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61-560: Many of the world's largest crystals are found within pegmatites. These include crystals of microcline , quartz , mica , spodumene , beryl , and tourmaline . Some individual crystals are over 10 m (33 ft) long. Most pegmatites are thought to form from the last fluid fraction of a large crystallizing magma body. This residual fluid is highly enriched in volatiles and trace elements, and its very low viscosity allows components to migrate rapidly to join an existing crystal rather than coming together to form new crystals. This allows

122-556: A pegmatitic gabbro ) is a coarse-grained rock containing patches of much coarser-grained rock of essentially the same composition. Individual crystals in pegmatites can be enormous in size. It is likely that the largest crystals ever found were feldspar crystals in pegmatites from Karelia with masses of thousands of tons. Quartz crystals with masses measured in thousands of pounds and micas over 10 meters (33 ft) across and 4 meters (13 ft) thick have been found. Spodumene crystals over 12 meters (40 ft) long have been found in

183-406: A composition similar to granite , so that their most common minerals are quartz , feldspar , and mica . However, other pegmatite compositions are known, including compositions similar to nepheline syenite or gabbro . The term pegmatite is thus purely a textural description. Geologists typically prefix the term with a compositional description, so that granitic pegmatite is a pegmatite with

244-523: A few very large crystals to form. While most pegmatites have a simple composition of minerals common in ordinary igneous rock, a few pegmatites have a complex composition, with numerous unusual minerals of rare elements. These complex pegmatites are mined for lithium , beryllium , boron , fluorine , tin , tantalum , niobium , rare earth elements , uranium , and other valuable commodities. The word pegmatite derives from Homeric Greek , πήγνυμι ( pēgnymi ), which means “to bind together”, in reference to

305-633: A grating-like structure that is unmistakable. Perthite is either microcline or orthoclase with thin lamellae of exsolved albite. Amazon stone, or amazonite , is a green variety of microcline. It is not found anywhere in the Amazon Basin , however. The Spanish explorers who named it apparently confused it with another green mineral from that region. The largest documented single crystals of microcline were found in Devil's Hole Beryl Mine, Colorado , US and measured ~50 × 36 × 14 m. This could be one of

366-498: A growing crystal, thus increasing the number of crystals in the system. So both primary and secondary nucleation increase the number of crystals in the system but their mechanisms are very different, and secondary nucleation relies on crystals already being present. It is typically difficult to experimentally study the nucleation of crystals. The nucleus is microscopic, and thus too small to be directly observed. In large liquid volumes there are typically multiple nucleation events, and it

427-604: A higher aluminium content (peraluminous granites). Intermediate pegmatites (NYF + LCT pegmatites) are known and may have formed by contamination of an initially NYF magma body with melted undepleted supracrustral rock. Pegmatites often contain rare elements and gemstones . Examples include aquamarine , tourmaline, topaz, fluorite, apatite, and corundum , often along with tin , rare earth, and tungsten minerals, among others. Pegmatites have been mined for both quartz and feldspar. For quartz mining, pegmatites with central quartz masses have been of particular interest. Pegmatites are

488-698: A larger intrusion. Pegmatites in low-grade metamorphic rock tend to be dominated by quartz and carbonate minerals . Pegmatites in metamorphic rock of higher grade are dominted by alkali feldspar . Gabbroic pegmatites typically occur as lenses within bodies of gabbro or diabase . Nepheline syenite pegmatites are common in alkaline igneous complexes. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Microcline Microcline (KAlSi 3 O 8 )

549-457: A nucleus that may be only of order ten molecules across it is not always clear that we can treat something so small as a volume plus a surface. Also nucleation is an inherently out of thermodynamic equilibrium phenomenon so it is not always obvious that its rate can be estimated using equilibrium properties. However, modern computers are powerful enough to calculate essentially exact nucleation rates for simple models. These have been compared with

610-444: A rate ranging from 1 m to 10 m per day. Pegmatites are the last part of a magma body to crystallize. This final fluid fraction is enriched in volatile and trace elements. The residual magma undergoes phase separation into a melt phase and a hydrous fluid phase saturated with silica , alkalis , and other elements. Such phase separation requires formation from a wet magma, rich enough in water to saturate before more than two-thirds of

671-1035: A simple composition, often being composed entirely of minerals common in granite, such as feldspar, mica, and quartz. The feldspar and quartz often show graphic texture . Rarely, pegmatites are extremely enriched in incompatible elements , such as lithium , caesium , beryllium , tin , niobium , zirconium , uranium , thorium , boron, phosphorus, and fluorine. These complex pegmatites contain unusual minerals of these elements, such as beryl, spodumene, lepidolite, amblygonite, topaz, apatite, fluorite, tourmaline, triphylite , columbite , monazite , and molybdenite . Some of these can be important ore minerals. Some gemstones , such as emerald , are found almost exclusively in pegmatites. Nepheline syenite pegmatites typically contain zirconium, titanium , and rare earth element minerals. Gabbroic pegmatites typically consist of exceptionally coarse interlocking pyroxene and plagioclase . Pegmatites are enriched in volatile and incompatible elements , consistent with their likely origin as

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732-499: A substance or mixture . Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled (at atmospheric pressure ) significantly below 0   °C, it will tend to freeze into ice , but volumes of water cooled only a few degrees below 0   °C often stay completely free of ice for long periods ( supercooling ). At these conditions, nucleation of ice

793-515: Is Wise's (2022) pegmatite classification, which focuses mostly on the source of the magma from which the pegmatite crystalizes. Pegmatites form under conditions in which the rate of new crystal nucleation is much slower than the rate of crystal growth . Large crystals are favored. In normal igneous rocks, coarse texture is a result of slow cooling deep underground. It is not clear if pegmatite forms by slow or rapid cooling. In some studies, crystals in pegmatitic conditions have been recorded to grow at

854-848: Is a chilled margin whose composition is representative of the original melt. Pegmatites derived from batholiths can be divided into a family of NYF pegmatites, characterized by progressive enrichment in niobium , yttrium , and fluorine as well as enrichment in beryllium, rare earth elements, scandium , titanium, zirconium, thorium, and uranium; and a family of LCT pegmatites, characterized by progressive accumulation of lithium, caesium , and tantalum, as well as enrichment in rubidium , beryllium, tin, barium, phosphorus, and fluorine. The NYF pegmatites likely fractionated from A- to I-type granites that were relatively low in aluminium (subaluminous to metaluminous granites). These granites originated from depleted crust or mantle rock. LCT pegmatites most likely formed from S-type granites or possibly I-type granites, with

915-471: Is an important igneous rock -forming tectosilicate mineral . It is a potassium -rich alkali feldspar . Microcline typically contains minor amounts of sodium . It is common in granite and pegmatites . Microcline forms during slow cooling of orthoclase ; it is more stable at lower temperatures than orthoclase. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, white, pale-yellow, brick-red, or green; it

976-426: Is broad agreement on the basic mechanisms by which they form, the details of pegmatite formation remain enigmatic. Pegmatites have characteristics inconsistent with other igneous intrusions. They are not porphyritic , and show no chilled margin . On the contrary, the largest crystals are often found on the margins of the pegmatite body. While aplites are sometimes found on the margins, they are as likely to occur within

1037-514: Is delayed until the system enters the unstable region where a small perturbation in composition leads to a decrease in energy and, thus, spontaneous growth of the perturbation. This region of a phase diagram is known as the spinodal region and the phase separation process is known as spinodal decomposition and may be governed by the Cahn–Hilliard equation . In many cases, liquids and solutions can be cooled down or concentrated up to conditions where

1098-418: Is difficult to disentangle the effects of nucleation from those of growth of the nucleated phase. These problems can be overcome by working with small droplets. As nucleation is stochastic , many droplets are needed so that statistics for the nucleation events can be obtained. To the right is shown an example set of nucleation data. It is for the nucleation at constant temperature and hence supersaturation of

1159-441: Is either slow or does not occur at all. However, at lower temperatures nucleation is fast, and ice crystals appear after little or no delay. Nucleation is a common mechanism which generates first-order phase transitions , and it is the start of the process of forming a new thermodynamic phase. In contrast, new phases at continuous phase transitions start to form immediately. Nucleation is often very sensitive to impurities in

1220-473: Is followed by deposition of albite , lepidolite , gem tourmaline , beryl, spodumene, amblygonite , topaz , apatite , and fluorite , which may partially replace some of the minerals in the earlier zone. The center of the pegmatite may have cavities lined with spectacular gemstone crystals. Some pegmatites have more complex zoning. Five distinct zones are recognized in the Harding Pegmatite in

1281-485: Is found. Pegmatites are found as irregular dikes , sills , or veins , and are most common at the margins of batholiths (great masses of intrusive igneous rock). Most are closely related spatially and genetically to large intrusions. They may take the form of veins or dikes in the intrusion itself, but more commonly, they extend into the surrounding country rock, especially above the intrusion. Some pegmatites surrounded by metamorphic rock have no obvious connection to

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1342-403: Is generally characterized by cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline. The chemical compound name is potassium aluminium silicate, and it is known as E number reference E555 . Microcline may be chemically the same as monoclinic orthoclase, but because it belongs to the triclinic crystal system, the prism angle

1403-415: Is illustrated in the animation to the right. This shows nucleation of a new phase (shown in red) in an existing phase (white). In the existing phase microscopic fluctuations of the red phase appear and decay continuously, until an unusually large fluctuation of the new red phase is so large it is more favourable for it to grow than to shrink back to nothing. This nucleus of the red phase then grows and converts

1464-459: Is on the order of magnitude of one to a few hundred meters. Compared to typical igneous rocks they are rather inhomogeneous and may show zones with different mineral assemblages. Crystal size and mineral assemblages are usually oriented parallel to the wall rock or even concentric for pegmatite lenses. Modern pegmatite classification schemes are strongly influenced by the depth-zone classification of granitic rocks published by Buddington (1959), and

1525-489: Is slightly less than right angles; hence the name "microcline" from the Greek "small slope". It is a fully ordered triclinic modification of potassium feldspar and is dimorphous with orthoclase . Microcline is identical to orthoclase in many physical properties, and can be distinguished by x-ray or optical examination. When viewed under a polarizing microscope , microcline exhibits a minute multiple twinning which forms

1586-441: Is the very first nucleus of that phase to form, or because the nucleus forms far from any pre-existing piece of the new phase. Particularly in the study of crystallisation, secondary nucleation can be important. This is the formation of nuclei of a new crystal directly caused by pre-existing crystals. For example, if the crystals are in a solution and the system is subject to shearing forces, small crystal nuclei could be sheared off

1647-516: The Black Hills of South Dakota , and beryl crystals 8.2 meters (27 ft) long and 1.8 meters (6 ft) in diameter have been found at Albany, Maine . The largest beryl crystal ever found was from Malakialina on Madagascar, weighing about 380 tons, with a length of 18 m (59 ft) and a crosscut of 3.5 m (11 ft). Pegmatite bodies are usually of minor size compared to typical intrusive rock bodies. Pegmatite body size

1708-556: The EFSA . In 2008, it (along with other Aluminum compounds) was the subject of a Scientific Opinion of the Panel on Food Additives, Flavourings, Processing Aids and Food Contact Materials from the EFSA. Nucleation In thermodynamics , nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within

1769-509: The Picuris Mountains of northern New Mexico , US. These are: Large crystals nucleate on the margins of pegmatites, becoming larger as they grow inward. These include very large conical alkali feldspar crystals. Aplites are commonly present. These may cut across the pegmatite, but also form zones or irregular patches around coarser material. The aplites are often layered, showing evidence of deformation. Xenoliths may be found in

1830-495: The Ginsburg & Rodionov (1960) and Ginsburg et al. (1979) classification which categorized pegmatites according to their depth of emplacement and relationship to metamorphism and granitic plutons. Cerny’s (1991) revision of that classification scheme is widely used, Cerny’s (1991) pegmatite classification, which is a combination of emplacement depth, metamorphic grade and minor element content, has provided significant insight into

1891-702: The Kibara Belt of Rwanda and Democratic Republic of the Congo , the Kenticha mine of Ethiopia the Alto Ligonha Province of Mozambique , and the Mibra (Volta) mine of Minas Gerais , Brazil. Notable pegmatite occurrences are found worldwide within the major cratons , and within greenschist -facies metamorphic belts. However, pegmatite localities are only well recorded when economic mineralisation

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1952-453: The body of the pegmatite, but their original mineral content is replaced by quartz and alkali feldspar, so that they are difficult to distinguish from the surrounding pegmatite. Pegmatite also commonly replaces part of the surrounding country rock. Because pegmatites likely crystallize from a fluid-dominated phase, rather than a melt phase, they straddle the boundary between hydrothermal mineral deposits and igneous intrusions . Although there

2013-487: The body of the pegmatite. The crystals are never aligned in a way that would indicate flow, but are perpendicular to the walls. This implies formation in a static environment. Some pegmatities take the form of isolated pods, with no obvious feeder conduit. As a result, metamorphic or metasomatic origins have sometimes been suggested for pegmatites. A metamorphic pegmatite would be formed by removal of volatiles from metamorphic rocks, particularly felsic gneiss , to liberate

2074-617: The classical theory, for example for the case of nucleation of the crystal phase in the model of hard spheres. This is a model of perfectly hard spheres in thermal motion, and is a simple model of some colloids . For the crystallization of hard spheres the classical theory is a very reasonable approximate theory. So for the simple models we can study, classical nucleation theory works quite well, but we do not know if it works equally well for (say) complex molecules crystallising out of solution. Phase-transition processes can also be explained in terms of spinodal decomposition , where phase separation

2135-425: The classification is the petrogenetic component of the classification, which shows the association of LCT pegmatites with mainly orogenic plutons, and NYF pegmatites with mainly anorogenic plutons. Lately, there have been a few attempts to create a new classification for pegmatites less dependent on mineralogy and more reflective of their geological setting. On this issue, one of the most notable efforts on this matter

2196-491: The composition of granite while nepheline syenite pegmatite is a pegmatite with the composition of nepheline syenite. However, the British Geological Survey (BGS) discourages this usage, preferring terms like biotite-quartz-feldspar pegmatite for a pegmatite with a typical granitic composition, dominated by feldspar with lesser quartz and biotite. Under BGS terminology, a pegmatitic rock (for example,

2257-414: The concentration of dissolved chemicals in the water increases. Thus small droplets of water, as found in clouds, may remain liquid far below 0   °C. An example of experimental data on the freezing of small water droplets is shown at the right. The plot shows the fraction of a large set of water droplets, that are still liquid water, i.e., have not yet frozen, as a function of temperature. Note that

2318-489: The crystal phase in small droplets of supercooled liquid tin; this is the work of Pound and La Mer. Nucleation occurs in different droplets at different times, hence the fraction is not a simple step function that drops sharply from one to zero at one particular time. The red curve is a fit of a Gompertz function to the data. This is a simplified version of the model Pound and La Mer used to model their data. The model assumes that nucleation occurs due to impurity particles in

2379-444: The energy barrier for nucleation. The time until the appearance of the first crystal is also called primary nucleation time, to distinguish it from secondary nucleation times. Primary here refers to the first nucleus to form, while secondary nuclei are crystal nuclei produced from a preexisting crystal. Primary nucleation describes the transition to a new phase that does not rely on the new phase already being present, either because it

2440-443: The existing theories including the classical nucleation theory explain well the steady nucleation state when the crystal nucleation rate is not time dependent, the initial non-steady state transient nucleation, and even more mysterious incubation period, require more attention of the scientific community. Chemical ordering of the undercooling liquid prior to crystal nucleation was suggested to be responsible for that feature by reducing

2501-401: The final melt fraction of a crystallizing body of magma. However, it is difficult to get a representative composition of a pegmatite, due to the large size of the constituent mineral crystals. Hence, pegmatite is often characterised by sampling the individual minerals that compose the pegmatite, and comparisons are made according to mineral chemistry. A common error is to assume that the wall zone

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2562-552: The hydrous phase is completely depolymerized, existing almost entirely as orthosilicate , with all oxygen bridges between silicon ions broken. The low viscosity promotes rapid diffusion through the fluid, allowing growth of large crystals. When this hydrous fluid is injected into the surrounding country rock , minerals crystallize from the outside in to form a zoned pegmatite, with different minerals predominating in concentric zones. A typical sequence of deposition begins with microcline and quartz, with minor schorl and garnet . This

2623-510: The intertwined crystals of quartz and feldspar in the texture known as graphic granite . The term was first used by René Just Haüy in 1822 as a synonym for graphic granite . Wilhelm Karl Ritter von Haidinger first used the term in its present meaning in 1845. Pegmatites are exceptionally coarse-grained igneous rocks composed of interlocking crystals , with individual crystals usually over 1 centimeter (0.4 in) in size and sometimes exceeding 1 meter (3 ft). Most pegmatites have

2684-406: The largest crystals of any material found so far. Microcline is exceptionally active ice-nucleating agent in the atmosphere. Recently it has been possible to understand how water binds to the microcline surface. The chemical compound name is potassium aluminium silicate, and it is known as E number reference E555 . It was the subject in 2018 of a Call for technical and toxicological data from

2745-554: The liquid or solution is significantly less thermodynamically stable than the crystal, but where no crystals will form for minutes, hours, weeks or longer; this process is called supercooling . Nucleation of the crystal is then being prevented by a substantial barrier. This has consequences, for example cold high altitude clouds may contain large numbers of small liquid water droplets that are far below 0   °C. In small volumes, such as in small droplets, only one nucleation event may be needed for crystallisation. In these small volumes,

2806-400: The liquid tin droplets, and it makes the simplifying assumption that all impurity particles produce nucleation at the same rate. It also assumes that these particles are Poisson distributed among the liquid tin droplets. The fit values are that the nucleation rate due to a single impurity particle is 0.02/s, and the average number of impurity particles per droplet is 1.2. Note that about 30% of

2867-454: The magma is crystallized. Otherwise, the separation of the fluid phase is difficult to explain. Granite requires a water content of 4 wt% at a pressure of 0.5  GPa (72,500  psi ), but only 1.5 wt% at 0.1 GPa (14,500 psi) for phase separation to take place. The volatiles (primarily water, borates , fluorides , chlorides , and phosphates ) are concentrated in the hydrous phase, greatly lowering its viscosity. The silica in

2928-415: The nucleation of crystals in that there is clear evidence for heterogeneous nucleation, and that nucleation is clearly stochastic. The freezing of small water droplets to ice is an important process, particularly in the formation and dynamics of clouds. Water (at atmospheric pressure) does not freeze at 0   °C, but rather at temperatures that tend to decrease as the volume of the water decreases and as

2989-418: The nucleus at a surface, is much more common than homogeneous nucleation. For example, in the nucleation of ice from supercooled water droplets, purifying the water to remove all or almost all impurities results in water droplets that freeze below around −35   °C, whereas water that contains impurities may freeze at −5   °C or warmer. This observation that heterogeneous nucleation can occur when

3050-652: The origin of pegmatitic melts and their relative degrees of fractionation. Granitic pegmatites are commonly ranked into three hierarchies (class – family – type – subtype) depending upon their mineralogical-geochemical characteristics and depth of emplacement according to Cerny (1991). Classes are Abyssal, Muscovite, Rare-Element and Miarolitic. The Rare-Element Class is subdivided based on composition into LCT and NYF families: LCT for Lithium, Cesium, and Tantalum enrichment and NYF for Niobium, Yttrium, and Fluorine enrichment. Most authors classify pegmatites according to LCT- and NYF-types and subtypes. Another important contribution of

3111-471: The primary source of lithium either as spodumene, lithiophyllite or usually from lepidolite. The primary source for caesium is pollucite , a mineral from a zoned pegmatite. The majority of the world's beryllium is sourced from non-gem quality beryl within pegmatite. Tantalum, niobium, and rare-earth elements are sourced from a few pegmatites worldwide, such as the Greenbushes Pegmatite ,

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3172-405: The rate of homogeneous nucleation is essentially zero, is often understood using classical nucleation theory . This predicts that the nucleation slows exponentially with the height of a free energy barrier ΔG*. This barrier comes from the free energy penalty of forming the surface of the growing nucleus. For homogeneous nucleation the nucleus is approximated by a sphere, but as we can see in

3233-438: The right constituents and water, at the right temperature. A metasomatic pegmatite would be formed by hydrothermal circulation of hot alteration fluids upon a rock mass, with bulk chemical and textural change. Metasomatism is currently not favored as a mechanism for pegmatite formation and it is likely that metamorphism and magmatism are both contributors toward the conditions necessary for pegmatite genesis. Most pegmatites have

3294-456: The schematic of macroscopic droplets to the right, droplets on surfaces are not complete spheres and so the area of the interface between the droplet and the surrounding fluid is less than a sphere's 4 π r 2 {\displaystyle 4\pi r^{2}} . This reduction in surface area of the nucleus reduces the height of the barrier to nucleation and so speeds nucleation up exponentially. Nucleation can also start at

3355-426: The surface of a liquid. For example, computer simulations of gold nanoparticles show that the crystal phase sometimes nucleates at the liquid-gold surface. Classical nucleation theory makes a number of assumptions, for example it treats a microscopic nucleus as if it is a macroscopic droplet with a well-defined surface whose free energy is estimated using an equilibrium property: the interfacial tension σ. For

3416-489: The system is not evolving with time and nucleation occurs in one step, then the probability that nucleation has not occurred should undergo exponential decay . This is seen for example in the nucleation of ice in supercooled small water droplets. The decay rate of the exponential gives the nucleation rate. Classical nucleation theory is a widely used approximate theory for estimating these rates, and how they vary with variables such as temperature. It correctly predicts that

3477-479: The system to this phase. The standard theory that describes this behaviour for the nucleation of a new thermodynamic phase is called classical nucleation theory . However, the CNT fails in describing experimental results of vapour to liquid nucleation even for model substances like argon by several orders of magnitude. For nucleation of a new thermodynamic phase, such as the formation of ice in water below 0   °C, if

3538-523: The system. These impurities may be too small to be seen by the naked eye, but still can control the rate of nucleation. Because of this, it is often important to distinguish between heterogeneous nucleation and homogeneous nucleation. Heterogeneous nucleation occurs at nucleation sites on surfaces in the system. Homogeneous nucleation occurs away from a surface. Nucleation is usually a stochastic (random) process, so even in two identical systems nucleation will occur at different times. A common mechanism

3599-470: The time until the first crystal appears is usually defined to be the nucleation time. Calcium carbonate crystal nucleation depends not only on degree of supersaturation but also the ratio of calcium to carbonate ions in aqueous solutions. In larger volumes many nucleation events will occur. A simple model for crystallisation in that case, that combines nucleation and growth is the KJMA or Avrami model . Although

3660-495: The time you have to wait for nucleation decreases extremely rapidly when supersaturated . It is not just new phases such as liquids and crystals that form via nucleation followed by growth. The self-assembly process that forms objects like the amyloid aggregates associated with Alzheimer's disease also starts with nucleation. Energy consuming self-organising systems such as the microtubules in cells also show nucleation and growth. Heterogeneous nucleation, nucleation with

3721-535: The tin droplets never freeze; the data plateaus at a fraction of about 0.3. Within the model this is assumed to be because, by chance, these droplets do not have even one impurity particle and so there is no heterogeneous nucleation. Homogeneous nucleation is assumed to be negligible on the timescale of this experiment. The remaining droplets freeze in a stochastic way, at rates 0.02/s if they have one impurity particle, 0.04/s if they have two, and so on. These data are just one example, but they illustrate common features of

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