Granite

Stones

( /ˈɡrænᵻt/) is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word “granite” comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term “granite” is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar.

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The term “granitic” means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. 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 known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids.[1] The extrusive igneous rock equivalent of granite is rhyolite.[2]

Granite is nearly always massive (lacking any internal structures), hard and tough, and therefore it has gained widespread use throughout human history as a construction stone. The average density of granite is between 2.65[3] and 2.75 g/cm3 (165.4 – 171.7 lb/ft3), its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3–6 • 1019 Pa·s.[4]

The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F);[5] it is strongly reduced in the presence of water, down to 650 °C at a few kBar pressure.[6]

Granite has poor primary permeability, but strong secondary permeability.[citation needed]

QAPF diagram for classification of plutonic rocks

Mineral assemblage of igneous rocks
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 feldspar granite. When a granitoid contains less than 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.

Chemical composition[edit]
A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses:[7]

SiO2 72.04% (silica)

Al2O3 14.42% (alumina)

K2O 4.12%

Na2O 3.69%

CaO 1.82%

FeO 1.68%

Fe2O3 1.22%

MgO 0.71%

TiO2 0.30%

P2O5 0.12%

MnO 0.05%

Occurrence[edit]

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The Cheesewring, a granite tor

A granite peak at Huangshan, China
Granite containing rock is widely distributed throughout the continental crust.[8] Much of it was intruded during the Precambrian age; it is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents. 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 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.

Origin[edit]
Granite has a felsic composition and is more common in recent geologic time in contrast to Earth’s ultramafic ancient igneous history. Felsic rocks are less dense than mafic and ultramafic rocks, and thus they tend to escape subduction, whereas basaltic or gabbroic rocks tend to sink into the mantle beneath the granitic rocks of the continental cratons. Therefore, granitic rocks form the basement of all land continents.

Geochemical origins[edit]
Granitoids 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.

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 that 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 that 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. Granite has a slow cooling process which forms larger crystals.

Chappell & White classification system[edit]
The letter-based Chappell & White classification system was proposed initially to divide granites into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite.[9] Both of these types of granite are formed by the melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively.

M-type or mantle derived granite was later proposed to cover those granites that 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 a peculiar mineralogy and geochemistry. These granites are formed by the melting of the lower crust under conditions that are usually extremely dry. A-type granites occur in the Koettlitz Glacier Alkaline Province in the Royal Society Range, Antarctica. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite.[10][11]

H-type or hybrid granites are formed following a mixing of two granitic magmas from different sources, e.g. M-type and S-type.

Granitization[edit]
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 migmatites. 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[edit]
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.[12] 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.

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 fracture or fault systems and networks of active shear zones.[13] 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.

Weathering[edit]
Further information: Weathering

Grus sand and granitoid it derived from
Physical weathering occurs on a large scale in the form of exfoliation joints, which are the result of granite’s expanding and fracturing as pressure is relieved when overlying material is removed by erosion or other processes.

Chemical weathering of granite occurs when dilute carbonic acid, and other acids present in rain and soil waters, alter feldspar in a process called hydrolysis.[14][15] As demonstrated in the following reaction, this causes potassium feldspar to form kaolinite, with potassium ions, bicarbonate, and silica in solution as byproducts. An end product of granite weathering is grus, which is often made up of coarse-grained fragments of disintegrated granite.

2 KAlSi3O8 + 2 H2CO3 + 9 H2O → Al2Si2O5(OH)4 + 4 H4SiO4 + 2 K+ + 2 HCO3−
Climatic variations also influence the weathering rate of granites. For about two thousand years, the relief engravings on Cleopatra’s Needle obelisk had survived the arid conditions of its origin before its transfer to London. Within two hundred years, the red granite has drastically deteriorated in the damp and polluted air there.[16]

Natural radiation[edit]
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.

Potassium-40 is a radioactive isotope of weak emission, and a constituent of alkali feldspar, which in turn is a common component of granitic rocks, more abundant in alkali feldspar granite and syenites. Naturally, a geiger counter should register this low effect.

Some granites contain around 10 to 20 parts per million (ppm) of uranium. By contrast, more mafic rocks, such as tonalite, gabbro and diorite, have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Many large granite plutons are 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. Cellars and basements built into soils over granite can become a trap for radon gas, which is formed by the decay of uranium.[17] Radon gas poses significant health concerns and is the number two cause of lung cancer in the US behind smoking.[18]

Thorium occurs in all granites as well.[19] Conway granite has been noted for its relatively high thorium concentration of 56±6 ppm.[20]

There is some concern that some granite sold as countertops or building material may be hazardous to health. Dan Steck of St. Johns University has stated[21] that approximately 5% of all granite is of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slab types have been 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. In this test, all of the 39 full-size granite slabs that were measured for the study showed radiation levels well below the European Union safety standards (section 4.1.1.1 of the National Health and Engineering study) and radon emission levels well below the average outdoor radon concentrations in the US.[22]

Industry[edit]
Granite and related marble industries are considered one of the oldest industries in the world; existing as far back as Ancient Egypt.[23]

Major modern exporters of granite include China, India, Italy, Brazil, Canada, Germany, Sweden, Spain and the United States .[24]

Indian granite quarries have been mired in controversy over child labor and slavery.[25][26]

Uses[edit]
Antiquity[edit]

Cleopatra’s Needle, London
The Red Pyramid of Egypt (c. 26th century BC), named for the light crimson hue of its exposed limestone 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, which is 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.[27] How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt[28] has postulated that the Egyptians used emery, which has greater hardness on the Mohs scale.

Rajaraja Chola I of the Chola Dynasty in South India built the world’s first temple entirely of granite in the 11th century AD in Tanjore, India. The Brihadeeswarar Temple dedicated to Lord Shiva was built in 1010. The massive Gopuram (ornate, upper section of shrine) is believed to have a mass of around 81 tonnes. It was the tallest temple in south India.[29]

Imperial Roman granite was quarried mainly in Egypt, and also in Turkey, and on the islands of Elba and Giglio. Granite became “an integral part of the Roman language of monumental architecture”.[30] The quarrying ceased around the third century CE. Beginning in Late Antiquity the granite was reused, which since at least the early 16th century became known as spoliation. Through the process of case-hardening, granite becomes harder with age. The technology required to make tempered steel chisels was largely forgotten during the Middle Ages. As a result, Medieval stoneworkers were forced to use saws or emery to shorten ancient columns or hack them into discs. Giorgio Vasari noted in the 16th century that granite in quarries was “far softer and easier to work than after it has lain exposed” while ancient columns, because of their “hardness and solidity have nothing to fear from fire or sword, and time itself, that drives everything to ruin, not only has not destroyed them but has not even altered their colour.”[30]

Modern[edit]
Sculpture and memorials[edit]

Various granites (cut and polished surfaces)
In some areas, granite is used for gravestones and memorials. Granite is a hard stone and requires skill to carve by hand. Until the early 18th century, in the Western world, granite could be carved only by hand tools with generally poor results.

A key breakthrough was the invention of steam-powered cutting and dressing tools by Alexander MacDonald of Aberdeen, inspired by seeing ancient Egyptian granite carvings. In 1832, the first polished tombstone of Aberdeen granite to be erected in an English cemetery was installed at Kensal Green Cemetery. It caused a sensation in the London monumental trade and for some years all polished granite ordered came from MacDonald’s.[31] As a result of the work of sculptor William Leslie, and later Sidney Field, granite memorials became a major status symbol in Victorian Britain. The royal sarcophagus at Frogmore was probably the pinnacle of its work, and at 30 tons one of the largest. It was not until the 1880s that rival machinery and works could compete with the MacDonald works.

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.

The stone known as “black granite” is usually gabbro, which has a completely different chemical composition.[32][33]

Buildings[edit]
Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Aberdeen in Scotland, which is constructed principally from local granite, is known as “The Granite City”. Because of its abundance in New England, granite was commonly used to build foundations for homes there. 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.

Engineering[edit]
Engineers have traditionally used polished granite surface plates 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 as the material of the tracks of the Haytor Granite Tramway, Devon, England, in 1820. Granite block is usually processed into slabs, which can be cut and shaped by a cutting center. Granite tables are used extensively as bases for optical instruments because of granite’s rigidity, high dimensional stability, and excellent vibration characteristics. In military engineering, Finland planted granite boulders along its Mannerheim Line to block invasion by Russian tanks in the winter war of 1940.

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