Granite is one of the most durable
stones, which has been incorporated well in infrastructures of the present
times. The word "Granite" originates from the Latin word 'granum',
referring to the coarse-grained structure of this crystalline rock. It is
composed of quartz, feldspars and micas, as well as traces of a variety of
other minerals, which contribute to the color and texture of natural granite
stone. These granite stones are available in pink, dark gray or even black,
depending on their chemistry and mineralogy. A broad range of elegant
patterns and colors makes granite the most versatile of all stones. The
crystal size of the natural granite stone is somewhat determined by the rate
at which the granite cools: the slower the cooling process, the larger the
crystals grow. Occasionally some individual crystals (phenocrysts) are
larger than the ground mass in which the texture is known as porphyritic.
Indian Granite Stone is a common and widely occurring type of intrusive,
felsic, igneous rock with great strength and value. The density of granite
is 2.75g/cm3 (avg) and its viscosity at standard temperature & pressure
is ~4.5 • 1019 Pa·s. Natural Granite stone is typically the
hardest of the dimensional stones and can withstand the vagaries of nature,
effectively. The natural properties of this stone makes it the real "maintenance-free"
stone.
Occurrence
It is widely spread throughout the continental crust of the Earth and is
generously found as a basement rock that underlies in the relatively thin
sedimentary veneer. This light colored stone often occurs as relatively
small, less as 100 sq. km stock masses (stocks). In batholiths they are
often associated with organic mountain ranges. In some areas very
coarse-grained pegmatite masses are found with granite. Outcrops of granite
usually form into tors, and rounded massifs. Granites also occur in circular
depressions that are surrounded by a range of hills and are formed by the
metamorphic aureole or hornfels.
Origin
This versatile rock has been intruded into the crust of our planet during
all geologic periods, though much of it is of Precambrian age. Granite is an
igneous rock which is formed from magma and is currently found only on Earth
where it forms a major part of continental crust. Granitic magma has many
potential origins but it must intrude other rocks. Most granite intrusions
are located deep within the crust, which is usually more than 1.5 kilometers
and up to 50 km depth within thick continental crust. Small embankments of
granitic composition known as aplites are often affiliated with the margins
of granitic intrusions. The origin of granite is contentious and has led to
varied schemes of classification. Classification schemes are regional and
include French, British, and American systems.
Geochemical Origins
Granite is a ubiquitous component of the crust that has crystallized from
magma and has compositions at or near a eutectic point. Magmas evolve to the
eutectic owing to the igneous differentiation, or because it represents low
degrees of partial melting. Fractional or partial crystallization serves to
reduce a melt in iron, titanium, magnesium, sodium and calcium. They also
enrich the melt in silicon and potassium, which is and alkali feldspar (rich
in potassium) and quartz (SiO2), are the two of the defining constituents of
granite.
This process functions is indifferent to the origin of the parental magma
and its chemistry. However, the composition of the magma which is different
in the final product- granite leaves certain geochemical and mineral
evidence as granite's parental rock. The absolute mineralogy, texture and
chemical composition of the granite is often unique as its origin. For
example, a granite that has been formed from melted sediments may have a
larger portion of alkali feldspar, whereas a granite, which has been derived
from melted basalt may be richer in plagioclase feldspar.
Classification
Granite is a common name for all Igneous and Metamorphic Rocks
commercially, but geologically “Granite” is a term only for
acidic, plutonic Igneous Rock. They can be classified based on Geological
and Commercial Value, broadly into three groups.
Genesis - A medium/fine-grained rock of Gneissic structure is
available in various colors. Plagioclase, microcline and quartz are the
vital minerals, while titanite, biotite, apatite, epidote, zircon and garnet
are found in lesser quantities. Small amount of hematite, pyroxene and
sericite are also present in this rock, which is of a granitic composition,
contains numerous crystallites. Gneisses can be recognized by the wavy
patterns or zig zag movements. Others in this category are Paradiso, Kashmir
White, Red Multi Color, Madura Gold, Colombo Juprana, Imperial White,
Shivakashi Gold, Kuppam Green, Vizag Blue, Lavender Blue, Tropical Green
etc.
Porphyry- Some individual crystals known as phenocrysts are bigger
than the ground mass, in such cases the texture is known as porphyritic and
the granite is called as porphyry. Porphyries are granites which have
dominant Feldspar Crystal or Flowers. Tan Brown, Sapphire Blue, Crystal
Yellow, Steel Grey, Ruby Red, Cats eye belong to this class.
Dyke- Dykes are the Black Granites, which commonly occurs as
Dolerite Dykes. The Black Granites which form a Ring Dyke is a classic
example of the Dyke. Other Black Granites or Dykes include Absolute Black in
Warangal, Chamrajnagar and other places.
Granite is classified on the basis of the QAPF diagram for coarse grained
plutonic rocks (granitoids). They are 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. A pure granite
according to modern petrologic convention consists of both plagioclase and
alkali feldspars. When a granite is devoid or nearly bereft of plagioclase,
it is referred to as alkali granite. When a granitoid contains <10% of
orthoclase it is called tonalite; pyroxene and amphibole are common in
tonalite. A granite which contains both muscovite and biotite micas is known
as binary or two-mica granite. These granites typically have high quantities
of potassium and are low in plagioclase. They are usually S or A-type
granites. The volcanic equivalent of plutonic granite is rhyolite. Granite
has poor primary permeability but strong secondary permeability.
