling, the “oddball” elements can become sufficiently concentrated that minerals bearing these elements begin to crystallize. One element that is often concentrated in pegmatite is lithium, which forms the pale purple mica mineral lepidolite. Other elements that tend to concentrate in pegmatite minerals include the rare earth elements, uranium, boron, beryllium, and many other uncommon atoms.
The slow crystallization of pegmatite mineral grains also allows for the formation of unusually pure and well-formed crystals of gem quality. Semi-precious minerals such as topaz and toumaline are often found in pegmatite; as are precious gems including emerald, the gem-quality form of the beryllium-rich silicate mineral beryl. Precious metals such as gold, silver and platinum are also associated with certain pegmatite bodies. Those metals often crystallize into pure flecks and nuggets that weather out of the pegmatite and form placer deposits in streams draining the area.
Although the gems and precious metals are more glamorous, pegmatites are typically mined for the large crystals of their more common minerals. Large crystals of muscovite; a clear, sheetlike mineral that is also an excellent electrical insulator, have been mined for centuries. Muscovite, also known as isinglass, has long been used in peepholes and small windows in furnaces and stoves. The concentration of unusual elements in the minerals can make for rich deposits of elements that are rare in other rocks, but even huge crystals of potassium feldspar are mined for use in ceramics.
Pegmatites are found throughout the world, generally in the exposed cores of mountain ranges and uplifted regions. In North America, pegmatite deposits are well-known from large mines in the Appalachian and Rocky Mountain ranges. Other pegmatite exposures have been described, and some have been mined, in the Sierra Nevada and the Black Hills.