Granite

Mineralogy

Granite is classified according to the nepheline, and are classified on the A-F-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 rhyolite.

Chemical composition

A worldwide average of the average proportion of the different chemical components in granites, in descending order by weight percent, is:^[1]

• SiO₂ — 72.04%
• Al₂O₃ — 14.42%
• 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%
  • 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 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.

Despite being fairly common throughout the world, the areas with the most commercial granite quarries are located in Finland, Norway and Sweden (Bohuslän), northern Portugal in Chaves and Vila Pouca de Aguiar, Spain (mostly Galicia and Extremadura), Brazil, India and several countries in southern Africa, namely Angola, Namibia, Zimbabwe and South Africa.

Origin

Granite is an 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; 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.

Geochemical origins

Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have compositions at or near a 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 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. For instance, a granite which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted plagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes are based.

Alphabet Soup Classification

The 'alphabet soup' scheme of Chappell & White was proposed initially to divide granites into I-type granite (or 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 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.^{[3] [4]}

Granitization

An old, and largely discounted theory, granitization 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 solidus temperature of the rock.

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. 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 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, 1997).^{[citation needed]} 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 by one or another mechanism.

Natural Radiation

Granite is a normal, geological, source of sedimentary rocks usually have equally low amounts.

Many large granite plutons are the sources for pegmatites.

In buildings constructed primarily from natural granite, it is possible to be exposed to approximately 200 mrems per year.^[5]

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 communites.^[6] Cellars and basements sunk into soils formed over or from particularly uraniferous granites can become a trap for radon gas, which is heavier than air.

However, in the majority of cases, although granite is a significant source of natural radiation as compared to other rocks it is not often an acute health threat or significant risk factor. Various resources from national geological survey organisations 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.

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. Menkaure's Pyramid, likely dating to the same era, was constructed of 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. [3]

Modern

Granite has been extensively used as a 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.

Engineers have traditionally used polished granite surfaces to establish a plane of reference, since they are relatively impervious and inflexible. Sandblasted 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. 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.[4]

Rock climbing

  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 Aiguille du Midi and the Grandes Jorasses), the Bregaglia, Corsica, parts of the Karakoram, the Towers of Paine, 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. Most, however, are made from manufactured materials, given the fact that granite is generally too heavy for portable rock climbing walls, as well as the buildings in which stationary walls are located.

See also

• List of minerals
• List of rock types
• Igneous rocks
• Dimension stone
• Skarn
• Greisen
• Aplite
• Batholith
• New Hampshire, the "Granite State"
• Barre (town), Vermont "Granite Capital of the World", home of the Rock of Ages Corporation
• Elberton, Georgia, the "Granite Capital of the World"
• Aberdeen, Scotland's third largest city nicknamed "The Granite City"
• Quartz monzonite

References