You Might Like
Granite containing <a href="/content/Potassium_feldspar" style="color:blue">potassium feldspar</a>, <a href="/content/Plagioclase_feldspar" style="color:blue">plagioclase feldspar</a>, <a href="/content/Quartz" style="color:blue">quartz</a>, and <a href="/content/Biotite" style="color:blue">biotite</a> and/or <a href="/content/Amphibole" style="color:blue">amphibole</a>
Granite containing potassium feldspar, plagioclase feldspar, quartz, and biotite and/or amphibole

Granite ( /ˈɡrænɪt/) 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.

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.

Granite is nearly always massive (i.e., lacking any internal structures), hard, and tough.

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 overall, but strong secondary permeability through cracks and fractures if they are present.

Mineralogy


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.

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

Occurrence


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


Granite has a felsic composition and is more common in continental crust than in oceanic crust.

Granitoids have crystallized from felsic magmas that have compositions at or near a eutectic point (or a temperature minimum on a cotectic curve).

There are also peritectic and residual minerals in granitic magmas.

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.

The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was.

The letter-based Chappell & White classification system was proposed initially to divide granites into I-type (igneous source) granite and S-type (sedimentary sources).[9] Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks.

M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle.

A-type granites were defined as to occur in anorogenic setting, have alkaline and anhydrous compositions.

H-type granites were suggested for hybrid granites, which were hypothesized to form by mixing between mafic and felsic from different sources, e.g. M-type and S-type.

An old, and largely discounted process, granitization states that granite is formed in place through extreme metasomatism by fluids bringing in elements, e.g. potassium, and removing others, e.g. calcium, to transform a metamorphic rock into a granite.

After more than 50 years of studies, it becomes clear that granitic magmas have separated from their sources and experienced fractional crystallization during their ascent toward the surface.

In nature, metamorphic rocks may undergo partial melting to transform into migmatites through peritectic reactions, with anatectic melts to crystallize as leucosomes.

After the extraction of anatectic melts, the migmatites become a kind of granulites.

Ascent and emplacement


The ascent and emplacement of large volumes of granite within the upper continental crust is a source of much debate amongst geologists.

Of these two mechanisms, Stokes diapir was favoured for many years in the absence of a reasonable alternative.

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


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.

Climatic variations also influence the weathering rate of granites.

Soil development on granite reflects the rock's high quartz content and dearth of available bases, with the base-poor status predisposing the soil to acidification and podzolization in cool humid climates as the weather-resistant quartz yields much sand.[17] Feldspars also weather slowly in cool climes, allowing sand to dominate the fine-earth fraction. In warm humid regions, the weathering of feldspar as described above is accelerated so as to allow a much higher proportion of clay with the Cecil soil series a prime example of the consequent Ultisol great soil group.[18]

Natural radiation


Granite is a natural source of radiation, like most natural stones.

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.

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.[19] Radon gas poses significant health concerns and is the number two cause of lung cancer in the US behind smoking.[20]

Thorium occurs in all granites.[21] Conway granite has been noted for its relatively high thorium concentration of 56±6 ppm.[22]

There is some concern that some granite sold as countertops or building material may be hazardous to health.

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.

Industry


Granite and related marble industries are considered one of the oldest industries in the world; existing as far back as Ancient Egypt.[25]

Major modern exporters of granite include China, India, Italy, Brazil, Canada, Germany, Sweden, Spain and the United States.[26]

Uses


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. 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 AD. 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]

In some areas, granite is used for gravestones and memorials.

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]

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.

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.

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 rarity of this granite, the best stones can cost as much as US$1,500. Between 60 and 70 percent of the stones used today are made from Ailsa Craig granite, although the island is now a wildlife reserve and is still used for quarrying under license for Ailsa granite by Kays of Scotland for curling stones.[33]

Rock climbing


Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction.

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.

See also


You Might Like