Specification of granite


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What is Granite
Granite is a felsic intrusive igneous rock with grit ranging from medium to coarse and occasionally may present megacristalli. Its name derives from the Latin granum (grain), with clear reference to its structure holocrystalline.
Granite is classified by the diagram QAPF, in which is placed in the field of rocks supersaturated, ie with a content of quartz between 20 and 60%. The other essential minerals present are feldspar (orthoclase, sanidine and microcline) and plagioclase (with compositions more albitiche) and micas (biotite and in some types of granite muscovite).If the plagioclase is almost entirely albite granite takes the name of granite sodium, if the rock is also present pyroxene rhombic the rock takes the name of granite charnockitico.Another variety of granite are leucograniti, in which mafic minerals are extremely reduced. The average density of granite is 2.75 g/cm3 with a range from 1.74 g/cm3 to 2.80 g/cm3.
Chemical composition
The average composition of granite is (in descending order):
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%
To obtain this composition were analyzed 2485 samples from all over the world. The percentage of SiO2 refers to all silicon oxides present (and therefore also those present in the silicate) and not only to the quartz.
Granite is an intrusive igneous rock, is then formed as a result of the slow cooling of a magma intruded at depths between 1.5 and 50 km. The process of formation of granite is however still under debate and has generated several hypotheses and classifications of the granites.
Classification Alphabetical
The alphabetic classification is divided into:
I-granites, derived from igneous rocks, then metalluninosi-alluminosaturi.
S-granites, derived from sedimentary or metamorphic crustal rocks are peralluminosi.
M-granite, resulting from fractional crystallization of the mantle.
A-granites, arising from the interaction of a hot spot with the lower part of the crust.
Classification of Pitcher
The classification of Pitcher (1979) distinguishes two types of granite associations: the 'ercinotipa and andinotipa. Further studies have led us to define three classes of granites:
I-cordillera or Andinotipi, represented by the granites of the Andes, characterized by plutonism of long duration, have origins calcalcaline
I-Caledonian, plutonism come from short and intensive,
S-ercinotipi, plutonism come from brief, intense but are distinguished by the abundant presence of muscovite, this mineral because of their protolito sediment.
Origin of granite
The origin of granite is for many decades been a source of controversy and heated discussions. The currently most widespread hypothesis is that the genesis by fractional crystallization. Another hypothesis is that the genesis for extreme metamorphism.
Genesis by fractional crystallization
According to this theory the formation of granite is attributed to the slow process of fractional crystallization occurring within the magma chamber. Through this theory it is possible to explain the different associations of rocks that are in various stages of evolution of plutons as magma. Plutonic associations can usefully be outlined in two main categories:
Plutonic associations calcalcaline
Associations in plutonic granites dominant
In both groups, however, granite is a culmination of evolution, because the rock is more differentiated. This evolution of the magma is well represented in the diagram of the system granite, where through a graph ternary SiO2, albite and orthoclase you can follow the route of differentiation of a magma oversaturated.
Genesis metamorphism
In contrast to the genesis by fractional crystallization few geologists have hypothesized for the genesis of metamorphism. This theory explains the formation of the granite due to the extreme metamorphism of amphibolites and granulites.
Ascent and emplacement
The ascent of magmas from the area of ​​origin (usually the asthenosphere) to the surface crust is the major contrast in density. The movement of the magma nell'astenosfera can therefore be effectively described with a flow pattern in an environment saturated with collapse of the environment through porous (Turcotte and Ahern, 1979). Upon reaching the lithosphere movement of magma changes the behavior of the medium more rigid then starting to follow preferential pathways.
Depending on the characteristics of the magma (density, viscosity, mass and heat loss) the intrusion will stop at different depths. We distinguish then epiplutoni, plutons to another level, and that cataplutoni plutons of medium-deep crust. The putting into place can, schematically, occur in two ways:
Forced intrusion, intrusions are carrying a boost incassanti deforming rocks.
Permitted intrusions, intrusions that are advancing to the collapse of the rocks above the magma.
A typical mechanism of intrusion is the permissive 'incassanti stoping in which the rocks are fragmented at the edges of the intrusion roof and freeing up space for the lift. The stoping is clearly visible at the edges of the pluton to the large presence of xenoliths.
Distribution: Location of granite was found in Italy
In Italy are very common granitic rocks located in the Alps, in Calabria and Sardinia. They belong to two separate intrusive cycles, associated with the Hercynian orogeny (late Paleozoic era) and the 'Alpine orogeny (Tertiary age) and are often associated, in the same mass of intrusive igneous rocks with members of other families (syenites, diorites, etc.).
The granitic rocks intruded during the Hercynian orogeny cycle are those outcropping over large areas in Sardinia and Corsica, mostly granites normal, but sometimes anfibolici or alkaline with sodic amphiboles, and also the granite of the Sila and Aspromonte, and those located mainly in the outer part of the Alps (Mont Blanc, Gotthard, etc..), these granites often show ercinici gneissic textures, formed by regional metamorphism occurring phenomena, perhaps several times, after solidification of the rocks.
Another series of acidic igneous rocks, granitic, has intruded during the Alpine orogeny cycle (Val Masino, Adamello, Rieserferner, Island of Elba). The Baveno granite is widely used as an ornamental and building stone, similar to the'''' ghiandone Val Masino, a porphyritic structure, tertiary age. The granites of the lower Val Sesia are of Hercynian age. The Adamello plutons, Rieserferner, Cima d'Asta, Bressanone in South Tyrol, all aged Tertiary, consist largely of adamelliti, granodiorite and tonalite.
Uses of granite
Some Egyptian pyramids were built partly of granite: the red pyramid (2600 BC) is named for the red color of the granite surface. The pyramid of Mycerinus is instead built from blocks of granite and limestone. The Great Pyramid of Giza has a majestic granite sarcophagus.
Many Hindu temples in South India are made of granite, especially those built during the 11 th century under the leadership of King Rajaraja Chola I.
Granite is widely used in modern buildings, especially in the pavement. Granite is also appreciated in the construction of monuments to its excellent resistance to acids.
The polished granite and clean is also used in cooking in many kitchens for its durability and aesthetics.
Climbing on the granite
Granite is one of the most popular with rock climbers, thanks to its adherence to its systems and fractures. Some of the most famous places to climb on the granite are: Yosemite, the Mont Blanc massif with many of his points (Aiguille du Dru, Aiguille du Midi, Grandes Jorasses), Val Masino, Corsica and the Karakorum. The granite is so popular in climbing gyms thanks to the artificial, in which many structures are built to recreate the feeling of climbing on granite.


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