Granites

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.

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.

The chemical composition of granite, by weight percent:

Other Properties of Granite are:

• Porosity/permeability:
  This hard rock has almost negligible porosity ranging between 0.2 to 4%.
• Thermal Stability:
  Granite is highly thermal stability and is impervious to weathering from temperature. Even air borne chemicals have no effect on it. It is highly resistant to chemical erosion making granite useful for making tanks to store highly caustic material.
• Co-efficient of expansion:
  The co-efficient of expansion for granite varies from 4.7×10-6 – 9.0×10-6(inch x inch).
• Variegation:
  Granite has high consistency in color and texture.
• Hardness:
  Hardness of granite lends it excellent wear, making it excellent building stone.

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.