Difference between revisions of "Granite" - New World Encyclopedia

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[[Image:Granite-Mt_Bago-USGS-mfe00742.jpg|thumb|400px|right|Jagged blades and pinnacles of granite on the south side of Mount Bago, Kings Canyon National Park, California.]]
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'''Granite''' is a hard, tough, [[igneous rock|igneous]] [[Rock (geology)|rock]] that is widely distributed in the [[Earth]]'s continental crust. It is medium- to coarse-grained and consists of a number of [[mineral]]s, especially members of the [[feldspar]] group and [[quartz]]. It varies in composition and comes in a range of colors, such as white, pink, buff, gray, and black, often occurring in combination. Given its ruggedness and wide distribution, it has been used as a construction stone since antiquity. For example, it was used to build some of the [[pyramid]]s of [[ancient Egypt]] and temples of ancient southern [[India]]. Today, granite continues to be used as a dimension stone (stone available in large quantities cut to specific sizes) in buildings and monuments. In addition, given its strength and aesthetic appeal, it is used for kitchen countertops and flooring tiles. Polished granite stones are used in the team sport known as [[curling]].
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{{toc}}
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== Origin and occurrence ==
 +
[[Image:Granite-Mt_Bago-USGS-mfe00742.jpg|thumb|400px|right|Jagged blades and pinnacles of granite on the south side of Mount Bago, Kings Canyon National Park, California]]
  
'''Granite''' is a common and widely-occurring type of [[Intrusion (geology)|intrusive]], [[felsic]], [[igneous rock|igneous]] [[rock (geology)|rock]].
+
The word ''granite'' comes from the [[Latin]] ''granum'', for grain, referring to the common, coarse-grained structure for this type of [[crystal]]line rock. In [[geology]], granite is called an intrusive, igneous rock. An igneous rock is one that is formed when molten rock (magma) cools and solidifies. The term "intrusive" indicates that the rock solidified below the [[Earth]]'s surface after it first intruded into other rocks. The average [[density]] of granite is 2.75&nbsp;g•cm<sup>&minus;3</sup> with a range of 1.74&nbsp;g•cm<sup>&minus;3</sup> to 2.80&nbsp;g•cm<sup>&minus;3</sup>.
  
Granites are usually a white, black or buff color and are medium to coarse [[grain size|grained]], occasionally with some individual crystals larger than the groundmass forming a rock known as [[Porphyry (geology)|porphyry]]. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy.  
+
Granite is currently known only on Earth, where it forms a major part of the [[continental crust]]. It has been found as relatively small (less than 100 km<sup>2</sup> ), stock-like masses and also as huge formations called "[[batholith]]s," often associated with [[mountain]] ranges formed by "orogeny" (the mountain-forming processes of plate tectonics). Most granite intrusions are emplaced at depths from 1.5 - 50 km (.9 - 30 miles) in the continental crust. Small [[dike (geology)|dikes]] of granitic composition called "aplites" are associated with granite margins.
  
Outcrops of granite tend to form [[tor]]s, rounded massifs, and terrains of rounded boulders cropping out of flat, sandy soils. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the [[metamorphic]] [[aureole]] or [[hornfels]].  
+
Granite has been intruded into the [[Earth|Earth's crust]] during all [[Geologic ages|geologic periods]]; much of it is of [[Precambrian]] age. The origin of granite is contentious and has led to varied schemes of classification. There is a French scheme, a British scheme, and an American scheme. This confusion arises because the classification schemes define granite by different means. The "alphabet-soup" classification is often used because it classifies granite based on origin of the "parental" magma from which the granite was formed.
  
Granite is nearly always massive, hard and tough, and it is for this reason it has gained widespread use as a construction stone.  
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<center> <gallery>
 +
Image:Granite azul noce.jpg|<center>Azul Noce ([[Spain]])</center>
 +
Image:Granite giallo.jpg|<center>Giallo Veneziano ([[Brazil]])</center>
 +
Image:Granite_gran_violet.jpg|<center>Gran Violet ([[Brazil]])</center>
 +
Image:Granite lavanda blue.jpg|<center>Lavanda Blue ([[Brazil]])</center> </gallery> </center>
  
The average [[density]] of granite is 2.75&nbsp;g·cm<sup>&minus;3</sup> with a range of 1.74&nbsp;g·cm<sup>&minus;3</sup> to 2.80&nbsp;g·cm<sup>&minus;3</sup>.
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===Geochemical origins===
  
The word granite comes from the [[Latin]] ''granum'', a grain, in reference to the coarse-grained structure of such a [[crystal]]line rock.
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As magma in the Earth's crust cools, it undergoes the process of "fractional crystallization"&mdash;a process by which some minerals crystallize before others. This process enriches the melt in silicon, aluminum, and potassium (the major constituents of granite), and lowers the content of iron, magnesium, titanium, calcium, and sodium.
  
==Mineralogy==
+
This process operates regardless of the chemistry and origin of the magma, but it leaves geochemical and mineralogical evidence of the composition and origin of the parental magma from which granite was formed. The final mineralogy, texture, and chemical composition of a granite are often distinctive, based on its origin. For instance, a granite formed from melted sediments will contain more alkali [[feldspar]] (oligoclase), whereas a granite derived from melted [[basalt]] will be richer in [[plagioclase]] feldspar. Modern classification schemes are built on this basis.
[[Image:QAPF diagram granite.svg|450px|right|thumb|Figure 1. QAPF diagram of granitoids and phaneritic foidolites (plutonic rocks).]]
 
  
Granite primarily consists of [[orthoclase]] and [[plagioclase]] [[feldspar]]s, [[quartz]], [[hornblende]], [[muscovite]] and/or [[biotite]] [[mica]]s, and minor accessory minerals such as [[magnetite]], [[garnet]], [[zircon]] and [[apatite]]. Rarely, a [[pyroxene]] is present.
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===Alphabet Soup Classification===
  
Granite is classified according to the [[QAPF diagram]] for coarse grained plutonic rocks (granitoids) and is named according to the percentage of Quartz, Alkali feldspar (orthoclase) and Plagioclase Feldspar on the A-Q-P half of the diagram. Highly peralkaline forms of granite which are silica undersaturated may have a [[feldspathoid]] such as [[nepheline]], and are classified on the A-F-P half of the diagram. See Figure 1, below.  
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The "alphabet soup" scheme of Chappell and White was proposed initially to divide granites into "I-type" (igneous protolith) and "S-type" (sedimentary protolith). Both of these types of granite were formed by the melting of precursor rock called the ''protolith''.
  
True granite according to modern petrology contains both plagioclase and orthoclase feldspars. When a granitoid is devoid of orthoclase the rock is referred to as alkali granite or ''adamellite''. When a granitoid contains <5% orthoclase it is known as a granodiorite, or [[tonalite]] when pyroxene is present.  
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Later, "M-type" ([[Mantle (geology)|mantle]]-derived) granite was proposed, to cover granites that clearly arose from magmas sourced from the mantle. This type of granite is rare.
  
A granite containing both muscovite and biotite [[mica]]s 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.  
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"A-type" (or ''anorogenic'') granites are formed above volcanic "hot spot" activity and have a peculiar mineralogy and [[geochemistry]]. These granites are formed by melting of the lower [[crust]] under conditions that are usually extremely dry. An example of an A-type granite is a [[caldera]] at [[Yellowstone National Park]].
  
The [[volcanic]] equivalent of [[plutonic]] granite is [[rhyolite]].
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===Granitization===
  
=== Chemical Composition ===
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The granitization theory states that granite (or some types of granite) is formed in place by extreme [[metamorphism]]&mdash;that is, the solid-state recrystallization of pre-existing rock, under conditions of extreme heat and pressure. The production of granite by metamorphic heat is difficult, but it is observed to occur in certain [[amphibolite]] and [[granulite]] terrains. These rocks are technically not granite as they do not actually intrude into other rocks.
A worldwide average of the average proportion of the different chemical components in granites, in descending order, is approximately<ref name="LG">{{cite book
 
| author = E. H. Shackleton
 
| year = 1971
 
| title = Lakeland Geology
 
| publisher = Dalesman
 
| location = Clapham (via Lancaster)
 
| edition = 3rd edition
 
| pages = 49}}</ref>:
 
* [[Silicon dioxide|SiO<sub>2</sub>]] &mdash; 70.18%
 
* [[Aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]] &mdash; 14.47%
 
* [[Potassium oxide|K<sub>2</sub>O]] &mdash; 4.11%
 
* [[Sodium oxide|Na<sub>2</sub>O]] &mdash; 3.48%
 
* [[Calcium oxide|CaO]] &mdash; 1.99%
 
* [[Iron(II) oxide|FeO]] &mdash; 1.78%
 
* [[Iron(III) oxide|Fe<sub>2</sub>O<sub>3</sub>]] &mdash; 1.57%
 
* [[Magnesium oxide|MgO]] &mdash; 0.88%
 
* [[Water (molecule)|H<sub>2</sub>O]] &mdash; 0.84%
 
* [[Titanium dioxide|TiO<sub>2</sub>]] &mdash; 0.39%
 
* [[Diphosphorus pentoxide|P<sub>2</sub>O<sub>5</sub>]] &mdash; 0.19%
 
* [[Manganese Oxide|MnO]] &mdash; 0.12%
 
  
==Occurrence==
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==Mineralogy==
Granite is currently known only on Earth where it forms a major part of [[continental crust]]. Granite occurs as relatively small, less than 100 km<sup>2</sup> stock-like masses and as large [[batholith]]s often associated with [[orogeny|orogenic]] [[mountain]] ranges and is frequently of great extent. Small [[dike (geology)|dikes]] of granitic composition called [[aplite]]s are associated with granite margins. In some locations very coarse-grained [[pegmatite]] masses occur with granite.
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[[Image:QAPF diagram granite.svg|450px|right|thumb|Figure 1. QAPF diagram of granitoids and phaneritic foidolites (plutonic rocks).]]
  
Granite has been intruded into the [[Crust (geology)|crust]] of the [[Earth]] during all [[Geologic ages|geologic periods]]; much of it is of [[Precambrian]] age. Granite is widely distributed throughout the [[continental crust]] of the Earth and is the most abundant ''basement rock'' that underlies the relatively thin [[sedimentary rock|sedimentary]] veneer of the continents.
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Granite comprises a number of minerals, primarily [[orthoclase]] and [[plagioclase]] [[feldspar]]s, [[quartz]], [[hornblende]], and [[muscovite]] or [[biotite]] [[mica]]s. In addition, it contains minor amounts of other minerals, such as [[magnetite]], [[garnet]], [[zircon]], and [[apatite]]. On rare occasions, a [[pyroxene]] is present.
  
==Origin==
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The naming of various types of granite is based on their mineralogic composition, using what is called the "QAPF diagram" for coarse-grained plutonic rocks (granitoids). QAPF is an acronym for four mineral groups: Quartz, Alkali feldspar (orthoclase), Plagioclase feldspar, and Feldspathoid (foid). Plutonic rocks are rocks formed when magma solidifies underground (see the QAPF diagram on the right).
Granite is an [[igneous]] rock and is formed from magma. Granite 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&nbsp;[[kilometre|km]] and up to 50&nbsp;km depth within thick continental crust.
 
  
The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional; there is a French scheme, a British scheme and an American scheme. This confusion arises because the classification schemes define granite by different means. Generally the 'alphabet-soup' classification is used because it classifies based on genesis or origin of the magma.  
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According to modern petrology, true granite contains both plagioclase and orthoclase feldspars. When a granitoid is extremely rich in orthoclase, the rock is referred to as alkali granite. If quartz is either absent or present in relatively small amounts (under five percent), the rock is called syenite. A granite containing both muscovite and biotite [[mica]]s is called a binary or "two-mica" granite. Two-mica granites are typically high in potassium and low in plagioclase.
  
===Geochemical origins===
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=== Chemical Composition ===
Granitoids are a ubiquitous component of the crust because they represent a [[eutectic]] point to which magmas will evolve via [[igneous differentiation]]. This occurs because fractional crystallisation serves to reduce a melt in iron, magnesium, titanium, calcium and sodium, and enrich the melt in potassium, aluminium and silicon - which are the major constituents of a granite.
 
  
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 mineralogical 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.
+
The average proportions of the different chemical components in granites, estimated on a worldwide basis, is approximately as follows (in descending order)<ref name="LG">{{cite book
 +
| author = Shackleton, E. H.
 +
| year = 1971
 +
| title = Lakeland Geology
 +
| publisher = Dalesman
 +
| location = Clapham (via Lancaster)
 +
| edition = 3rd edition}}</ref>:
 +
* Silicon dioxide (SiO<sub>2</sub>) &mdash; 70.18%
 +
* Aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) &mdash; 14.47%
 +
* Potassium oxide (K<sub>2</sub>O) &mdash; 4.11%
 +
* Sodium oxide (Na<sub>2</sub>O) &mdash; 3.48%
 +
* Calcium oxide (CaO) &mdash; 1.99%
 +
* Iron(II) oxide (FeO) &mdash; 1.78%
 +
* Iron(III) oxide (Fe<sub>2</sub>O<sub>3</sub>) &mdash; 1.57%
 +
* Magnesium oxide (MgO) &mdash; 0.88%
 +
* Water (H<sub>2</sub>O) &mdash; 0.84%
 +
* Titanium dioxide (TiO<sub>2</sub>) &mdash; 0.39%
 +
* Diphosphorus pentoxide (P<sub>2</sub>O<sub>5</sub>) &mdash; 0.19%
 +
* Manganese Oxide (MnO) &mdash; 0.12%
  
For instance, a granite which is formed from melted sediments will have more alkali feldspar, whereas a granite derived from melted [[basalt]] will be richer in [[plagioclase]] feldspar. It is on this basis that the modern classification shemes are based.
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==Emplacement mechanisms==
  
===Alphabet Soup Classification===
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For over a century, geologists have been grappling with the problem of how large volumes of molten rock are emplaced within the solid Earth. This problem has still not been entirely resolved. Granite magma must make room for itself or be intruded into other rocks, and several mechanisms have been proposed to explain how large batholiths have been emplaced.
The 'alphabet soup' scheme of Chappell & White was proposed initially to divide granites into ''I-type'' granite (or [[igneous]] protolith) granite and ''S-type'' or sedimentary [[protolith]] granite. 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.  
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* Stoping: The granite cracks the wall rocks and pushes upward as it removes blocks of the overlying crust.
 +
* Diapirism: The granite of lower density causes relative buoyancy and pushes upward, warping and folding the rock above it.
 +
* Assimilation: The granite melts its way up into the crust, thereby removing overlying material.
 +
* Inflation: The granite body inflates under pressure and is injected into position.
  
M-type or [[Mantle (geology)|mantle]] derived granite was proposed later, to cover those granites which were clearly sourced from crystallised [[mafic]] magmas, generally sourced from the mantle. These are rare, because it is difficult to turn [[basalt]] into granite via [[fractional crystallisation]].  
+
Most geologists today accept that a combination of these processes may operate to produce granite intrusions, as no single mechanism provides an adequate explanation.
  
A-type or ''anorogenic'' granites are formed above [[hot spot]] activity and have peculiar mineralogy and [[geochemistry]]. These granites are formed by melting of the lower [[crust]] under conditions which are usually too dry. The granite [[caldera]] of [[Yellowstone]] National Park is an example of an A-type granite.
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==Uses==
 +
===Uses in antiquity===
  
===Granitization===
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The [[Red Pyramid]] of [[ancient Egypt]] (about twenty-sixth century B.C.E..), 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]] (about 2580 B.C.E..) 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]]. Other uses of granite in ancient Egypt include [[column]]s, door [[lintel]]s, [[sill]]s, [[jamb]]s, and wall and floor [[veneer]].<ref>Harrell, James A. [http://www.eeescience.utoledo.edu/Faculty/Harrell/Egypt/Mosques/CAIRO_Rocks_1.htm Preliminary Description of Stone Varieties Used in Medieval Islamic Buildings of Cairo, Egypt.] University of Toledo. Retrieved June 12, 2007.</ref>
The granitization theory states that granite is formed in place by extreme [[metamorphism]]. 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 [[gneiss]]es. 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 volatiles which act as a [[catalyst]] by lowering the [[solidus]] temperature of the rock.
 
  
==Emplacement mechanisms==
+
How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt has postulated that the Egyptians used [[Emery (mineral)|emery]], which has greater hardness on the [[Mohs scale of mineral hardness|Mohs scale]].<ref>Corbin, Nancy. [http://hebsed.home.comcast.net/hunt.htm Egyptian Genius: Stoneworking for Eternity.] American Research Center in Egypt, Northern California Chapter. Retrieved June 12, 2007.</ref>
The problem of emplacing large volumes of molten rock within the solid Earth has faced geologists for over a century, and is not entirely resolved. Granite 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
 
* [[Diapir|Diapirism]] where the density of the lighter granite causes relative buoyancy and the granite pushes upwards, warping and folding the rock above it
 
* 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 phenomenon can be used to explain granite intrusions, and that not all granites can be explained by one or another mechanism.
+
Many large Hindu temples in southern India, particularly those built by the eleventh-century king [[Rajaraja Chola I]], were made of granite. The amount of granite in them is said to be comparable to that used in the Great Pyramid of Giza.<ref>“The Lost Temples of India.” Discovery. [http://video.google.com/videoplay?docid=8931191297840928556&q=Lost+temples+India Available online at Google Video.] Retrieved June 12, 2007.</ref>
  
==Uses==
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===Modern uses===
===Antiquity===
 
The [[Red Pyramid]] of [[Ancient Egypt]] (c.[[26th century B.C.E.]]), 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.[[26th century B.C.E.|2580 B.C.E.]]) 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]], [http://www.eeescience.utoledo.edu/Faculty/Harrell/Egypt/Mosques/CAIRO_Rocks_1.htm] include [[column]]s, door [[lintel]]s, [[sill]]s, [[jamb]]s, and wall and floor [[veneer]].
 
  
How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt [http://hebsed.home.comcast.net/hunt.htm] has postulated that the Egyptians used [[Emery (mineral)|emery]] shown to have higher [[Hardenability|hardness]] on the [[Mohs scale of mineral hardness|Mohs scale]].
+
* Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Given the problem of [[acid rain]] in parts of the world, granite has begun to supplant [[marble]] as a monument material, because it is much more durable. The Black Galaxy granites from the Cheemakurthy area of [[Andhra Pradesh]], [[India]], are renowned for their elegance.
  
Many large Hindu temples in southern India, particularly those built by the 11th century king [[Rajaraja Chola I]], were made of granite. In fact, the amount of granite in them is comparable to the Great Pyramid of Giza. [http://video.google.com/videoplay?docid=8931191297840928556&q=Lost+temples+India]
+
* Polished granite has become a popular choice for [[kitchen]] countertops, given its high durability and aesthetic qualities.
  
===Modern===
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* Engineers have traditionally used polished granite surfaces to establish a [[Plane (mathematics)|plane]] of reference, because they are relatively impervious and inflexible.
<!-- Unsourced image removed: [[Image:Granitebuilding.jpg|right|thumb|200px|A building clad in polished granite]] —>
 
Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. 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 has been a popular choice for [[kitchen]] countertops due to its high durability and aesthetic qualities. The Black Galaxy granites from the Cheemakurthy area of [[Andhra Pradesh]] in [[India]] are world known for their elegance.
 
  
[[Engineers]] have traditionally used polished granite surfaces to establish a [[Plane (mathematics)|plane]] of reference, since they are relatively impervious and inflexible.
+
* The team sport called "[[curling]]" uses stones that are traditionally made of polished granite.
  
In the world of sports, [[curling]] rocks are traditionally fashioned of granite.
+
* 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.
 
 
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.
 
 
 
<center> <gallery>
 
Image:Granite azul noce.jpg|<center>Azul Noce ([[Spain]])</center>
 
Image:Granite giallo.jpg|<center>Giallo Veneziano ([[Brazil]])</center>
 
Image:Granite_gran_violet.jpg|<center>Gran Violet ([[Brazil]])</center>
 
Image:Granite lavanda blue.jpg|<center>Lavanda Blue ([[Brazil]])</center> </gallery> </center>
 
  
 
==See also==
 
==See also==
* [[List of minerals]]
+
* [[Igneous rock]]
* [[List of rocks]]
+
* [[Mineral]]
* [[Igneous rocks]]
 
 
* [[Rock (geology)]]
 
* [[Rock (geology)]]
  
==References==
+
== Notes ==
 
 
 
<references/>
 
<references/>
  
 
==External links==
 
==External links==
 
+
All links retrieved January 9, 2014.
*[http://www.geologynet.com/granite1.htm The Emplacement and Origin of Granite]
+
*[http://www.geologynet.com/granite1.htm The Emplacement and Origin of Granite] by Rob Kanen.
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]

Revision as of 17:28, 9 January 2014

Granite is a hard, tough, igneous rock that is widely distributed in the Earth's continental crust. It is medium- to coarse-grained and consists of a number of minerals, especially members of the feldspar group and quartz. It varies in composition and comes in a range of colors, such as white, pink, buff, gray, and black, often occurring in combination. Given its ruggedness and wide distribution, it has been used as a construction stone since antiquity. For example, it was used to build some of the pyramids of ancient Egypt and temples of ancient southern India. Today, granite continues to be used as a dimension stone (stone available in large quantities cut to specific sizes) in buildings and monuments. In addition, given its strength and aesthetic appeal, it is used for kitchen countertops and flooring tiles. Polished granite stones are used in the team sport known as curling.

Origin and occurrence

File:Granite-Mt Bago-USGS-mfe00742.jpg
Jagged blades and pinnacles of granite on the south side of Mount Bago, Kings Canyon National Park, California

The word granite comes from the Latin granum, for grain, referring to the common, coarse-grained structure for this type of crystalline rock. In geology, granite is called an intrusive, igneous rock. An igneous rock is one that is formed when molten rock (magma) cools and solidifies. The term "intrusive" indicates that the rock solidified below the Earth's surface after it first intruded into other rocks. The average density of granite is 2.75 g•cm−3 with a range of 1.74 g•cm−3 to 2.80 g•cm−3.

Granite is currently known only on Earth, where it forms a major part of the continental crust. It has been found as relatively small (less than 100 km2 ), stock-like masses and also as huge formations called "batholiths," often associated with mountain ranges formed by "orogeny" (the mountain-forming processes of plate tectonics). Most granite intrusions are emplaced at depths from 1.5 - 50 km (.9 - 30 miles) in the continental crust. Small dikes of granitic composition called "aplites" are associated with granite margins.

Granite has been intruded into the Earth's crust during all geologic periods; much of it is of Precambrian age. The origin of granite is contentious and has led to varied schemes of classification. There is a French scheme, a British scheme, and an American scheme. This confusion arises because the classification schemes define granite by different means. The "alphabet-soup" classification is often used because it classifies granite based on origin of the "parental" magma from which the granite was formed.

Geochemical origins

As magma in the Earth's crust cools, it undergoes the process of "fractional crystallization"—a process by which some minerals crystallize before others. This process enriches the melt in silicon, aluminum, and potassium (the major constituents of granite), and lowers the content of iron, magnesium, titanium, calcium, and sodium.

This process operates regardless of the chemistry and origin of the magma, but it leaves geochemical and mineralogical evidence of the composition and origin of the parental magma from which granite was formed. The final mineralogy, texture, and chemical composition of a granite are often distinctive, based on its origin. For instance, a granite formed from melted sediments will contain more alkali feldspar (oligoclase), whereas a granite derived from melted basalt will be richer in plagioclase feldspar. Modern classification schemes are built on this basis.

Alphabet Soup Classification

The "alphabet soup" scheme of Chappell and White was proposed initially to divide granites into "I-type" (igneous protolith) and "S-type" (sedimentary protolith). Both of these types of granite were formed by the melting of precursor rock called the protolith.

Later, "M-type" (mantle-derived) granite was proposed, to cover granites that clearly arose from magmas sourced from the mantle. This type of granite is rare.

"A-type" (or anorogenic) granites are formed above volcanic "hot spot" activity and have a peculiar mineralogy and geochemistry. These granites are formed by melting of the lower crust under conditions that are usually extremely dry. An example of an A-type granite is a caldera at Yellowstone National Park.

Granitization

The granitization theory states that granite (or some types of granite) is formed in place by extreme metamorphism—that is, the solid-state recrystallization of pre-existing rock, under conditions of extreme heat and pressure. The production of granite by metamorphic heat is difficult, but it is observed to occur in certain amphibolite and granulite terrains. These rocks are technically not granite as they do not actually intrude into other rocks.

Mineralogy

Figure 1. QAPF diagram of granitoids and phaneritic foidolites (plutonic rocks).

Granite comprises a number of minerals, primarily orthoclase and plagioclase feldspars, quartz, hornblende, and muscovite or biotite micas. In addition, it contains minor amounts of other minerals, such as magnetite, garnet, zircon, and apatite. On rare occasions, a pyroxene is present.

The naming of various types of granite is based on their mineralogic composition, using what is called the "QAPF diagram" for coarse-grained plutonic rocks (granitoids). QAPF is an acronym for four mineral groups: Quartz, Alkali feldspar (orthoclase), Plagioclase feldspar, and Feldspathoid (foid). Plutonic rocks are rocks formed when magma solidifies underground (see the QAPF diagram on the right).

According to modern petrology, true granite contains both plagioclase and orthoclase feldspars. When a granitoid is extremely rich in orthoclase, the rock is referred to as alkali granite. If quartz is either absent or present in relatively small amounts (under five percent), the rock is called syenite. 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.

Chemical Composition

The average proportions of the different chemical components in granites, estimated on a worldwide basis, is approximately as follows (in descending order)[1]:

  • Silicon dioxide (SiO2) — 70.18%
  • Aluminum oxide (Al2O3) — 14.47%
  • Potassium oxide (K2O) — 4.11%
  • Sodium oxide (Na2O) — 3.48%
  • Calcium oxide (CaO) — 1.99%
  • Iron(II) oxide (FeO) — 1.78%
  • Iron(III) oxide (Fe2O3) — 1.57%
  • Magnesium oxide (MgO) — 0.88%
  • Water (H2O) — 0.84%
  • Titanium dioxide (TiO2) — 0.39%
  • Diphosphorus pentoxide (P2O5) — 0.19%
  • Manganese Oxide (MnO) — 0.12%

Emplacement mechanisms

For over a century, geologists have been grappling with the problem of how large volumes of molten rock are emplaced within the solid Earth. This problem has still not been entirely resolved. Granite magma must make room for itself or be intruded into other rocks, and several mechanisms have been proposed to explain how large batholiths have been emplaced.

  • Stoping: The granite cracks the wall rocks and pushes upward as it removes blocks of the overlying crust.
  • Diapirism: The granite of lower density causes relative buoyancy and pushes upward, warping and folding the rock above it.
  • Assimilation: The granite melts its way up into the crust, thereby removing overlying material.
  • Inflation: The granite body inflates under pressure and is injected into position.

Most geologists today accept that a combination of these processes may operate to produce granite intrusions, as no single mechanism provides an adequate explanation.

Uses

Uses in antiquity

The Red Pyramid of ancient Egypt (about twenty-sixth century B.C.E.), 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 (about 2580 B.C.E.) 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. Other uses of granite in ancient Egypt include columns, door lintels, sills, jambs, and wall and floor veneer.[2]

How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt has postulated that the Egyptians used emery, which has greater hardness on the Mohs scale.[3]

Many large Hindu temples in southern India, particularly those built by the eleventh-century king Rajaraja Chola I, were made of granite. The amount of granite in them is said to be comparable to that used in the Great Pyramid of Giza.[4]

Modern uses

  • Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Given the problem of acid rain in parts of the world, granite has begun to supplant marble as a monument material, because it is much more durable. The Black Galaxy granites from the Cheemakurthy area of Andhra Pradesh, India, are renowned for their elegance.
  • Polished granite has become a popular choice for kitchen countertops, given its high durability and aesthetic qualities.
  • Engineers have traditionally used polished granite surfaces to establish a plane of reference, because they are relatively impervious and inflexible.
  • The team sport called "curling" uses stones that are traditionally made of polished granite.
  • 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.

See also

Notes

  1. Shackleton, E. H. (1971). Lakeland Geology, 3rd edition, Clapham (via Lancaster): Dalesman. 
  2. Harrell, James A. Preliminary Description of Stone Varieties Used in Medieval Islamic Buildings of Cairo, Egypt. University of Toledo. Retrieved June 12, 2007.
  3. Corbin, Nancy. Egyptian Genius: Stoneworking for Eternity. American Research Center in Egypt, Northern California Chapter. Retrieved June 12, 2007.
  4. “The Lost Temples of India.” Discovery. Available online at Google Video. Retrieved June 12, 2007.

External links

All links retrieved January 9, 2014.

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