A coarse-grained, acid (high in silica) igneous rock containing orthoclase feldspar, quartz, and mica (and/or hornblende); pale pink or grey in colour, its durability makes it an important building stone.
For other uses, see granite (disambiguation).Granite (IPA: /ˈgranɪt/) is a common and widely occurring type of intrusive, felsic, igneous rock.
Granites are usually a white, black or buff color and are medium to coarse grained, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry.
Outcrops of granite tend to form tors, rounded massifs, and terrains of rounded boulders cropping out of flat, sandy soils.
Granite is nearly always massive, hard and tough, and it is for this reason it has gained widespread use as a construction stone.
The average density of granite is 2.75 g·cm to 2.80 g·cm−3.
The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.
Mineralogy
Granite primarily consists of orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, and minor accessory minerals such as magnetite, garnet, zircon and apatite.
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, 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.
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.
Chemical Composition
A worldwide average of the average proportion of the different chemical components in granites, in descending order, is approximately:
SiO2 — 70.18% Al2O3 — 14.47% K2O — 4.11% Na2O — 3.48% CaO — 1.99% FeO — 1.78% Fe2O3 — 1.57% MgO — 0.88% H2O — 0.84% TiO2 — 0.39% P2O5 — 0.19% MnO — 0.12%Occurrence
Granite is currently known only on Earth where it forms a major part of continental crust.
Granite has been intruded into the crust of the Earth during all geologic periods; Granite 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.
Despite being fairly common throughout the world, the areas with the most commercial granite quarries are located in the Scandinavian Peninsula (mostly in Finland and Norway), Spain (mostly Galicia and Asturias), Brazil, India and several countries in the South end of the African continent, namely Angola, Namibia, Zimbabwe and South Africa.
Origin
Granite is an igneous rock and is formed from magma.
The origin of granite is contentious and has led to varied schemes of classification.
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.
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. 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.
M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallised mafic magmas, generally sourced from the mantle.
A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy and geochemistry. The granite caldera of Yellowstone National Park is an example of an A-type granite.
Granitization
The granitization theory states that granite is formed in place by extreme metamorphism. 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. 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 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 positionMost geologists today accept that a combination of these phenomena can be used to explain granite intrusions, and that not all granites can be explained by one or another mechanism.
Uses
Antiquity
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. The Great Pyramid of Giza (c.2580 BC) contains a huge granite sarcophagus fashioned of "Red Aswan Granite."
How the Egyptians worked the solid granite is still a matter of debate.
Many large Hindu temples in southern India, particularly those built by the 11th century king Rajaraja Chola I, were made of granite.
Modern
Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments.
Engineers have traditionally used polished granite surfaces to establish a plane of reference, since they are relatively impervious and inflexible.
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.
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