Granite
Granite is a common and widely occurring type of intrusive,
felsic, igneous rock. Granite has a medium to coarse texture,
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 and rounded massifs. Granites sometimes occur
in circular depressions surrounded by a range of hills, formed by the
metamorphic aureole or hornfels.
Granite is nearly always massive (lacking internal structures), hard and
tough, and therefore it has gained widespread use as a construction
stone. The average density of granite is 2.75 g/cm3 and its viscosity at
standard temperature and pressure is ~4.5 • 1019 Pa·s [1] . The word
granite comes from the Latin granum, a grain, in reference to the
coarse-grained structure of such a crystalline rock
Mineralogy
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. 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. Granite has poor primary permeability but strong
secondary permeability.
Occurrence
The Stawamus Chief is a granite monolith in British Columbia Granite is
currently known only on Earth where it forms a major part of continental
crust. Granite often occurs as relatively small, less than 100 km² stock
masses (stocks) and in batholiths that are often associated with
orogenic mountain ranges. Small dikes of granitic composition called
aplites are often associated with the margins of granitic intrusions. 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, although much of it is of Precambrian age. Granitic rock 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, certain areas are well
known for their commercial granite quarries:
Origin
Close-up of granite exposed in Chennai, India. Granite is an igneous rock
and is formed from magma. Granitic 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 kilometers 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.
From
Wikipedia