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.
Classification
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.
