Granite

see also: Etymology of Granite

The granite is a magmatic Roche plutonic with grained structure, i.e. entirely crystallized, formed by slow, in-depth cooling of a magma resulting from the fusion partial of the continental crust. It is made of minerals in grains (crystals) all visible with the naked eye, mainly of the quartz, the Mica S (Biotite or Muscovite), of the potassic Feldspath S (Orthose S) and of the Plagioclase S. It can also contain Hornblende, Magnétite, Grenat, Zircon and Apatite. This appearance is worth to him its name derived from Latin granum , grain. Almost all the countries of the world exploit one or of the granite careers. (Canada-Quebec, the United States, Norway, Brazil, India, China,…) In all, one counts so far more than 500 different granite colors.

It is a rock Acide made up mainly of quartz, of potassic feldspars, sodic plagioclases and of Mica S. the volcanic rocks corresponding are the Rhyolite S.

The average chemical composition of the granite is: SiO₂: 72.5%, Al₂O₃: 14%, (Na₂O, K₂O): 9.5%, oxides (Fe, mn, Mg, Ca): 2 %. The granite is a dense rock (average density: 2,7).

The granite and its associated rocks form the essence of the continental Croûte of planet. The largest granitic monoliths of the world are in the park of the Yosemite, in California.

Difference between granite and granite

One should not confuse the granite with the Granit. This last term (used by the monumental masons) does not define a specific rock: the granite is a type of nonporous rock ( IE . the Eau does not penetrate there), impermeable ( IE . water does not cross it) and coherent (it is not exhausted under the pressure of the fingers, because it is made of elements fortements welded between them). The small granite of the Ardennes is, for example, a Calcaire.

Genesis of the granites

The granites are of plutonic origin, in opposition to the effusive rocks of ic origin Volcan like the Basalte. They are formed in-depth by very slow cooling of the magma, mixed with other rocks. The minerals crystallize then in a certain order: initially micas, then feldspars, finally quartz. Certain granites are born from the fusion of the continental crust at the time of a collision between two tectonic plates.

The granites in the broad sense represent an important component of the terrestrial continental crust. In certain areas of the world (South Africa, Northern Is Brazil, Northern West of the Australia), they constitute up to 75% of the surface of the exposed rocks. For example, the collision of continental plates causes essential the formation of great zones of deformation, but also the production of granites. It is one of the most effective means to evacuate the energy of the collision, either thermically (fusion of the crust), or mechanically (vertical or horizontal shearings). The granites represent the principal mode of transfer of the elements, in particular those producing of heat (Th, U) of the crust lower than the higher crust. They thus lead to a chemical differentiation of the crust. Moreover they are often at the origin of mineralizations whose economic interest is obvious.

Typology

Calc-alkali granites

August 1st

Subalkaline granites

August 1st

Alkaline granites

August 1st

Leucogranites

The leucogranites (of the Greek leucos , white) are at the origin of the ear Syénite S.

Granite of anatexie

The granite of anatexie (of the Greek ana , in top and texis , childbirth, fusion) has an aspect different from the other granites. It often has heterogeneities, with directed minerals. It is resulting from the fusion of the continental crust, often thanks to the presence of fluids.

Use

Construction material, macadamization, sculpture, counter. Noble material is used by the world with the manufacture of monuments, pavement, borders pavement, since the years 1980 ornament of the kitchens and bathroom, the uses multiple and are varied.

Deteriorations and modelled granite

During its crystallization in the crust, the granite remains immobilized at this precise place, which forms granitic Pluto then. The rising of the ground and the erosion of the roadbases make observable granitic Pluto. The granitic outcrops are numerous in the world; in France, one can observe them in Brittany, in the the Vosges, the Massif Central or the the Alps.

The deterioration of the granite generally begins along Diaclase S, i.e. more or less broad cracks. It leads, in moderated climate, with the formation of granite balls then of a granitic chaos with the foot of which a granitic arena is observed (see for example the site of the “Pierre to the nine steps” Soubrebost, in the department of the Creuse). The arena is made mainly of faded mica and faded feldspar but quartz grains, also. The speed of disintegration of the granite depends on the climate.

In mountain, the erosion of the granitic solid masses gives place to different modelled: Needle S, arrows…

The disintegration of the granite, having thus released feldspar, quartz and the mica, is the origin of layers of which one can extract these various minerals.

The feldspar can evolve/move until the kaolinic stage of clay.

the deterioration of the arena granite

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