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49-722: The Lapland Granulite Belt is an elongate and arcuate zone of granulite rock in the Cap of the North spanning areas within Norway , Finland and Murmansk Oblast in Russia . At most the belt is 80 km broad. The main rocks of the belt are migmatized greywacke and argillites . Studies of detrital zircon show that the sedimentary protolith of the metamorphic rocks of the belt could not be older than 2900–1940 million years . The belt has norite and enderbite intrusions of calc-alkaline chemistry . It

98-399: A polarizing microscope . The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member (albite 2.62) increases 0.02 per 10% increase in anorthite (2.75). Plagioclase is

147-421: A considerable part of these rocks, they may contain also biotite, hornblende and quartz. Around the garnets there is often a radial grouping of small grains of pyroxene and hornblende in a clear matrix of feldspar: these centric structures are frequent in granulites. The rocks of this group accompany gabbro and serpentine , but the exact conditions under which they are formed and the significance of their structures

196-524: A crushed grain mount can be obtained by the Tsuboi method, which yields an accurate measurement of the minimum refractive index that in turn gives an accurate composition. In thin section , the composition can be determined by either the Michel Lévy or Carlsbad-albite methods. The former relies on accurate measure of minimum index of refraction, while the latter relies on measuring the extinction angle under

245-403: A fair number of minute, rounded, pale-red garnets . Among English and American geologists the term is generally employed in this sense. The granulites are very closely allied to the gneisses , as they consist of nearly the same minerals, but they are finer-grained, have usually less perfect foliation, are more frequently garnetiferous, and have some special features of microscopic structure. In

294-619: A linear pattern resembling gneiss or migmatite banding. [REDACTED] Granulites form at crustal depths, typically during regional metamorphism at high thermal gradients of greater than 30 °C/km. In continental crustal rocks, biotite may break down at high temperatures to form orthopyroxene + potassium feldspar + water, producing a granulite. Other possible minerals formed at dehydration melting conditions include sapphirine, spinel, sillimanite, and osumilite. Some assemblages such as sapphirine + quartz indicate very high temperatures of greater than 900 °C. Some granulites may represent

343-400: A more calcium-rich rim on a more sodium-rich core. Plagioclase also sometimes shows oscillatory zoning, with the zones fluctuating between sodium-rich and calcium-rich compositions, though this is usually superimposed on an overall normal zoning trend. Plagioclase is very important for the classification of crystalline igneous rocks. Generally, the more silica is present in the rock, the fewer

392-535: A plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages. The distinction between these minerals cannot easily be made in the field . The composition can be roughly determined by specific gravity, but accurate measurement requires chemical or optical tests. The composition in

441-477: A silica content of 60.7 wt%; and 1,275 °C (2,327 °F) in dacite with a silica content of 69.9 wt%. These values are for dry magma. The liquidus is greatly lowered by the addition of water, and much more for plagioclase than for mafic minerals. The eutectic (minimum melting mixture) for a mixture of anorthite and diopside shifts from 40 wt% anorthite to 78 wt% anorthite as the water vapor pressure goes from 1 bar to 10 kbar. The presence of water also shifts

490-427: A solid condition, grinding them down and breaking up their minerals, while the pressure to which they were subjected welded them together into coherent rock. It is now believed, however, that they are comparatively recent and include sedimentary rocks , partly of Palaeozoic age, and intrusive masses which may be nearly massive or may have gneissose, flaser or granulitic structures. These have been developed largely by

539-442: A thermal character or due to pressure and crushing. The granites pass into gneiss and granulite; the gabbros into flaser gabbro and amphibolite; the slates often contain andalusite or chiastolite , and show transitions to mica schists. At one time these rocks were regarded as Archean gneisses of a special type. Johannes Georg Lehmann propounded the hypothesis that their present state was due principally to crushing acting on them in

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588-502: A useful estimate of composition if measured accurately. The index of refraction likewise varies smoothly from 1.53 to 1.58, and, if measured carefully, this also gives a useful composition estimate. Plagioclase almost universally shows a characteristic polysynthetic twinning that produces twinning striations on [010]. These striations allow plagioclase to be distinguished from alkali feldspar. Plagioclase often also displays Carlsbad, Baveno, and Manebach Law twinning. The composition of

637-444: Is 6 to 6.5, and cleavage is perfect on [001] and good on [010], with the cleavage planes meeting at an angle of 93 to 94 degrees. It is from this slightly oblique cleavage angle that plagioclase gets its name, Ancient Greek plágios ( πλάγιος 'oblique') + klásis ( κλάσις 'fracture'). The name was introduced by August Breithaupt in 1847. There is also a poor cleavage on [110] rarely seen in hand samples. The luster

686-540: Is also common as a detrital mineral in sedimentary rock . It is not a single mineral , but is a solid solution of two end members , albite or sodium feldspar ( NaAlSi 3 O 8 ) and anorthite or calcium feldspar ( CaAl 2 Si 2 O 8 ). These can be present in plagioclase in any proportion from pure anorthite to pure albite. The composition of plagioclase can thus be written as Na 1−x Ca x Al 1+x Si 3−x O 8 where x ranges from 0 for pure albite to 1 for pure anorthite. This solid solution series

735-590: Is an intrusive rock composed of at least 90% plagioclase. Albite is an end member of both the alkali and plagioclase series. However, it is included in the alkali feldspar fraction of the rock in the QAPF classification. Plagioclase is also common in metamorphic rock. Plagioclase tends to be albite in low-grade metamorphic rock, while oligoclase to andesine are more common in medium- to high-grade metamorphic rock. Metacarbonate rock sometimes contains fairly pure anorthite. Feldspar makes up between 10 and 20 percent of

784-469: Is believed that the belt formed by the closure of an ancient Lapland-Kola Ocean and the continental collision of two continents of Archean age. The granulite belt with its arcuate shape is part of the larger Inari orocline . This orocline include also the Tana and Karasjok belts. Two ideas on origin of the orocline have been proposed; that it originated at the same time as the southwest directed thrusting of

833-450: Is defined by these reaction isograds : Hornblende granulite subfacies is a transitional coexistence region of anhydrous and hydrated ferromagnesian minerals, so the above-mentioned isograds mark the boundary with pyroxene granulite subfacies – facies with completely anhydrous mineral assemblages. Granulite ( Latin granulum , "a little grain") is a name used by petrographers to designate two distinct classes of rocks . According to

882-559: Is known as the alkali feldspar series. Thus, almost all feldspar found on Earth is either plagioclase or alkali feldspar, with the two series overlapping for pure albite. When a plagioclase composition is described by its anorthite mol% (such as An40 in the previous example) it is assumed that the remainder is albite, with only a minor component of potassium feldspar. Plagioclase of any composition shares many basic physical characteristics, while other characteristics vary smoothly with composition. The Mohs hardness of all plagioclase species

931-452: Is known as the plagioclase series. The composition of a particular sample of plagioclase is customarily expressed as the mol% of anorthite in the sample. For example, plagioclase that is 40 mol% anorthite would be described as An40 plagioclase. The ability of albite and anorthite to form solid solutions in any proportions at elevated temperature reflects the ease with which calcium and aluminium can substitute for sodium and silicon in

980-520: Is not very clearly understood. Plagioclase Plagioclase is a major constituent mineral in Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks . Plagioclase is also a major constituent of rock in the highlands of the Moon . Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase

1029-434: Is the most abundant mineral in the crust of Mars. Its name comes from Ancient Greek πλάγιος ( plágios )  'oblique' and κλάσις ( klásis )  'fracture', in reference to its two cleavage angles. Plagioclase is the most common and abundant mineral group in the Earth's crust . Part of the feldspar family of minerals, it is abundant in igneous and metamorphic rock , and it

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1078-428: Is usually white to greyish-white in color, with a slight tendency for more calcium-rich samples to be darker. Impurities can infrequently tint the mineral greenish, yellowish, or flesh-red. Ferric iron (Fe ) gives a pale yellow color in plagioclase feldspar from Lake County, Oregon . The specific gravity increases smoothly with calcium content, from 2.62 for pure albite to 2.76 for pure anorthite, and this can provide

1127-566: Is vitreous to pearly and the diaphaneity is transparent to translucent. The tenacity is brittle, and the fracture is uneven or conchoidal, but the fracture is rarely observed due to the strong tendency of the mineral to cleave instead. At low temperature, the crystal structure belongs to the triclinic system , space group P 1 Well-formed crystals are rare and are most commonly sodic in composition. Well-shaped samples are instead typically cleavage fragments. Well-formed crystals are typically bladed or tabular parallel to [010]. Plagioclase

1176-431: The Earth's crust and the upper mantle , is thought to be the depth where feldspar disappears from the rock. While plagioclase is the most important aluminium-bearing mineral in the crust, it breaks down at the high pressure of the upper mantle, with the aluminium tending to be incorporated into clinopyroxene as Tschermak's molecule ( CaAl 2 SiO 6 ) or in jadeite NaAlSi 2 O 6 . At still higher pressure,

1225-646: The Lapland Granulite Belt or that it former by lateral compression after the thrusting event. Granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism . They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals , with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of

1274-518: The aluminium is incorporated into garnet . At very high temperatures, plagioclase forms a solid solution with potassium feldspar, but this becomes highly unstable on cooling. The plagioclase separates from the potassium feldspar, a process called exsolution . The resulting rock, in which fine streaks of plagioclase ( lamellae ) are present in potassium feldspar, is called perthite . The solid solution between anorthite and albite remains stable to lower temperatures, but ultimately becomes unstable as

1323-528: The asthenospheric mantle in continental rifting settings, which can cause the regional metamorphism at the high thermal gradients of greater than 30 °C/km. The granulite facies is determined by the lower temperature boundary of 700 ± 50 °C and the pressure range of 2–15 kb. The most common mineral assemblage of granulite facies consists of antiperthitic plagioclase , alkali feldspar containing up to 50% albite and Al 2 O 3 -rich pyroxenes . Transition between amphibolite and granulite facies

1372-401: The beautiful play of colors known as chatoyance . In addition to its importance to geologists in classifying igneous rocks, plagioclase finds practical use as construction aggregate , as dimension stone , and in powdered form as a filler in paint, plastics, and rubber. Sodium-rich plagioclase finds use in the manufacture of glass and ceramics. Anorthosite could someday be important as

1421-481: The composition of the crystallizing plagioclase towards anorthite. The eutectic for this wet mixture drops to about 1,010 °C (1,850 °F). Crystallizing plagioclase is always richer in anorthite than the melt from which it crystallizes. This plagioclase effect causes the residual melt to be enriched in sodium and silicon and depleted in aluminium and calcium. However, the simultaneous crystallization of mafic minerals not containing aluminium can partially offset

1470-401: The composition with which plagioclase crystallizes also depends on the other components of the melt, so it is not by itself a reliable thermometer. The liquidus of plagioclase (the temperature at which the plagioclase first begins to crystallize) is about 1,215 °C (2,219 °F) for olivine basalt , with a composition of 50.5 wt% silica; 1,255 °C (2,291 °F) in andesite with

1519-502: The continents contains pyroxene , plagioclase feldspar and accessory garnet , oxides and possibly amphiboles . Both clinopyroxene and orthopyroxene may be present, and in fact, the coexistence of clino- and orthopyroxene in a metabasite (metamorphed basalt) defines the granulite facies. A granulite may be visually quite distinct with abundant small pink or red pyralspite garnets in a 'granular' holocrystalline matrix. Concentrations of garnets, micas , or amphiboles may form along

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1568-483: The deep continental crust . Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth. The minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure conditions experienced during metamorphism. A common type of granulite found in high-grade metamorphic rocks of

1617-565: The depletion in aluminium. In volcanic rock, the crystallized plagioclase incorporates most of the potassium in the melt as a trace element. New plagioclase crystals nucleate only with difficulty, and diffusion is very slow within the solid crystals. As a result, as a magma cools, increasingly sodium-rich plagioclase is usually crystallized onto the rims of existing plagioclase crystals, which retain their more calcium-rich cores. This results in compositional zoning of plagioclase in igneous rocks. In rare cases, plagioclase shows reverse zoning, with

1666-453: The following may be mentioned: hornblende (not common), riebeckite (rare), epidote and zoisite , calcite , sphene , andalusite , sillimanite , kyanite , hercynite (a green spinel), rutile , orthite and tourmaline . Though occasionally we may find larger grains of feldspar, quartz or epidote, it is more characteristic of these rocks that all the minerals are in small, nearly uniform, imperfectly shaped individuals. On account of

1715-419: The framework grains in typical sandstones . Alkali feldspar is usually more abundant than plagioclase in sandstone because Alkali feldspars are more resistant to chemical weathering and more stable, but sandstone derived from volcanic rock contains more plagioclase. Plagioclase weathers relatively rapidly to clay minerals such as smectite . The Mohorovičić discontinuity , which defines the boundary between

1764-541: The granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible with the eye as pink spots on the broken surfaces of the rock. They usually are filled with enclosed grains of the other minerals. The feldspar of the granulites is mostly orthoclase or cryptoperthite; microcline, oligoclase and albite are also common. Basic feldspars occur only rarely. Among accessory minerals, in addition to apatite , zircon , and iron oxides ,

1813-495: The group name of Moine gneisses. Along with the typical acid granulites above described, in Saxony, India , Scotland and other countries there occur dark-colored basic granulites (trap granulites). These are fine-grained rocks, not usually banded, nearly black in color with small red spots of garnet. Their essential minerals are pyroxene, plagioclase and garnet: chemically they resemble the gabbros. Green augite and hypersthene form

1862-401: The injection of semi-consolidated highly viscous intrusions, and the varieties of texture are original or were produced very shortly after the crystallization of the rocks. Meanwhile, however, Lehmanns advocacy of post-consolidation crushing as a factor in the development of granulites has been so successful that the terms granulitization and granulitic structures are widely employed to indicate

1911-402: The larger. This is especially true of the quartz and feldspar which are the predominant minerals; mica always appears as flat scales (irregular or rounded but not hexagonal). Both muscovite and biotite may be present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a rudimentary schistosity , and they may be aggregated into bands in which case

1960-531: The mafic minerals, and the more sodium-rich the plagioclase. Alkali feldspar appears as the silica content becomes high. Under the QAPF classification , plagioclase is one of the three key minerals, along with quartz and alkali feldspar, used to make the initial classification of the rock type. Low-silica igneous rocks are further divided into dioritic rocks having sodium-rich plagioclase (An<50) and gabbroic rocks having calcium-rich plagioclase (An>50). Anorthosite

2009-616: The minuteness with which it has been described and the important controversies on points of theoretical geology which have arisen regarding it, the granulite district of Saxony (in the area of Rosswein and Penig) in Germany may be considered the typical region for rocks of this group. It should be remembered that though granulites are probably the commonest rocks of this country, they are mingled with granites, gneisses, gabbros , amphibolites , mica schists and many other petrographical types. All of these rocks show more-or-less metamorphism either of

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2058-399: The plagioclase crystal structure. Although a calcium ion has a charge of +2, versus +1 for a sodium ion, the two ions have very nearly the same effective radius. The difference in charge is accommodated by the coupled substitution of aluminium (charge +3) for silicon (charge +4), both of which can occupy tetrahedral sites (surrounded by four oxygen ions). This contrasts with potassium, which has

2107-438: The primary aluminium-bearing mineral in mafic rocks formed at low pressure. It is normally the first and most abundant feldspar to crystallize from a cooling primitive magma . Anorthite has a much higher melting point than albite, and, as a result, calcium-rich plagioclase is the first to crystallize. The plagioclase becomes more enriched in sodium as the temperature drops, forming Bowen's continuous reaction series . However,

2156-446: The residues of partial melting at extraction of felsic melts in variable amounts, and in extreme cases represent rocks that all constituent minerals are anhydrous and thus look as if they did not melt at ultrahigh temperature conditions. Therefore, very high temperatures of 900 to 1150 °C are even necessary to produce the granulite-facies mineral assemblages. Such high temperatures at crustal depths only can be delivered by upwelling of

2205-443: The results of dynamometamorphism acting on rocks at a period long after their solidification. The Saxon granulites are apparently for the most part igneous and correspond in composition to granites and porphyries . There are, however, many granulites which undoubtedly were originally sediments ( arkoses , grits and sandstones ). A large part of the highlands of Scotland consists of paragranulites of this kind, which have received

2254-411: The rock approaches ambient surface temperatures. The resulting exsolution results in very fine lamellar and other intergrowths, normally detected only by sophisticated means. However, exsolution in the andesine to labradorite compositional range sometimes produces lamellae with thicknesses comparable to the wavelength of visible light. This acts like a diffraction grating , causing the labradorite to show

2303-428: The rocks of this group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a closely fitted mosaic. The individual crystals never have perfect form, and indeed traces of it are rare. In some granulites they interlock, with irregular borders; in others they have been drawn out and flattened into tapering lenticles by crushing. In most cases they are somewhat rounded with smaller grains between

2352-429: The same charge as sodium, but is a significantly larger ion. As a result of the size and charge difference between potassium and calcium, there is a very wide miscibility gap between anorthite and potassium feldspar , ( KAlSi 3 O 8 ), the third common rock-forming feldspar end member. Potassium feldspar does form a solid solution series with albite , due to the identical charges of sodium and potassium ions, which

2401-726: The terminology of the French school it signifies a granite in which both kinds of mica ( muscovite and biotite) occur, and corresponds to the German Granit , or to the English muscovite biotite granite. This application has not been accepted generally. [ This granitic meaning of granulite is now obsolete. ] To the German petrologists granulite means a more or less banded fine-grained metamorphic rock , consisting mainly of quartz and feldspar in very small irregular crystals and usually also containing

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