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Granite
 —  Igneous Rock  —
Granite Image
Granite containing potassium feldspar, plagioclase feldspar, quartz, and biotite and/or amphibole
Composition
Potassium feldspar, plagioclase feldspar, and quartz; differing amounts of muscovite, biotite, and hornblende-type amphiboles.

Granite (pronounced /ˈɡrænɪt/) is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone. The average density of granite is 2.75 g/cm3 and its viscosity at standard temperature and pressure is ~4.5 • 1019 Pa·s.[1]

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Granitoid is used as a discriptive field term for general, light colored, coarse-grained igneous rocks for which a more specific name requires petrographic examination.[2]

Contents

Mineralogy

Orbicular granite near the town of Caldera, northern Chile

Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modern petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called tonalite; pyroxene and amphibole are common in tonalite. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite. Granite has poor primary permeability but strong secondary permeability.

Chemical composition

A worldwide average of the chemical composition of granite, by weight percent: [3]

The Stawamus Chief is a granite monolith in British Columbia

Based on 2485 analyses

Occurrence

Granite is currently known only on Earth where it forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations very coarse-grained pegmatite masses occur with granite.

Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.

Origin

Close-up of granite exposed in Chennai, India.

Granite is an igneous rock and is formed from magma. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional and include French, British, and American systems.

Geochemical origins

Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have compositions at or near a eutectic point (or a temperature minimum on a cotectic curve). Magmas will evolve to the eutectic because of igneous differentiation, or because they represent low degrees of partial melting. Fractional crystallisation serves to reduce a melt in iron, magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon - alkali feldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite.

Close-up of granite from Yosemite National Park, valley of the Merced River

This process operates regardless of the origin of the parental magma to the granite, and regardless of its chemistry. However, the composition and origin of the magma which differentiates into granite, leaves certain geochemical and mineral evidence as to what the granite's parental rock was. The final mineralogy, texture and chemical composition of a granite is often distinctive as to its origin. For instance, a granite which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted basalt may be richer in plagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes are based.

Chappell & White classification system

The letter-based Chappell & White classificiation system was proposed initially to divide granites into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite.[4] Both of these types of granite are formed by melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively.

M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basalt into granite via fractional crystallisation.

A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy and geochemistry. These granites are formed by melting of the lower crust under conditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite.[5][6]

Granitization

An old, and largely discounted theory, granitization states that granite is formed in place by extreme metasomatism by fluids bringing in elements e.g. potassium and removing others e.g. calcium to transform the metamorphic rock into a granite. This was supposed to occur across a migrating front. The production of granite by metamorphic heat is difficult, but is observed to occur in certain amphibolite and granulite terrains. In-situ granitisation or melting by metamorphism is difficult to recognise except where leucosome and melanosome textures are present in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as a transitional between the two, but are not technically granite as they do not actually intrude into other rocks. In all cases, melting of solid rock requires high temperature, and also water or other volatiles which act as a catalyst by lowering the solidus temperature of the rock.

Ascent and emplacement

Roche Rock, Cornwall
The Cheesewring, a granite tor on the southern edge of Bodmin Moor, Cornwall

The ascent and emplacement of large volumes of granite within the upper continental crust is a source of much debate amongst geologists. There is a lack of field evidence for any proposed mechanisms, so hypotheses are predominantly based upon experimental data. There are two major hypotheses for the ascent of magma through the crust:

  • Stokes Diapir
  • Fracture Propagation

Of these two mechanisms, Stokes diapir was favoured for many years in the absence of a reasonable alternative. The basic idea is that magma will rise through the crust as a single mass through buoyancy. As it rises it heats the wall rocks, causing them to behave as a power-law fluid and thus flow around the pluton allowing it to pass rapidly and without major heat loss.[7] This is entirely feasible in the warm, ductile lower crust where rocks are easily deformed, but runs into problems in the upper crust which is far colder and more brittle. Rocks there do not deform so easily: for magma to rise as a pluton it would expend far too much energy in heating wall rocks, thus cooling and solidifying before reaching higher levels within the crust.

Nowadays fracture propagation is the mechanism preferred by many geologists as it largely eliminates the major problems of moving a huge mass of magma through cold brittle crust. Magma rises instead in small channels along self-propagating dykes which form along new or pre-existing fault systems and networks of active shear zones (Clemens, 1998).[8] As these narrow conduits open, the first magma to enter solidifies and provides a form of insulation for later magma.

Granitic magma must make room for itself or be intruded into other rocks in order to form an intrusion, and several mechanisms have been proposed to explain how large batholiths have been emplaced:

  • Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust
  • Assimilation, where the granite melts its way up into the crust and removes overlying material in this way
  • Inflation, where the granite body inflates under pressure and is injected into position

Most geologists today accept that a combination of these phenomena can be used to explain granite intrusions, and that not all granites can be explained entirely by one or another mechanism.

Natural radiation

Granite is a natural source of radiation, like most natural stones. However, some granites have been reported to have higher radioactivity thereby raising some concerns about their safety.

Some granites contain around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where the uranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities.[9] Cellars and basements sunk into soils over granite can become a trap for radon gas, which is formed by the decay of uranium.[10] Radon can also be introduced into houses by wells drilled into granite.[11] Radon gas poses significant health concerns, and is the #2 cause of lung cancer in the US behind smoking.[11]

There is some concern that materials sold as granite countertops or as building material may be hazardous to health. One expert, Dr. Dan Steck of St. Johns University, has stated[12] that approximately 5% of all granites will be of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slabs have been actually tested. Various resources from national geological survey organizations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.

"A study of Granite Countertops was done (initiated and paid for by the Marble Institute of America) in November 2008 by National Health and Engineering Inc of USA,[13] and found that 18 of the 39 full size granite slabs that were measured for the study failed to meet the European Union safety standards (section 4.1.1.1 of the National Health and Engineering study).

Furthermore, all but one of the 39 full size slabs tested in the E,H,& E study had Activity Concentration Indexes above that which the EU regulations require dose assements (Section 4.3.1 of the E,H,&E study). The Marble Institute dealt with this issue by stating that the European Union granite countertop regulations were flawed. The stones tested include types of granite that comprise approximately 80 percent of the annual U.S. market share for granite countertops, based on the most recent market data available."

Other researchers and organizations do not agree with the Marble Institute's stated position on granite safety, including AARST (American Association of Radon Scientists and Technicians) and the CRCPD (Conference of Radiation Control Program Directors, an organization of state radiation protection officials). Both organizations have committees currently setting maximum allowed levels of radiation/radon as well as protocols for measuring radiation/radon from granite countertops. The European Union regulations will likely serve as the basis for new EPA based regulations for granite building materials in the U.S.

Uses

Antiquity

Life-size elephant and other creatures carved in granite; Mahabalipuram, India.

The Red Pyramid of Egypt (c.26th century BC), named for the light crimson hue of its exposed granite surfaces, is the third largest of Egyptian pyramids. Menkaure's Pyramid, likely dating to the same era, was constructed of limestone and granite blocks. The Great Pyramid of Giza (c.2580 BC) contains a huge granite sarcophagus fashioned of "Red Aswan Granite." The mostly ruined Black Pyramid dating from the reign of Amenemhat III once had a polished granite pyramidion or capstone, now on display in the main hall of the Egyptian Museum in Cairo (see Dahshur). Other uses in Ancient Egypt include columns, door lintels, sills, jambs, and wall and floor veneer.[14] How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt[15] has postulated that the Egyptians used emery shown to have higher hardness on the Mohs scale.

Many large Hindu temples in southern India, particularly those built by the 11th century king Rajaraja Chola I, were made of granite. There is a large amount of granite in these structures. They are comparable to the Great Pyramid of Giza.[16]

Modern

Building

Quarrying granite for the Mormon Temple, Utah Territory, in Little Cottonwood Canyon
Polished red granite tombstone
Granite was used for cobblestones on the St. Louis riverfront and for the piers of the Eads Bridge (background).

Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Because of its abundance, granite was commonly used to build foundations for homes in New England. The Granite Railway, America's first railroad, was built to haul granite from the quarries in Quincy, Massachusetts, to the Neponset River in the 1820s. With increasing amounts of acid rain in parts of the world, granite has begun to supplant marble as a monument material, since it is much more durable. Polished granite is also a popular choice for kitchen countertops due to its high durability and aesthetic qualities. In building and for countertops, the term "granite" is often applied to all igneous rocks with large crystals, and not specifically to those with a granitic composition.

Other uses

Curling stones are traditionally fashioned of Ailsa Craig granite. The first stones were made in the 1750s, the original source being Ailsa Craig in Scotland. Because of the particular rarity of the granite, the best stones can cost as much as US$1,500. Between 60–70 percent of the stones used today are made from Ailsa Craig granite, although the island is now a wildlife reserve and is no longer used for quarrying.[17]

In some areas granite is used for gravestones and memorials. Granite is a hard stone and requires skill to carve by hand. Modern methods of carving include using computer-controlled rotary bits and sandblasting over a rubber stencil. Leaving the letters, numbers and emblems exposed on the stone, the blaster can create virtually any kind of artwork or epitaph.

Granite gravestone in Hingham, Massachusetts

Engineering

Engineers have traditionally used polished granite surfaces to establish a plane of reference, since they are relatively impervious and inflexible. Sandblasted concrete with a heavy aggregate content has an appearance similar to rough granite, and is often used as a substitute when use of real granite is impractical. A most unusual use of granite was in the construction of the rails for the Haytor Granite Tramway, Devon, England, in 1820.

Rock climbing

The granite peaks of the Torres del Paine in the Chilean Patagonia

Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction. Well-known venues for granite climbing include Yosemite, the Bugaboos, the Mont Blanc massif (and peaks such as the Aiguille du Dru, the Mountains of Mourne, the Aiguille du Midi and the Grandes Jorasses), the Bregaglia, Corsica, parts of the Karakoram (especially the Trango Towers), the Fitzroy Massif, Patagonia, Baffin Island, the Cornish coast and the Cairngorms.

Granite rock climbing is so popular that many of the artificial rock climbing walls found in gyms and theme parks are made to look and feel like granite.

Half Dome, Yosemite, a classic granite dome and popular rock climb

See also

References

  1. ^ Kumagai, Naoichi; Sadao Sasajima, Hidebumi Ito (15 February 1978). "Long-term Creep of Rocks: Results with Large Specimens Obtained in about 20 Years and Those with Small Specimens in about 3 Years". Journal of the Society of Materials Science (Japan) (Japan Energy Society) 27 (293): 157–161. http://translate.google.com/translate?hl=en&sl=ja&u=http://ci.nii.ac.jp/naid/110002299397/&sa=X&oi=translate&resnum=4&ct=result&prev=/search%3Fq%3DIto%2BHidebumi%26hl%3Den. Retrieved 2008-06-16. 
  2. ^ http://geology.about.com/od/more_igrocks/a/granitoids.htm
  3. ^ Harvey Blatt and Robert J. Tracy (1997). Petrology (2nd ed.). New York: Freeman. pp. 66. ISBN 0716724383. 
  4. ^ Chappell, B.W. and White, A.J.R., 2001. Two contrasting granite types: 25 years later. Australian Journal of Earth Sciences v.48, p.489-499.
  5. ^ Boroughs, S., Wolff, J., Bonnichsen, B., Godchaux, M., and Larson, P., 2005, Large-volume, low-δ18O rhyolites of the central Snake River Plain, Idaho, USA: Geology 33: 821–824.
  6. ^ C.D. Frost, M. McCurry, R. Christiansen, K. Putirka and M. Kuntz, Extrusive A-type magmatism of the Yellowstone hot spot track 15th Goldschmidt Conference Field Trip AC-4. Field Trip Guide, University of Wyoming (2005) 76 pp., plus an appended map.
  7. ^ Weinberg, R. F., and Podladchikov, Y., Diapiric ascent of magmas through power-law crust and mantle, 1994, J. Geophys. Res., 99, 9543-9559
  8. ^ Clemens, John (1998). "Observations on the origins and ascent mechanisms of granitic magmas". Journal of the Geological Society of London 155 (Part 5): 843–51. doi:10.1144/gsjgs.155.5.0843. 
  9. ^ "Radiation and Life". World Nuclear Association. July 2002. http://world-nuclear.org/education/ral.htm. Retrieved 2010-02-04. 
  10. ^ "Decay series of Uranium". http://www.world-nuclear.org/images/info/decayseries.gif. Retrieved 2008-10-19. 
  11. ^ a b "Radon and Cancer: Questions and Answers". National Cancer Institute. http://www.cancer.gov/cancerTopics/factsheet/Risk/radon. Retrieved 2008-10-19. 
  12. ^ Steck, Daniel J. (2009). "Pre- and Post-Market Measurements of Gamma Radiation and Radon Emanation from a Large Sample of Decorative Granites". http://www.aarst.org/proceedings/2009/PRE-AND_POST-MARKET_MEASUREMENTS_OF_GAMMA_RADIATION_AND_RADON_EMANATION_FROM_A_LARGE_SAMPLE_OF_DECORATIVE_GRANITES.pdf. 
  13. ^ http://www.marble-institute.com/industryresources/assessingexposureexecutivesummary.pdf
  14. ^ James A. Harrell. "Decorative Stones in the Pre-Ottoman Islamic Buildings of Cairo, Egypt". http://www.eeescience.utoledo.edu/Faculty/Harrell/Egypt/Mosques/CAIRO_Rocks_1.htm. Retrieved 2008-01-06. 
  15. ^ "Egyptian Genius: Stoneworking for Eternity". http://hebsed.home.comcast.net/hunt.htm. Retrieved 2008-01-06. 
  16. ^ "The Lost Temples of India" (video). http://video.google.com/videoplay?docid=8931191297840928556&q=Lost+temples+India. Retrieved 2008-01-06. 
  17. ^ "National Geographic News — Puffins Return to Scottish Island Famous for Curling Stones". News.nationalgeographic.com. http://news.nationalgeographic.com/news/2004/10/1027_041027_curling_stones.html. Retrieved 2009-07-30. 

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

GRANITE (adapted from the Ital. rgranito, grained; Lat. granum, grain), the group designation for a family of igneous. rocks whose essential characteristics are that they are of acid. composition (containing high percentages of silica), consist principally of quartz and felspar, with some mica, hornblendeor augite, and are of holocrystalline or "granitoid" structure. In popular usage the term is given to almost any crystalline rock which resembles granite in appearance or properties. Thus. syenites, diorites, gabbros, diabases, porphyries, gneiss, and even. limestones and dolomites, are bought and sold daily as "granites." True granites are common rocks, especially among the older strata of the earth's crust. They have great variety in colour and general appearance, some being white or grey, while others are pink, greenish or yellow: this depends mainly on the state of preservation of their felspars, which are their most abundant minerals, and partly also on the relative proportion in which they contain biotite and other dark coloured silicates. Many granites have large rounded or angular crystals of felspar (Shap granite, many Cornish granites), well seen on polished faces. Others show an elementary foliation or banding (e.g. Aberdeen granite). Rounded or oval dark patches frequently appear in the granitic matrix of many Cornish rocks of this group.

In the field granite usually occurs in great masses, covering wide areas. These are generally elliptical or nearly circular and may be 20 m. in diameter or more. In the same district separate areas or "bosses" of granite may be found, all having much in common in their mineralogical and structural features, and such groups have probably all proceeded from the same focus or deep-seated source. Towards their margins these granite outcrops often show modifications by which they pass into diorite or syenite, &c.; they may also be finer grained (like porphyries) or rich in tourmaline, or intersected by many veins of pegmatite. From the main granite dikes or veins often run out into the surrounding rocks, thus proving that the granite is intrusive and has forced its way upwards by splitting apart the strata among which it lies. Further evidence of this is afforded by the alteration which the granite has produced through a zone which varies from a few yards to a mile or more in breadth around it. In the vicinity of intrusive granites slates become converted into hornfelses containing biotite, chiastolite or andalusite, sillimanite and a variety of other minerals; limestones recrystallize as marbles, and all rocks, according to their composition, are more or less profoundly modified in such a way as to prove that they have been raised to a high temperature by proximity to the molten intrusive mass. Where exposed in cliffs and other natural sections many granites have a rudely columnar appearance. Others weather into large cuboidal blocks which may produce structures resembling cyclopean masonry. The tors of the west of England are of this nature. These differences depend on the disposition of the joint cracks which traverse the rock and are opened up by the action of frost and weathering.

The majority of granites are so coarse in grain that their principal component minerals may be identified in the hand specimens by the unaided eye. The felspar is pearly, white or pink, with smooth cleaved surfaces; the quartz is usually transparent, glassy with rough irregular fractures; the micas appear as shining black or white flakes. Very coarse granites are called pegmatite or giant granite, while very fine granites are known as microgranites (though the latter term has also been applied to certain porphyries). Many granites show pearly scales of white mica; others contain dark green or black hornblende in small prisms. Reddish grains of sphene or of garnet are occasionally visible. In the tourmaline granites prisms of black schorl occur either singly or in stellate groups. The parallel banded structures of many granites, which may be original or due to crushing, connect these rocks with the granite gneisses or orthogneisses.

Under the microscope the felspar is mainly orthoclase with perthite or microcline, while a small amount of plagioclase (ranging from oligoclase to albite) is practically never absent. These minerals are often clouded by a deposit of fine mica and kaolin, due to weathering. The quartz is transparent, irregular in form, destitute of cleavage, and is filled with very small cavities which contain a fluid, a mobile bubble and sometimes a minute crystal. The micas, brown and white, are often in parallel growth. The hornblende of granites is usually pale green in section, the augite and enstatite nearly colourless. Tourmaline may be brown, yellow or blue, and often the same crystal shows zones of different colours. Apatite, zircon and iron oxides, in small crystals, are always present. Among the less common accessories may be mentioned pinkish garnets; andalusite in small pleochroic crystals; colourless grains of topaz; six-sided compound crystals of cordierite, which weather to dark green pinite; blue-black hornblende (riebeckite), beryl, tinstone, orthite and pyrites.

The sequence of crystallization in the granites is of a normal type, and may be ascertained by observing the perfection with which the different minerals have crystallized and the order in which they enclose one another. Zircon, apatite and iron oxides are the first; their crystals are small, very perfect and nearly free from enclosures; they are followed by hornblende and biotite; if muscovite is present it succeeds the brown mica. Of the felspars the plagioclase separates first and forms wellshaped crystals of which the central parts may be more basic than the outer zones. Last come orthoclase, quartz, microcline and micropegmatite, which fill up the irregular spaces left between the earlier minerals. Exceptions to this sequence are unusual; sometimes the first of the felspars have preceded the hornblende or biotite which may envelop them in ophitic manner.

An earlier generation of felspar, and occasionally also of quartz, may be represented by large and perfect crystals of these minerals giving the rock a porphyritic character.

Many granites have suffered modification by the action of vapours emitted during cooling. Hydrofluoric and boric emanations exert a profound influence on granitic rocks; their felspar is resolved into aggregates of kaolin, muscovite and quartz; tourmaline appears, largely replacing the brown mica; topaz also is not uncommon. In this way the rotten granite or china stone, used in pottery, originates; and over considerable areas kaolin replaces the felspar and forms valuable sources of china clay. Veins of quartz, tourmaline and chlorite may traverse the granite, containing tinstone often in workable quantities. These veins are the principal sources of tin in Cornwall, but the same changes may appear in the body of the granite without being restricted to veins, and tinstone occurs also as an original constituent of some granite pegmatites.

Granites may also be modified by crushing. Their crystals tend to lose their original forms and to break into mosaics of interlocking grains. The latter structure is very well seen in the quartz, which is a brittle mineral under stress. White mica develops in the felspars. The larger crystals are converted into lenticular or elliptical "augen," which may be shattered throughout or may have a peripheral seam of small detached granules surrounding a still undisintegrated core. Streaks of "granulitic" or pulverized material wind irregularly through the rock, giving it a roughly foliated character.

The interesting structural variation of granite in which there are spheroidal masses surrounded by a granitic matrix is known as "orbicular granite." The spheroids range from a fraction of an inch to a foot in diameter, and may have a felspar crystal at the centre. Around this there maybe several zones, alternately lighter and darker in colour, consisting of the essential minerals of the rock in different proportions. Radiate arrangement is sometimes visible in the crystals of the whole or part of the spheroid. Spheroidal granites of this sort are found in Sweden, Finland, Ireland, &c. In other cases the spheroids are simply dark rounded lumps of biotite, in fine scales. These are probably due to the adhesion of the biotite crystals to one another as they separated from the rock magma at an early stage in its crystallization. The Rapakiwi granites of Finland have many round or ovoidal felspar crystals scattered through a granitic matrix. These larger felspars have no crystalline outlines and consist of orthoclase or microcline surrounded by borders of white oligoclase. Often they enclose dark crystals of biotite and hornblende, arranged zonally. Many of these granites contain tourmaline, fluorite and monazite. In most granite masses, especially near their contacts with the surrounding rocks, it is common to find enclosures of altered sedimentary or igneous materials which are more or less dissolved and permeated by the granitic magma.

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The chemical composition of a few granites from different parts of the world is given below: I. Carn Brea, Cornwall (Phillips); II. Mazaruni, Brit. Guiana (Harrison); III. Rodo, near AlnO, Vesternorrland, Sweden (Holmquist); IV. Abruzzen, a group of hills in the Riesengebirge (Milch); V. Pikes Peak, Colorado (Matthews); VI. Wilson's Creek, near Omeo, Victoria (Howitt).

Only the most important components are shown in the table, but all granites contain also small amounts of zirconia, titanium oxide, phosphoric acid, sulphur, oxides of barium, strontium, manganese and water. These are in all cases less than I %, and usually much less than this, except the water, which may be 2 or 3% in weathered rocks. From the chemical composition it may be computed that granites contain, on an average, 35 to 55% of quartz, 20 to 30% of orthoclase, 20 to 30% of plagioclase felspar (including the albite of microperthite) and 5 to io % of ferromagnesian silicates and minor accessories such as apatite, zircon, sphene and iron oxides. The aplites, pegmatites, graphic granites and muscovite granites are usually richest in silica, while with increase of biotite and hornblende, augite and enstatite the analyses show the presende of more magnesia, iron and lime.

In the weathering of granite the quartz suffers little change; the felspar passes into dull cloudy, soft aggregates of kaolin, muscovite and secondary quartz, while chlorite, quartz and calcite replace the biotite, hornblende and augite. The rock often assumes a rusty brown colour from the liberation of the oxides of iron, and the decomposed mass is friable and can easily be dug with a spade; where the granite has been cut by joint planes not too close together weathering proceeds from their surfaces and large rounded blocks may be left embedded in rotted materials. The amount of water in the rock increases and part of the alkalis is carried away in solution; they form valuable sources of mineral food to plants. The chemical changes are shown by the following analyses: Analyses of I., fresh grey granite; II. brown moderately firm granite; III. residual sand, produced by the weathering of the same mass (anal. G. P. Merrill).

The differences are surprisingly small and are principally an increase in the water and a diminution in the amount of alkalis and lime together with the oxidation of the ferrous oxide. (J. S. F.)


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Wiktionary

Up to date as of January 14, 2010

Definition from Wiktionary, a free dictionary

See also granite

German

Noun

Granite

  1. Plural form of Granit.

Simple English

File:Aberdeen buildings
Nearly every building in Aberdeen, Scotland, is made of granite.

Granite is a kind of igneous rock. It is formed from hot, molten magma.

The magma is forced between other layers of rock by the pressure under the Earth's surface. The magma cools and turns slowly into solid stone. Granite has many different types of minerals in it. These include quartz, feldspar, hornblende, and sometimes mica. As the magma cools, these minerals form crystals. The crystals can be seen easily, if the granite is cut and polished.

Granite is a common stone on Earth, and makes up a big part of the crust (the Earth's outer layer). Although it forms under the surface of the Earth, there are many places where it has been forced upwards by tectonic movement. This is the cracking and folding of the Earth's crust. When this happens, granite mountains can be formed.

Contents

Uses

Kitchen benches are often made of polished granite. Granite is found in many countries of the world. Some countries have beautifully patterned granite which is quarried (cut in open mines) and sold for building material.

Gallery

Description

Granite is dense, and can be cut, carved and shaped. It is resistant to water and pollution, and has a range of different colors. [1]

References

  1. Learn Science intermediate grades 5 to 6, by Mike Evans and Linda Ellis
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