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) 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 band's with differing texture and mineral content. This large, complex pegmatite, approximately 80 m thick, is rich in lithium minerals and rare elements, but apparently crystallized at too great a depth in the crust to allow for the formation of gem-bearing pockets. The elongate, ligh t-colored crystals are common spodumene reaching 5 m in length. Photo by Richard H. Jahns. TABLE 3. Chemical and mineralogical comparison of pegmatites can exceed by several orders of magni- granite and granitic pegmatites, tude the amounts found in other rocks. In fact, the mining of pegmatites for rare elements, or even for Component/ Common Gem-bearing phase Granitea pegmatitec the common pegmatite minerals, has at times been of far greater economic importance than the Chemistry mining of pegmatites for gemstones. For instance, Si02 A1203 the Harding pegmatite (figure 6) was an important FeO + ~ei0; wartime source of beryl for beryllium, of lepidolite Ti02 - - and spodumene for lithium, and of microlite for MnO - 0.28 tantalum. "20 0.6 0.39 MgO trace CaO 0.3 1.36 DESCRIPTIVE CLASSIFICATION Na20 4.6 4.45 K20 4.2 2.85 Efforts over the past century to better understand Li20 - 1.49 granitic pegmatites and their distinctive features P2° 0.3 0.07 have led to numerous attempts to classify them F 0.1 0.11 - 0.18 according to some logical framework. Most early B2° Be0 trace trace schemes were descriptive in nature, and were Rb20 + Cs2O - trace based on various observable features such as shape Total 99.86 or key minerals (see Landes, 1933; Jahns, 1955; Mineralogy Vlasov, 1961; Cerny, 1982). One of the most Major phases Microcline Microcline Microcline widely accepted of these descriptive classifica- Quartz Quartz Quartz tions was summarized by Heinrich (1956). This Plagioclase Albite Albite Muscovite Muscovite Muscovite scheme, which is still useful today, divides grani- Biotite tic pegmatites into three types on the basis of in- Minor phases Beryl Beryl ternal structure: Tourmaline Tourmaline Apatite Apatite 1. Simple (figures 5 and 7a)-lack any segre- Garnet Garnet gation of minerals; may or may not display Spodurnene a systematic variation in texture; consist Rhodizite Lepidolite mostly of common silicate minerals with Hambergite accessory minerals on some occasions. Danburite 2. Zoned (figures 7b, 8, and 9)-possess dis- 'Westerly granite, Westerly, Rhode Island (Turtle and Bowen, 1958). tinct internal zones of contrasting mineral ^~iamondMica pegmatite, Keystone, South Dakota (Norton, 1970). cGem-bearingpegmatite, Manjaka, Madagascar (Schneiderhdhn, 1961). content and texture; often larger than the simple types; consist of common silicate Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 Granitic host rock - No internal zones: quartz-microcline-muscovite Wall zone: quartz-plagioclase-microcline-muscovite-biotite Figure 7. Idealized Intermedate zone: microcline-plagioclase-quartz-muscovite diagram showing internal structural relationships in Core zone: quartz simple (a), zoned (b), Core zone: quartz-spodumene complex (c), pegmatites. - Artwork by Christine IGem pockets-secondary replacement minerals Wilson; adapted from Johns (1953). minerals and various accessory species; fieldwork at numerous localities has documented minerals become more coarse grained to- a basic internal arrangement of minerals in ward the interior of the pegmatite. pegrnatites that is generalized in table 4. In the 3. Complex (figures 6 and 7c)-similar to the field, recognition of this mineral arrangement in a zoned types except also exhibit extensive pegmatite is of enormous help both to the student mineral alteration and replacement; often of pegmatites and to the miner (Sinlzankas, 1970). contain high concentrations of rare ele- ments and unusual minerals. GENETIC CLASSIFICATION Simple pegmatites are by far the most numer- Building on the earlier studies, more recent at- ous, while complex pegmatites are the least com- tempts to classify granitic pegmatites have em- mon but of greatest interest to the geologist and phasized the geologic environment of pegmatite miner. Giant crystals, rare elements, and gem formation rather than descriptive details. For ex- minerals are generally restricted to zoned and ample, an alternative scheme proposed by complex pegmatites. Ginzburg et al. ( 1979; summarized in Cernv, 1982) Minerals found in the latter two types of uses the conditions present at various levels in the pegmatites are arranged in layers or zones. As il- earth's crust to help explain observed differences lustrated in figures 8 and 9, in an ideal situation in the nature of pegmatites. They recognized four these shell-like zones are concentrically disposed classes of pegmatites: around an innermost core and tend to follow the 1. Those formed at very great depth (more exterior shape of the pegmatite body. Although the than 11 km), which are generally unzoned internal structure of many zoned pegmatites is and possess little economic mineralization rarely so uniform, and may be quite complicated, other than occasional concentrations of 70 Gem-Bearing Pegmatites GEMS &. GEMOLOGY Summer 1984 Figure 8. View of the Oregon NO. 2% pegmatite in the South Platte pegmatite district, Jefferson County, Colorado. I This zoned granitic pegmatite is a vertical, column-shooed body with an exceptionally well-developed internal arrangement of minerals. The feldspar- rich intermediate zones were removed during mining operations, while the central quartz core was left standing. The pegmatite body is approximately 70m in diameter. Photo by William B. Simmons, /r. uranium, thorium, and rare-earth ele- ments. 2. Those formed at great depth (approximately 7- 11 lzrn), which may be zoned, are gener- ally rich in mica, but have few rare ele- ments. 3. Those formed at intermediate depth (ap- proximately 3.5-7 lzrn), which are often zoked, may contain small crystal-lined pockets, and possess a number of rare ele- ments. 4. Those formed at shallow depth (less than 3.5 lzrn), which are zoned, and which some- times contain rare elements and gem pockets. Ginzburg et al. further contend that at very Granitic host rock great depth pegmatites are formed largely through Wall zone: quartz-microcline-biotite the partial melting or metamorphic recrystalliza- tion of existing roclzs essentially in place, while at Intermediate zone: microcline lesser depths pegmatite formation becomes more 1 Core zone: quartz and more an igneous process involving the injec- I tion of a magma (molten rock) and its subsequent crystallization. Figure 9. Idealized block diagram of a zoned granitic pegmatite similar to the one shown in OCCURRENCE figure 8. Note the horizontal and vertical zone Geologic Setting. Compared to most other rock structure and the apparent outcrop pattern types, pegmatites are relatively rare, yet they are resulting from the different levels of exposure widely scattered in the earth's crust and can be possible. Artwork by Christine Wilson; adapted from Simmons and Heinrich (1980). locally abundant. Pegmatites tend to be most common in particular geologic settings, generally where igneous or metamorphic rocks are exposed matites near Bancroft, Ontario). Those formed at at the earth's surface (Jahns,1955; Schneiderhohn, deep and intermediate depths occur in folded and 1961; terny, 1982). Those pegmatites formed at metamorphosed rocks in mountain belts (e.g., the very great depths are usually found in metamor- mica pegmatites in the Soviet Union, and the phosed roclzs that comprise the ancient cores of beryl-spodumene pegmatites of the Black Hills, continents (e.g., the uranium-rare-earth peg- South Dakota). The shallow-depth pegmatites are Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 71 associated with large, buried masses of intrusive million years ago. As might be expected from their igneous rocks, known as plutons or batholiths, frequent occurrence in mountainous regions, that frequently underlie mountainous areas (e.g., pegmatites can often be correlated in age with the the gem-bearing pegmatites of southern Califor- corresponding orogenic (mountain-building) nia, of the Hindu Rush region of Afghanistan, and periods. of Minas Gerais, Brazil). Pegmatites in each of these depth-of-formation categories gradually be- Size. In general, pegmatites are rather small rock come exposed at the surface over geologic time by bodies, although quite large ones do occur. Out- either erosion or large-scale mountain uplift, and crops of pegmatites have been observed to range thereby become accessible. from centimeters to meters in minimum dimen- sion and up to several kilometers in maximum Genetic Relationships. The close petrologic rela- dimension (e.g., compare figures 5 and 6). Typi- tionship that is sometimes apparent between cally, pegmatite bodies are completely enclosed by pegmatites and plutonic igneous rocks has been other kinds of rocks. The actual dimensions of taken as evidence that most pegmatites them- pegmatites are often difficult to estimate, not only selves result from the crystallization of silicate because they are frequently irregular in shape, but magmas. However, this connection is most obvi- also because very little of the pegmatite is exposed ous for the shallow pegmatites. As depth increases, on the surface. metamorphic processes seem to play a greater role in pegmatite formation. Shape. Pegmatites are among the least regular and most varied in shape of all rock bodies. This wide Age. The ages of pegmatites span much of the diversity can be attributed to a number of factors, geologic time scale from 3.9 billion to less than 100 including the depth of formation, the mechanical TABLE 4. Some generalized features of internally zoned granitic pegmatitesa Mineralogy Zone Thickness Texture (accessory phases) Other comments Border Usually a few centime- Plagioclase-quartz- May or may' not have ters, but sometimes muscovite (garnet, sharp contacts with the thicker tourmaline, other surrounding host rocks phases) Wall Usually on the order of Generally coarser than Plagioclase- May not be continuous several meters border zone microcline-quartz or of uniform thickness (muscovite, beryl, tour- around the entire maline, garnet, other pegmatite phases) Intermediate Each zone may reach Progressively coarser Microcline-quartz- May consist of a number (possibly several meters in thick- grain size proceeding spodumene-amblygonite- of distinct zones of dif- several ness depending on the inward; some giant muscovite-plagio- fering mineralogy; each zones) size and shape of the crystals; inner~riost clase (tourmaline, may or may not com- pegmatite zones may contain phosphate minerals, pletely enclose the some pocket-rich areas beryl, other phases) central core; interme- diate zones contain the giant crystals and comprise the bulk of the pegmatite; unusual minerals often con- centrated toward core Core Up to several meters in Variable-may contain Quartz-spodumene- Core zone may be thickness depending on both coarse- and fine- microcline (tourmaline, composed of several the size and shape of grained material; some lepidolite, beryl, topaz, segments; gem pockets the pegmatite giant crystals; may in- gem minerals, other often located on the clude pocket-rich areas phases) contact between the core and the enclosing intermediate zones aAdapted from Cameron el a/. (1949) and Norton (1983). 72 Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 internally zoned pegmatites, but even here they are infrequent. Although pocket-bearing granitic pegmatites are widespread in such regions as Minas Gerais, Brazil, and southern California, Jahns (1955)suggested that these cavities probably occur in less than one percent of all known pegmatites. Furthermore, few pocket-bearing pegmatites have the rare elements necessary for the formation of gem minerals in high-quality transparent crystals (figure 10). In fact, most con- tain only crystals of the basic constituents of the pegmatite itself, namely quartz, feldspars, micas, and schorl. Even within the same pegmatite body, pockets Figure 10. Portion of an open gem pocket in the can vary greatly in size and shape. Most are less famous Himalaya pegmatite of Mesa Grands, than several centimeters in diameter, but a few California, with multicolored crystals of gem several meters across have been reported. Never- tourmaline up to several centimeters in length theless, the total volume of pockets is usually triv- along with pale crystals of feldspar and quartz. ial in comparison to that of the enclosing Photo by Michael Havstad. pegmatite. There seems to be no particular rela- tionship between the dimensions of the pegmatite and the number, size, or shape of its pockets. Even properties of the host rock at that depth, and the within the same pegmatite region, some tectonic and metamorphic processes that took pegmatites are remarkably rich in pockets (e.g.,the place at the time of formation. Shallow pegmatites Himalaya pegmatite shown in figure 11)) while often fordsheet-like dikes, veins, or lenses that others have few if any. occur along faults or fractures in pre-existing host Pockets are usually found within the central rocks. Deeper pegmatites, on the other hand, tend core or along the margins between the core zone to be elliptical or ovoid as a result of the more and the enclosing intermediate zones. The mineral plastic character of the enclosing rocks at depth. content from one pocket to another, even in a given pegmatite, is likely to vary considerably. OCCURRENCE OF GEMSTONES Figure 12, a diagram of an actual gem pocket Gemstones never constitute more than a small found in a granitic pegmatite in southern Califor- portion of any pegmatite body. Certain of the more nia, illustrates several important characteristics of common gem minerals, such as tourmaline, beryl, gem pockets. Pocket crystals are firmly rooted in and spodumene, can occur as giant crystals in the the surrounding massive pegmatite, but in the intermediate or core zones of pegmatites. How- open space of the pocket they are able to grow ever, such large crystals are almost always highly freely and thereby develop regular crystal faces. fractured and clouded with inclusions, and conse- Crystals of feldspar and quartz are usually larger quently are of little or no gem value. The smaller, and more abundant than those of any gem min- transparent crystals of these and other gem min- erals that may be present. Pocket crystals are often erals are generally found only in pegmatite cavi- distributed nonuniformly, as is the case with the ties, or "pockets." pocket in figure 12, where tourmaline was only Pockets are irregular openings in otherwise found in place on the roof of the cavity. In some solid pegmatite (see figure 10). As found today, instances these crystals are etched or corroded as a these pockets are usually filled partly or complete- result of chemical attack (see figure 13) and may ly with clay, but during pegmatite crystallization exhibit replacement by secondary minerals. Bro- they provided the necessary open space into which ken crystals of feldspar, quartz, tourmaline, and crystals could grow unimpeded, thereby attaining other minerals that once grew from the walls are a very high degree of internal and external perfec- sometimes found scattered about in the pocket, tion. usually embedded in clays that formed after the Pockets are most common in the complex, crystals had finished growing. Few gem species are Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 Figure 1 1. Underground mine view of the Himalaya pegmatite. The pegmatite, only about 1 m thick, is remarkably rich in gem pockets and has been a noted producer of gem tourmaline. Thepegmatite consists of tan and grayish feldspars, grayish quartz, pink lepidolite, and blaclz schorl. The light-colored areas along the central portion of the pegmatite arenewly exposed but still intact gem pockets. Within the pockets, crystals of gem tourmaline are embedded in clays and other secondary alteration minerals. Photo by Michael Havstad. found in any one pocket-in this case tourmaline and project into a pocket. was the only gem mineral present. Miners often use the color changes in tourma- Some gem minerals exhibit compositional zo- line embedded in the massive pegmatite as an in- nation to a varying degree within individual crys- dication that a pocket area may be close by. Other tals. In an extreme example, such as tourmaline, indicators are the presence of lepidolite, the in- this zonation is reflected in color zoning. That is, creased transparency of quartz, the blaclz staining tourmaline crystals may have black, opaque of the pegmatite by manganese oxides, extensive (schorl) roots in the solid pegmatite and become rock alteration, and the presence of clays. Miners pink, green, blue, etc. (elbaite) as they approach quickly learn, however, that each pegmatite is Figure 12. Diagram of a mapped, vertical cross section through a small, tourmaline-bearing gem pocket in the Stewart pegmatite, Pala, California. Crystal fragments of quartz and gem tourmaline, broken during the final stages of pocket formation, are embedded in the pocket-filling clay material. Artwork by Christine Wilson; adapted from Johns (1979). 1 Microcline Feldspar Albite Feldspar 1 Quartz 1 Muscovite Lepidolite I] Pocket clay with numerous small crystal fragments 1 1 Tourmaline 2.5cm Massive Pegmatite surrounding pocket 74 Gem-Bearing Pegmatite8 GEMS & GEMOLOGY Summer 1984 unique, with its own set of pocket indicators. They also learn that, despite careful attention to these clues, the location of productive gem pockets is still largely a matter of luck and hard work. THE GENESIS OF GEM PEGMATITES Although in the past there has been considerable disagreement among geologists regarding the ori- gin of granitic pegmatites, there is a general con- sensus today that at least the shallower ones formed by crystallization from a magma. Numer- ous theories have been proposed to explain the formation of pegmatites via magmatic crystalli- zation (see Landes, 1933; Jahns, 19551, but perhaps the most widely accepted general model is that of Jahns and Burnham (1969; also see Jahns, 1953, 1979, 1982). Their model is based on the work of many early investigators, notably Fersman ( 1931). The key points of this genetic model relevant to internally zoned granitic pegmatites containing gem minerals are described below. Starting Materials. The starting material for a gem-beaii& pegmatite is a volatile- and rare element-!rich silicate magma derived from the final stages of crystallization of certain granitic magmas. 'Water is the most important volatile constituent of this pegmatite magma; however, other volatiles-such as fluorine, boron, lithium, carbon dioxide, and/or phosphorus-may be pres- Figure 13. This crystal of gem green beryl from ent. The rare elements include beryllium, cesium, the Ukraine region of the Soviet Union niobium, tantalum, and tin, among others. Both (9 x 17.5 cm) displays intriguing surface features caused by chemical etching that took place groups of constituents may be present at much subsequent to its formation in a pegmatite gem higher levels in pegmatite magmas than in the pocket. Photo 0 Harold o) Erica Van Pelt. parent granitic magmas. Emplacement and Initial Crystallization. This as feldspars, quartz, and micas. Because of their pegmatite magma is injected into fractured rock in chemical and structural makeup, the early-formed the upper portion of the crust; as the temperature minerals cannot, for the most part, incorporate the falls, the magma begins to crystallize. Mineral volatiles or rare elements present in the magma. formation begins at the outer margin of the As a result, these components are preferentially pegmatite magma chamber at temperatures retained in the magma, where they become pro- somewhat below 1000°CPlagioclase feldspar, gressively concentrated. As crystallization con- quartz, and muscovite mica crystallize first as the tinues with further cooling, the water content of fine-grained border zone. Later, these are joined in the magma eventually reaches a saturation level. the coarser-grained intermediate zones by mi- At this point, an aqueous fluid, rich in volatiles crocline feldspar and additional minerals, such as and certain rare elements, separates from the re- common spodumene and beryl. maining pegmatite magma at between 750 and 650°C Concentration of Volatiles. The major chemical elements of the pegmatite magma determine the Nature of Aqueous Fluid. The physical properties nature of the abundant, first-formed minerals such of this aqueous fluid are markedly different from Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 those of the magma and, therefore, the fluid greatly occurrence of abundant liquid inclusions in many affects the subsequent crystallization of pegmatite pocket crystals (and the gems faceted from them) minerals. The much lower viscosity of the aque- attests to their growth from an aqueous fluid. The ous fluid permits the rapid transport of chemical final temperature for mineral crystallization from nutrients to the growing crystals, thereby promot- this fluid may be as low as 250°C ing their growth in the innermost zones of the pegmatite. Its greater concentration of volatiles Evolution of the Pocket. Unfortunately, the chang- and many of the rare elements contributes to the ing fluid chemistry, coupled with decreasing tem- partitioning of elements between magma and perature and increasing pressure, can eventually fluid, and thereby to the segregation of minerals in lead to the destruction of many pocket crystals. In separate zones. In addition, vertical segregation of addition, earlier-formed minerals may become un- minerals has been attributed to the rise of the stable in contact with this highly reactive fluid. As less-dense fluid within the pegmatite magma a result, some pockets are found to contain only chamber. The aqueous fluid seems, in general, to the remnants of what may have been gem-quality be a superior solvent. Minerals crystallize from it crystals altered to secondary minerals such as at lower temperatures, and can grow to greater lepidolite mica and montmorillonite clay. Most of size, than from the magma. The fluid also tends to those pockets that ultimately produce gem min- redissolve some earlier-formed minerals with erals are thought by some to have undergone a which it comes in contact. This fluid is probably final, very important step. This step involves the responsible for much of the secondary mineral re- leakage of volatile fluids through breaks in the placement observed in complex pegmatites. pocket walls that may have resulted from one or more of a number of possible mechanisms: Intermediate Stages of Crystallization. The 1. Increase in fluid pressure within the pock- volatile-rich, aqueous fluid continues to exsolve ets that eventually exceeds the confining from the magma as crystallization proceeds, and pressure or strength of the surrounding minerals in the inner zones of the pegmatite form massive pegmatite. in the presence of both the fluid and the magma. 2. Cooling and contraction of the pegmatite The last remaining magma eventually disappears body. at temperatures between 600Âand 500°CThe in- nermost portions of the pegmatite are now occu- 3. Earth movements in the vicinity of the pied by large crystals of feldspar and quartz (and pegmatite body. possibly common spodumene or beryl) with a few If the leakage of volatiles is gradual, the pocket isolated, intervening "pockets" of trapped fluid. crystals will remain intact. Unfortunately, evi- dence indicates that the pocket fluid often escapes Formation of Pocket Crystals. The formation of rapidly, resulting in a very sudden drop in pressure minerals in open pockets is the final stage in the and a consequent dramatic decrease in tempera- primary crystallization of the pegmatite. With de- ture. The resultant "thermal shock" is thought to creasing temperature (600' to 400°Cand rising be responsible for much of the internal fracturing internal pressure (resulting from the release of vol- often observed in gem crystals, as well as for the atile~),crystallization continues from the fluid. shattered fragments of crystals found on the floors Euhedral crystals of various minerals are able to of many gem pockets or embedded in the pocket form from the fluid within the open space of the clay. The fluid lost from the pocket is injected into pockets. At this stage, the concentrations of cer- fractures in the surrounding pegmatite where it tain rare elements may reach sufficiently high results in the replacement of earlier-formed levels for the crystallization of corresponding unu- minerals. sual minerals. In some instances, non-gem-quality Depth of formation is an important factor in crystals projecting into the pocket continue to determining whether pocket rupture will occur. It grow and attain a more flawless, gemmy termina- also seems to be a factor in determining whether tion. This continued mineral growth is accompa- pockets will form at all, because pockets are only nied by changes in the chemical composition of found in the shallow and intermediate-depth the fluid which, in turn, are reflected in corre- pegmatites. It has been suggested that the separate, sponding changes in some minerals (i.e., as exem- less dense, aqueous fluid phase, thought to be re- plified by the color zonation of tourmaline). The quired for pocket formation, is only able to exsolve 76 Gem-Bearing Pegmatites GEMS & GEMOLOGY Summer 1984 from the magma under the lower confining pres- matite~.A small percentage of pegmatites contain sures experienced by pegmatites formed at shal- well-formed crystals of gem minerals in pockets in lower depths. This would explain the apparent their interiors. lack of pockets in deeper pegmatites and the ab- With the increased demand for colored gem- sence of pockets altogether from pegmatites in stones, the mining of pegmatites for gem material many parts of the world. will continue to be an activity of small scale but great economic importance. This demand is lead- SUMMARY ing to accelerated exploration of pegmatite regions Pegmatites are among the most geologically inter- for new sources not only in long-established areas esting and gemologically important types of rocks such as Brazil and southern California, but also in exposed at the earth's surface. They are crystalline newly discovered or recently accessible areas such rocks that are often characterized by highly varia- as East Africa, Madagascar, and Afghanistan. ble texture, giant-size crystals, unusual minerals, Gem-bearing pegmatites are most likely to be and concentrations of rare elements. Pegmatites found in geologic environments where the originate from residual magmas derived by partial pegmatite magmas crystallized at shallow crustal melting of crustal rocks or as products of the final depths. Pegmatites that form in other geologic stages of igneous crystallization. Crystallization of settings hold less promise as sources of gemstones these magmas, which are sometimes rich in vola- because physical and chemical conditions in these tiles and rare elements, gives rise to the distinctive instances seem to preclude the formation of open mineral content and textural features of peg- pockets and, therefore, gem crystals. Norton J.J. (1970)Composition of a pegmatite, Keystone, South Dakota. American Mineralogist, Vol. 55, pp. 98 1 - 1002. Cameron E.N., Jahns R.H., McNair A.M., Page L.R. (1949)In- Norton J.J.(1983) Sequence of mineral assemblages in differen- terrial Structure of Granitic Pegmatites. Monograph 2, Eco- tiated granitic pegmatites. Economic Geology, Vol. 78, nomic Geology, Urbana, IL. pp. 854-874. tern$ P., Ed. (1982)Granitic Pegmatites in Science and Indus- Rickwood P.C. (1981) The largest crystals. American Mineral- try. Short Course Handbook, Vol. 8, Mineralogical Associa- ogist, Vol. 66, pp. 885-907. tion of Canada, Winnipeg. SchneiderhOhn H. (1961)Die Erzlagerstiitlen cler Etde. II. Die Fersman A.E. (1931)Les Pegmatites, leur Importance Scien- Pegmatite. Gustav Fischer Verlag, Stuttgart. tifiqiie et Practique. Akademiya Nauk SSSR, Leningrad. Simmons W.B. Jr., Heinrich E.W. (1980)Rare-earth pegmatites Translated from Russian into French under the direction of of the South Platte district, Colorado. Resource Series 11, R. du Trieu cle Terdonck and J. Thoreau, University de Colorado Geological Survey, Denver, CO. Louvain, Louvain, Belgium, 1952. Sinkankas J. (1970)Prospecting, for Gemstones and Minerals. Ginzburg A.I., Timofeyev I.N., Feldman L.G. (1979)Principles Van Nostrand Reinhold, New York, NY. of Geology of Granitic Pegmatites (in Russian). Nedra, Sinkankas J. (1981)Emerald and Other Beryls. Chilton Book Moscow (summarized in Cerny, 1982). Co., Radnor, PA. Heinrich E.W. (1956)Radioactive pegmatite deposits-how to Tuttle O.F., Bowen N.L. (1958)Origin of Granite in Light of know them. Canadian Mining Journal, Vol. 77, No. 4, Experirnental Studies in the System NaAISiiOx-I Gem-Bearing Pegmati tes GEMS & GEMOLOGY Summer 1984 77