Pegmatite: An Intrusive Igneous Rock

How Pegmatite Forms

An igneous rock forms by the solidification of molten material. Pegmatite is called an intrusive igneous rock because it forms when magma solidifies under the Earth's surface. In contrast, when magma solidifies outside the Earth's surface, it forms an extrusive igneous rock.

The process by which pegmatite is formed explains why its crystals are so large:

• Pegmatite-forming magma tends to have low viscosity, which allows minerals to move within the fluid. Despite high diffusivity, nucleation rates are low, so a small number of large crystals forms (instead of a large number of small crystals).
• The melt contains water and often volatile carbon dioxide and fluorine. The high vapor pressure and mobility of water allows the melt to retain dissolved ions. As the water escapes, the ions deposit to form crystals.
• The melt commonly contains a high concentration of boron and lithium, which act as fluxing element to lower the temperature needed for solidification.
• The high temperature of surrounding rock and low thermal gradient allows for slow crystallization, which promotes larger crystal size.

Pegmatite occurs throughout the world within greenschist-facies metamorphic belts and major cratons, which tend to occur in the interior of tectonic plates. The rock tends to be associated with granite. In the United States, one excellent place to view pegmatite is at Black Canyon of the Gunnison National Park in Colorado. The park contains metamorphic gneiss and schist, with igneous pink pegmatite, dating back to the Precambrian era.

Mineralogy and Geochemistry

The most common minerals in pegmatite are feldspar, mica, and quartz. While the mineral chemistry is highly variable, the elemental composition often resembles that of granite. However, pegmatite is enriched with trace elements, which make it even more interesting and commercially important. trace elements that makes pegmatite so interesting and commercially important. 

Because the composition of pegmatites is so diverse, they may be classified according to the element or mineral of economic interest. For example, "lithian pegmatite" contains lithium, while "boron pegmatite" contains boron or yields tourmaline.

Uses and Economic Importance

Pegmatite may be cut and polished for architectural stone, but the true economic importance of the rock is as a source of elements and gemstones.

The minerals lepidolite, spodumene, and lithiophyllite in pegmatite are the primary source of the alkali metal lithium. The mineral pollucite is the main source of the metal cesium. Other elements that may be sourced from pegmatite include tantalum, niobium, bismuth, molybdenum, tin, tungsten, and the rare earths.

Sometimes pegmatite is mined for its minerals, including mica and feldspar. Mica is used to make optical elements in electronics. Feldspar is used to make glass and ceramics.

Pegmatites can also contain gemstone-quality minerals, including beryl (aquamarine, emerald), tourmaline, topaz, garnet, corundum (ruby and sapphire), fluorite, amazonite, kunzite, zircon, lepidolite, and apatite.

Pegmatite Key Takeaways

• Pegmatite is an extremely coarse-grained intrusive igneous rock composed of large interlocking crystals.
• There is no defined mineralogy for pegmatite; any plutonic rock may form pegmatite. The most common type of pegmatite is made of granite. Granite pegmatite typically contains feldspar, mica, and quartz.
• Pegmatite is an economically-important rock because it is the source material for lithium, cesium, and rare earth elements and because it may hold large gemstones.

Sources

• Linnen, R. L.; Lichtervelde, M. Van; Cerny, P. (2012-08-01). "Granitic Pegmatites as Sources of Strategic Metals". Elements. 8 (4): 275–280.
• London, David; Morgan, George B. (2012-08-01). "The Pegmatite Puzzle". Elements. 8 (4): 263–268.
• London, D. (2008): Pegmatites. Canadian Mineralogist Special Publication 10, 347 pp.
• Simmons, W. B.; Pezzotta, F.; Shigley, J. E.; Beurlen, H. (2012-08-01). "Granitic Pegmatites as Sources of Colored Gemstones". Elements. 8 (4): 281–287.