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. The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.
Chemical composition
A worldwide average of the average proportion of the different chemical components in granites, in descending order by weight percent, is
* 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%
Origin
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 kilometres 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.
