Pegmatite

pegmatite: see granite.

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pegmatite

Almost any wholly crystalline igneous rock that is at least in part very coarse-grained, the major constituents of which include minerals typically found in ordinary igneous rocks (such as granites) and in which extreme textural variations, especially in grain size, are characteristic. Usually found as irregular dikes, lenses, or veins, pegmatite deposits occur in all parts of the world and are the chief source of commercial feldspar, sheet mica, and beryllium, tantalum-niobium, and lithium minerals.

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pegmatite [′peg·mə‚tīt]

(petrology)

Any extremely coarse-grained, igneous rock with interlocking crystals; pegmatites are relatively small, are relatively light colored, and range widely in composition, but most are of granitic composition; they are principal sources for feldspar, mica, gemstones, and rare elements. Also known as giant granite; granite pegmatite.

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Pegmatite 

an igneous rock that is usually found in veins. A rock is classified as a pegmatite if it displays certain properties and characteristics: (1) the size of the constituent minerals must be large; (2) the rock must contain many minerals that have such highly volatile components as water, fluorine, chlorine, and bromine; (3) the mineral composition must be varied and complex, containing in addition to the principal minerals that are common to both the pegmatite and parent rock substantial quantities of minerals that are formed from rare and trace elements, for example, Li, Rb, Cs, Be, Nb, Ta, Zr, Hf, Th, U, and Sc; and (4) it must contain a large quantity of minerals formed as a result of the metasomatic substitution and hydrolysis of feldspar. The concentration of highly volatile, rare, and trace elements in pegmatites is sometimes hundreds and even thousands of times greater than in the parent rocks. The term “pegmatite” was first introduced in 1801 by the French scientist R. J. Haüy to designate graphic, or Hebraic, granite, a frequently encountered structural variety of pegmatite.

Pegmatites sometimes occur as pocket-like inclusions in the intrusive parent rock into which they grade marginally. However, they occur more commonly as veins of varying size and shape that intersect the various igneous, metamorphic, and sedimentary host rocks; the contacts between the pegmatite and the host rock are sharp. Pegmatite veins, which can form sheets, lenses, or pipes, are often several hundred meters thick and several kilometers long and can be traced at depths of more than 1 km. They occur in clusters that usually cover an area of up to tens of km² and that contain from several tens of individual veins to several thousand.

Apart from the relatively simple veins that contain rocks with a homogeneous, fine-grained, graphic texture, pegmatite veins can display a specific zonal structure. As a rule, the peripheral zones in zonal veins are composed of pegmatites with a graphic or granophyric texture; the central zones are composed of rocks with a coarse-grained texture, which is due to the presence of large potassium-sodium and calcium-sodium feldspar crystals, which can sometimes reach several m³ in size. The coarsegrained texture can also result from the presence of muscovite aggregations, the interstices of which are filled with quartz.

Granitic pegmatites, which are genetically associated with intrusions of granite, are the most widely distributed and commercially valuable pegmatites. Micaceous pegmatites are formed at great depths (more than 6 km) and consist of plagioclase, microcline, quartz, muscovite, biotite, black tourmaline, apatite, and beryl. The diversity of minerals in micaceous pegmatites is low in comparison to that of other pegmatites. Micaceous pegmatites serve as a source of sheet muscovite and the ceramic materials microcline and quartz. Rare-metal pegmatites are formed at average depths (4–6 km) and consist of microcline, quartz, and albite; they sometimes also contain other minerals, including spodumene, muscovite, lepidolite, and beryl, as well as various tourmalines, columbite, tantalite, cassiterite, and pollu-cite. Metasomatic substitution (albitization, greisenization) is characteristic of rare-metal pegmatites, which serve as a source of Li, Cs, Be, Ta, and Sn, as well as of such minerals as aquamarine, heliodor, and topaz. Crystal-bearing pegmatites occur at relatively shallow depths (3–4 km) and contain microcline, quartz, albite, muscovite, and biotite. They are the source of rock crystal (which has valuable piezoelectric and optical properties) and optical fluorite and are sometimes a suitable source of topaz, beryl, and amethyst, which occur along the walls of cavities in the quartz zones of crystal bearing pegmatite veins.

Pegmatites that are associated with ultrabasic rocks form veins that measure tens of centimeters to several meters in thickness; the veins constitute a series. The constituent minerals of the central sections of large pegmatite veins include plagioclase, corundum, fluorite, beryl, margarite, and zeolites. Phlogopite-biotite zones, which sometimes also contain emerald, are symmetrically distributed along both sides of the central section; moving outward, these are usually followed by actinolite and chlorite zones, which sometimes contain phenacite and chry-soberyl and which are oriented away from the center toward the enclosing rocks. The peripheral zones are composed of talc and gradually convert into the enclosing rocks, which can be, for example, serpentinite and peridotite.

Pegmatites that are associated with basic magma generally contain hornblende, whose crystals can reach a length of 1 m, and basic plagioclase, whose crystals can measure up to several tens of centimeters in length along the long axis; other constituent minerals include magnetite, ilmenite, the sulfides, and apatite.

Alkali pegmatites, which are genetically associated with alkali igneous rocks, form veins and lenses that are often zoned and measure several tens of meters in thickness and hundreds of meters along the strike. These formations are composed of large crystals, blocks, and pockets of nepheline, microcline, sodalite, natrolite, alkali hornblende, aegirine, and biotite. Among the secondary minerals found in alkali pegmatites are zircon, eudia-lyte, pyrochlore, sphene, and other minerals that contain a variety of such elements as Ti, Be, Th, Nb, and Li.

Several theories on pegmatite formation, or pegmatitization, exist. According to the one proposed by the Soviet scientist A. E. Fersman in the 1920’s, pegmatites form from residual magma that has been enriched with volatile components; the magma undergoes prolonged crystallization with segregation of various mineral associations during different stages of the formation process. The formation of replacement bodies in previously segregated minerals represents the final, definitive stage of pegmatitization.

The Soviet geologist K. A. Vlasov, while accepting the importance of fractional crystallization during pegmatitization, also ascribes a major role to emanation. The latter process causes accumulation of volatile compounds in the upper sections of intrusive masses; it also accounts for the formation of different pegmatite varieties and for the “secondary distillation of volatile compounds” that occurs during the penetration of pegmatite melts and solutions into enclosing rocks. A third theory has been advanced by the American scientists W. T. Schaller, K. Landes, and H. Hess, who relate pegmatitization to metasomatic processes. Finally, the Soviet geologists A. N. Zavaritskii, D. S. Korzhinskii, and V. D. Nikitin suggest that pegmatites are formed from fine-grained, igneous source rocks by recrystalliza-tion in the emergent postmagmatic hydrothermal solutions.

Pegmatites are highly valuable commercially. They are the main source of gems, of feldspar for the ceramics and glass-making industries, and of mica and quartz in electrotechnology. Granite-pegmatite deposits contain rare-metal and rare-earth minerals, for example, spodumene, beryl, columbite, tantalite, lepidolite, cassiterite, pollucite, and uranous and thoric minerals. In the USSR the best-known pegmatite deposits are located in Karelia, in the Ukraine, and in the Urals; other major pegmatite deposits are found in Sweden, Norway, and the United States.

REFERENCES

Fersman, A. E. Pegmatity, 3rd ed., vol. 1. Moscow-Leningrad, 1940.
Vlasov, K. A. “Teksturno-parageneticheskaia klassifikatsiia granitnykh pegmatitov.” Izv. AN SSSR: Ser. geologicheskaia, 1952, no. 2.
Uspenskii, N. M. Negranitnye pegmatity. Moscow, 1965.
Nikitin, V. D. “Pegmatitovye mestorozhdeniia.” In Genezis endogennykh rudnykh mestorozhdenii. Moscow, 1968.
Pegmatity (collection of articles). Leningrad, 1972.
Landes, K. K. “Origin and Classification of Pegmatites.” American Mineralogist, 1933, vol. 18, nos. 2–3.
Schaller, W. T. “The Genesis of Lithium Pegmatites.” American Journal of Science, September 1925, vol. 10.
Jahns, R. H., and C. W. Burnham. “Experimental Studies of Pegmatite Genesis: The Model for Derivation and Crystallization of Granitic Pegmatite.” Economic Geology, 1969, vol. 64, no. 8.

G. G. RODIONOV