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Granite

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Granite is the most common intrusive rock in Earth’s continental crust, It is familiar as a mottled pink, white, gray, and black ornamental stone. It is coarse- to medium-grained. Its three main minerals are feldspar, quartz, and mica, which occur as silvery muscovite or dark biotite or both. Of these minerals, feldspar predominates, and quartz usually accounts for more than 10 percent. The alkali feldspars are often pink, resulting in the pink granite often used as a decorative stone. Granite crystallizes from silica-rich magmas that are miles deep in Earth’s crust. Many mineral deposits form near crystallizing granite bodies from the hydrothermal solutions that such bodies release.

Name origin: The name appeared for the first time in works of the English botanists, physician and philosopher Caesalpinus in the 16th century.

Group – plutonic.

Colour: Pink-grey.

Structure: Massive, confining.

Texture: phaneritic (medium to coarse grained). , holocrystalline, pan-hypidiomorphically grained, porphyric in places.

Alterations: The rock is unaltered, feldspars are rarely sericitized

Major minerals of Granite: Orthoclase, quartz, biotite, muscovite and plagioclase, which is twinned according to the albite law and oscillatory zoned. Chemical composition of the core corresponds to oligoclase and andesine (An30-38), whereas more acidic oligoclase and andesine occur in the margin.

Accessory minerals of Granite: Zircon and apatite, mainly as inclusions in biotite, titanite, orthite, magnetite, pyrite.

Classification

In the upper part of QAPF classification of plutonic rocks (Streckeisen, 1976), the granite field is defined by the modal composition of quartz (Q 20 – 60 %) and the P/(P + A) ratio between 10 and 65. The granite field comprises two sub-fields: syenogranite and monzogranite. Only rocks projecting within the syenogranite are considered granites in the Anglo-Saxon literature. In the European literature, rocks projecting within both syenogranite and monzogranite are named granites. The monzogranite sub-field contained adamellite and quartz monzonite in older classifications. The Subcommission for Rock Cassification recommends most recently rejecting the term adamellite and to name as the quartz monzonite only the rocks projecting within the quartz monzonite field sensu stricto.

QAPF Diagram
QAPF Diagram

Chemical Composition

A worldwide average of the chemical composition of granite, by weight percent,

based on 2485 analyses:

  • 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%

It always consists of the minerals quartz and feldspar, with or without a wide variety of other minerals (accessory minerals). The quartz and feldspar generally give granite a light color, ranging from pinkish to white. That light background color is punctuated by the darker accessory minerals. Thus classic granite has a “salt-andpepper” look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende. Almost all these rocks are igneous (it solidified from a magma) and plutonic (it did so in a large, deeply buried body or pluton). The random arrangement of grains in granite— its lack of fabric—is evidence of its plutonic origin. Rock with the same composition as granite can form through long and intense metamorphism of sedimentary rocks. But that kind of rock has a strong fabric and is usually called granite gneiss.

Density + Melting Point

The average density of it is between 2.65 and 2.75 g/cm3, its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3–6 • 1019 Pa·s. Melting temperature is 1215–1260 °C. It has poor primary permeability but strong secondary permeability.

Occurrence of the Granite Rock

It is found in large plutons on the continents, in areas where the Earth’s crust has been deeply eroded. This makes sense, because granite must solidify very slowly at deeply buried locations to make such large mineral grains. Plutons smaller than 100 square kilometers in area are called stocks, and larger ones are called batholiths. Lavas erupt all over the Earth, but lava with the same composition as granite (rhyolite) only erupts on the continents. That means that granite must form by the melting of continental rocks. That happens for two reasons: adding heat and adding volatiles (water or carbon dioxide or both). Continents are relatively hot because they contain most of the planet’s uranium and potassium, which heat up their surroundings through radioactive decay. Anywhere that the crust is thickened tends to get hot inside (for instance in the Tibetan Plateau). And the processes of plate tectonics, mainly subduction, can cause basaltic magmas to rise underneath the continents. In addition to heat, these magmas release CO2 and water, which helps rocks of all kinds melt at lower temperatures. It is thought that large amounts of basaltic magma can be plastered to the bottom of a continent in a process called underplating. With the slow release of heat and fluids from that basalt, a large amount of continental crust could turn to granite at the same time.

Where is it found?

So far, it is known that it is found on Earth only as abundant in all continents as part of the continental crust. This rock is found in small, stock-like masses of less than 100 km², or in batholiths that are part of orogenic mountain ranges. Together with the other continent and sedimentary rocks, generally form the base underground slope. It is also found in lacolites, trenches and thresholds. As in the granite composition, other rock variations are alpids and pegmatites. Adhesives with finer particle size than occur at the boundaries of granitic attacks. More granular pegmatites than granite generally share granite deposits.

Granite Uses

  • The ancient Egyptians built the pyramids from granites and limestones.
  • Other uses in ancient Egypt are columns, door lintels, sills, moldings and wall and floor covering.
  • Rajaraja Chola The Chola Dynasty in South India, in the 11th century AD in the city of Tanjore in India, made the world’s first temple completely granite. The Brihadeeswarar Temple, dedicated to Lord Shiva, was built in 1010.
  • In the Roman Empire, granite became an integral part of the building material and monumental architectural language.
  • It is most used as a size stone. It is based on abrasions, has been a useful rock due to its structure that accepts hard and glossy and polish to carry obvious weights.
  • It is used in interior spaces for polished granite slabs, tiles, benches, tile floors, stair treads and many other practical and decorative features.

Modern

  • Used for tombstones and monuments.
  • Used for flooring purposes.
  • Engineers have traditionally used polished granite surface plates to create the reference plane because they are relatively impermeable and not flexible

Production of Granite

It is mined worldwide but most exotic colors are derived from granite deposits in Brazil, India, China, Finland, South Africa and North America. The this rock mining is a capital and labor intensive process. The granite pieces are removed from the deposits by cutting or spraying operations. Special slicers are used to cut granite-extracted pieces into portable plates, which are then packed and transported by rail or shipping services. China, Brazil and India are the leading granite manufacturers in the world.

Conclusion

  • Stone known as “black granite” is usually gabbro which has a completely different chemical structure.
  • It is the most abundant rock in the Earth continental crust. In large areas known as batholiths and in the core areas of the continents known as shields are found in the core of many mountainous areas.
  • Mineral crystals show that it slowly cools down from the molten rock material which is formed under the surface of the earth and requires a long time.
  • If the granite is exposed on the Earth’s surface, it is caused by the rise of granite rocks and the erosion of the sedimentary rocks above it.
  • Under sedimentary rocks, granites, metamorphosed granites or related rocks are usually below this cover. They are later known as basement rocks.
  • Definitions used for granite often lead to communication about the rock and sometimes cause confusion. Sometimes there are many definitions used. There are three ways of defining the granite.
  • A simple course on rocks, along with granite, mica and amphibole minerals, can be described as a coarse, light, magmatic rock consisting mainly of feldspar and quartz.
  • A rock expert will define the exact composition of the rock, and most experts will not use granite to identify the rock unless it meets a certain percentage of minerals. They might call it alkaline granite, granodiorite, pegmatite or aplite.
  • The commercial definition used by sellers and buyers is often referred to as granular rocks that are harder than granite. They can call the granite of gabro, basalt, pegmatite, gneiss and many other rocks.
  • It is generally defined as a “size stone” that can be cut to certain lengths, widths and thicknesses.
  • Granite is strong enough to withstand most abrasions, large weights, resist weather conditions and accept varnishes. A very desirable and useful stone.
  • Although the cost of granite is much higher than the price for other man-made materials for projects, it is considered a prestigious material used to influence others because of its elegance, durability and quality.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Softschools.com. (2019). Granite Facts. [online] Available at: http://www.softschools.com/facts/rocks/granite_facts/2976/ [Accessed 13 Mar. 2019].
  • Helsinki (2015) Introductıon: The Rock, Granıte: About The Rock, Granıte-Research-Book-Reduced
  • Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].
Cite this article as: Mahmut Mat, "Granite", in Geology Science, [online] Accessed 18th July 2019, Available at: http://geologyscience.com/rocks/granite/
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