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What is Granite?
Granite is a common and widely-occurring group of
intrusive folic igneous rocks that form at great depths
and pressures under continents. Granite consists of
orthoclase and plagioclase feldspars, quartz, hornblende,
biotitic, muscovite and minor accessory minerals such as
magnetite, garnet, zircon and apatite. Rarely, a pyroxene
is present. Ordinary granite always carries a small amount
of plagioclase, but when this is absent the rock is
referred to as alkali granite. An increasing proportion of
plagioclase feldspar causes granite to pass into
grandiosity. A rock consisting of equal proportions of
orthoclase and plagioclase plus quartz may be considered a
quartz monzonite. A granite containing both muscovite and
biotitic micas is called a binary granite. The average
density is 2.75 g/cm3 with a range of 1.74 to 2.80.Granite is a common and widely-occurring group of
intrusive felsic igneous rocks that form at great depths
and pressures under continents. Granite consists of
orthoclase and plagioclase feldspars, quartz, hornblende,
biotite, muscovite and minor accessory minerals such as
magnetite, garnet, zircon and apatite. Rarely, a pyroxene
is present. Ordinary granite always carries a small amount
of plagioclase, but when this is absent the rock is
referred to as alkali granite.An increasing proportion of
plagioclase feldspar causes granite to pass into
granodiorite. |
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A rock consisting of equal proportions of
orthoclase and plagioclase plus quartz may be considered a
quartz monzonite. A granite containing both muscovite and
biotite micas is called a binary granite. The average
density is 2.75 g/cm3 with a range of 1.74 to 2.80.
The word granite comes from the Latin granum, a grain, in
reference to the coarse-grained structure of such a
crystalline rock.
Granite is a common and widely occurring type of
intrusive, felsic, igneous rock.
Granites are usually medium to coarse grained,
occasionally with some individual crystals larger than the
groundmass forming a rock known as porphyry. Granites can
be pink to dark gray or even black, depending on their
chemistry and mineralogy.
Outcrops of granite tend to form tors, rounded massifs,
and terrains of rounded boulders cropping out of flat,
sandy soils. Granites sometimes occur in circular
depressions surrounded by a range of hills, formed by the
metamorphic aureole or hornfels.
Granite is nearly always massive, hard and tough, and it
is for this reason it has gained widespread use as a
construction stone.
The average density of granite is 2.75 g·cm−3 with a range
of 1.74 g·cm−3 to 2.80 g·cm−3.
The word granite comes from the Latin granum, a grain, in
reference to the coarse-grained structure of such a
crystalline rock. |
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 Mineralogy
Granite primarily consists of orthoclase and plagioclase
feldspars, quartz, hornblende, muscovite and/or biotite
micas, and minor accessory minerals such as magnetite,
garnet, zircon and apatite. Rarely, a pyroxene is present.
Very rarely, iron-rich olivine, fayalite, occurs.
Granite is classified according to the QAPF diagram for
coarse grained plutonic rocks (granitoids) and is named
according to the percentage of quartz, alkali feldspar
(orthoclase, sanidine, or microcline) and plagioclase
feldspar on the A-Q-P half of the diagram. Granite-like
rocks which are silica-undersaturated may have a
feldspathoid such as nepheline, and are classified on the
A-F-P half of the diagram (Figure 1).
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 tonalite; pyroxene
and amphibole are common in tonalite.
A granite containing both muscovite and biotite micas is
called a binary or two-mica granite. Two-mica granites are
typically high in potassium and low in plagioclase, and
are usually S-type granites or A-type granites.
The volcanic equivalent of plutonic granite is 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%
Occurrence of granite
Granite is currently known only on Earth where it forms
a major part of continental crust. Granite occurs as
relatively small, less than 100 km² stock-like masses
and as large batholiths often associated with orogenic
mountain ranges and is frequently of great extent. Small
dikes of granitic composition called aplites are
associated with granite margins. In some locations very
coarse-grained pegmatite masses occur with granite.
Granite has been intruded into the crust of the Earth
during all geologic periods; much of it is of
Precambrian age. Granite 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 the Scandinavian Peninsula (mostly in Finland
and Norway), Spain (mostly Galicia and Asturias),
Brazil, India and several countries in the South end of
the African continent, namely Angola, Namibia, Zimbabwe
and South Africa.
Origin of granite
Granite is an igneous rock and is formed from magma.
Granite 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 km 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 eutectic point (or a temperature minimum on a
cotectic curve). Magmas will evolve to the eutectic
because of igneous differentiation, or because they
represent low degrees of partial melting. Fractional
crystallisation serves to reduce a melt in iron,
magnesium, titanium, calcium and sodium, and enrich the
melt in potassium and silicon - alkali feldspar (rich in
potassium) and 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 basalt may be richer in
plagioclase feldspar. It is on this basis that the
modern "alphabet" classification schemes are based. |
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Uses of granite
Granite has been extensively used a dimension stone
and as flooring tiles in public and commercial
buildings and monuments.
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
limestone and granite blocks. The Great Pyramid of Giza
(c.2580 BC) contains a huge granite sarcophagus fashioned
of "Red Aswan Granite." The mostly ruined Black Pyramid
dating from the reign of Amenemhat III once had a polished
granite pyramidion or capstone, now on display in the main
hall of the Egyptian Museum in Cairo (see Dahshur). Other
uses in Ancient Egypt, include columns, door lintels,
sills, jambs, and wall and floor veneer. How the Egyptians worked the solid granite is still a
matter of debate. Dr. Patrick Hunthas postulated that the
Egyptians used emery shown to have higher hardness on the
Mohs scale. |
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Many large Hindu temples in southern India, particularly
those built by the 11th century king Rajaraja Chola I,
were made of granite. In fact, the amount of granite in
them is comparable to the Great Pyramid of Giza.
Modern
Granite has been extensively used as a dimension stone and
as flooring tiles in public and commercial buildings and
monuments. With increasing amounts of acid rain in parts
of the world, granite has begun to supplant marble as a
monument material, since it is much more durable. Polished
granite has been 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.
In the world of sports, curling rocks are traditionally
fashioned of granite.
Sandblasted concrete with a heavy 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 Haytor Granite Tramway, Devon, England, in 1820. |
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Granite