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Gem-Bearing Pegmatites

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Gem-Bearing Pegmatites GEM-BEARING PEGmTITES : A REVIEW By lames E. Shigley and Anthony R, Kampf Many of the important gem ~nineralsseen quamarine, tourmaline, topaz, lzunzite, mor- on today's marliet-aquamarine, tourma- A ganite-these are but a few of the gemstones fo~~nd he,and topaz, among others-come from in the remarlzable mineral deposits that geologists call an unzisual type of rock lznown as a pegmatites (figure 1).Of the many different lzinds of roclz pegmatite. Gem-bearing pegmatites are exposed at the earth's surface, pegmatites contain the crystalline igneous rocks that are distin- greatest ab~~ndanceand variety of gem minerals. Pegmatite guished by their large-size crystals, concen- trations of certain chemical elements deposits in various parts of the world have yielded spec- otherwise rare in the earth's crust, and var- tacular crystals of gem tourmaline (figure 2)) topaz (figure ious urjus~~alminerals. l'egmatites are typ- 31, and beryl (figure41, as well as a host of other minerals ically rather small bodies of roc)<that are occasionally used as gems (see tables 1 and 2). Most of found in particular geologic environments; these minerals are only rarely found in other geologic envi- the gem minerals occur jn open cavities or ronments in crystals suitable for faceting, In addition, "poclzets" within the pegmatite. This arti- pegmatites are a major source of certain rare elements of cle surveys our current understanding of great economic importance. pegmatites, beginning with a brief de- This article briefly summarizes current knowledge scription of their characteristics and fol- concerning the nature and formation of these fascinating lowing with a disc~issionof the occurrence roclzs and the occurrence of gem minerals within them. of gem minerals in them. The article con- cludes with a summary of the specific Individual pegmatite localities are not discussed in detail conditions necessary d~~ringpegmatite here. Rather, a broad overview is presented to demonstrate formation for the crystall~zaiionof abun- the remarlzable similarities between gem-producing dant gem minerals. pegmatites in diverse parts of the world. Armed with this perspective, the reader should be able to better appreciate detailed reports of pegmatite occurrences such as those in Brazil that are discussed elsewhere in this issue. WHAT IS A PEGMATITE? The famous mineralogist Hafly first used the word pegmatite in the early 1800s to refer to a roclz with a patterned geometric intergrowth of feldspar and quartz (now commonly termed graphic granite). Today it is ap- ABOUT THE AUTHORS plied to any crystalline roclz that is, at least in part, ex- Dr. Shigley is research scientisl in /he Depar/men/ tremely coarse grained. The term pegmatjte, then, of Research, Gemologica/ lns/ilute of America, Santa Mon~ca,California; and Dr. Kampfis curator primarily refers to the texture of a roclz, that is, the size, of gems and minerals, Los Angeles Counly Mu- shape, and arrangement of mineral grains. In practice, it seum of Natural History, Los Angeles, California. can be applied to a wide range of roclzs of igneous or meta- 07984 Gemological lnsti/ule of Amer~ca morphic origin that exhibit large crystals. 64 Gem-Bearing Pegmatites - GEMS & GEMOLOGY Summer 1984 Figure 1. A selection of gemstones of pegmatite origin ranging from 5.87 to 11.25 ct. Minerals shown (from left to right, and from front to baclz) are us follows: tourmuline (elbaite),garnet (spessartine), chrysoberyl; topaz, feldspar (or~hocluse),beryl (morgunite); beryl (aquumarine), tourmaline (elbaite), tourmaline (elbaite); beryl (heliodor),spodumene (kunzite). Photo by Tino Hammid. In reality! the vast majority of pegmatites are the mineral grains in a granite are quite uniform in found to be chemically and mineralogically simi- size and only rarely exceed several millimeters in lar to ordinary granites andl hence! are called diameter. Those in a pegmatite are usually several "granitic1' pegmatites (as opposed to "gabbroicl' centimeters across on average! but they can range pegmatites' lfsyeniticllpegmatites' etc.). Because from millimeters to meters in diameter (figure 5). gem minerals arel for the most part) found only in Typically there is an increase in crystal size from granitic pegmatites! these will be the focal point of the outer margins toward the interior of the the ensuing discussion. pegmatite body. The largest crystals ever found have come Texture. Although pegmatites are commonly from pegmatites (seel for example! Jahns! 1953; thought of as very coarse-grained roclzsf they ac- Rickwood! 1981; Sinkankas' 198 1). Outstanding tually vary considerably in grain size. This varia- examples include a 14-m-long spodumene crystal bility is important in distinguishing pegmatites from the Etta mine in the Black Hills of South from most other crystalline rocks. For example! Dakota and an 18-m-long beryl crystal from a Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 Figure 2. Multicolored crystal of to~~rmaline with albite feldspar and quartz from a pegmatite in Afghanistan (14 x 9 cm), Photo 0 Harold d Erica Van Pelt. Specimen courtesy of David Wilber. pegmatite at Malakialina! Madagascar. The fa- these pegmatites have generally been of limited mous Harding pegmatite in New Mexico contains economicl scientific! or gemologic interest. A spodumene crystals that are 5 m long (figure 6). small percentage of pegmatites! however! contain Unfortunatelyl these and most other giant crystals additional minerals! such as beryl and tourmaline) are not of gem quality. There are! however, excep- which incorporate certain rare elements. This lat- tions: for instance! a 300-lzg transparent gem topaz ter group of granitic pegmatites, which are the from a pegmatite in Minas Gerais, Brazil! is now on source of most gem rough! have been the principal display in the American Museum of Natural His- objects of pegmatite exploration and mining. tory in New York. Establishing the chemical composition of a rock provides important clues regarding its origin! Mineralogy and Chemistry. The minerals that geologic history, and relationships to other rock occur in any rock depend on the rock's overall types. Relatively fine-grained roclzsl such as gran- chemistry and the pressure and temperature con- ites! are easily sampled and analyzed to determine ditions under which it formed. Most granitic overall chemical composition; however, this is not pegmatites are composed of the same minerals generally the case for pegmatites. Their coarse and found in ordinary granites! that isJ feldspars variable grain size) nonuniform distribution of (microclinel plagio~lase)~quartz! micas (musco- minerals! and often poor surface exposure pose vite, biotite)! and on occasion some common ac- serious obstacles to accurate chemical analysis. cessory minerals (table 3; also figure 5). As such Nevertheless! painstalzing work at a number of 66 Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 crystals wjth smolzy quartz from a pegmatite in the Ural Mountains of the Soviet Union (9 cm wide). Photo 0 Harold etl Erica Van Pelt. Specimen courtesy of David Wilber. Figure 4. A superb crydt.01 of morganite beryl with albite feldspar from ;he White Q~~een r~~ine,Pala, California (4 x 5 cm). Photo 0Harold d Erica Vun Pelt. Table 1. Occurrence of gem minerals in pegmatites. m Relative Pegmalites the Mineral abundancea major source? Common Feldspar (gem varieties A no orthoclase, amazonite, moonstone) Quartz (gem varieties rock crystal, amethyst, smoky, rose, citrine) Unusual (containing rare elements such as Li, Be, B, P, F, Cs, etc.) Apatite Amblygonite Beryl (gem varieties aquamarine, morganite, heliodor, goshenite) Beryllonite Brazilianite Figure 5. Small pegmatite body cutting through Chrysoberyl granite at the Fletcher stone quarry near Danburite Westford, Massachusetts. Both the pegmatite and Euclase Garnet (spessartine) enclosing granite are composed of feldspars, Hambergite mica, and qLlartz, but within [he peg~nu~itethe Herderile crystals are m~~chlarger, In addition, the Lepidolite cryslals near the center of the pegmatite are Petalite Phenakite larger ~hanthose along the outer margins. Pollucile Pho~oby Richard H. Iahns. Spodumene (gem varieties kunzile, hiddenite) Topaz Tourmaline (elbaite-gem Common granitic pegmatites show little devi- varieties rubellite, ation from typical granite chemistry (compare achroite, indicolite; liddicoatite) columns 1 and 2 in table 3).In contrast, those that contain small amounts of unusual minerals ex- aRelatiave abundance in granitic pegmafiles: A =abundant and wide- hibit marlzed enrichments in a variety of rare ele- spread; C = common or locally abundant; R = rare or uncommon; VR = very rare. ments (table 3, column 3). Lithium, berylliuml boron, and fluorine, in particular, are essential constituents in several important gem minerals. localities has yielded meaningful estimates of the Despite their small total amounts (seldom over 2 chemical composition of granitic pegmatites (see wt. expressed as oxides), the concentrations of table 3). these and other rare elements in some granitic TABLE 2. Some of the more important gem pegmatite regions. -- -- Region Tourmaline Beryl Spodumene Topaz Quartz Garnet Afghanislanlpakistan Brazil Madagascar Mozambique Namibia Soviet Union Ural Mountains Transbaikalia Ukraine East Africa United States New England Colorado California 68 Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 Figure 6. View of the Haiding pegmatite in Taos County, New Mexico. Several internal zones are visible as colored
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