An Introduction to Granite Rocks

Granites are widely occurring felsic, intrusive and igneous type of rocks. The texture of granite is medium to coarse, rarely with some crystals bigger than the groundmass forming rock which is called as porphyry. Granites are found in different colors like pink, dark gray, green, yellow, red or sometimes even in black and gold depending upon its mineralogy and chemistry. Sometimes Granites occur in circular depressions encircled by a series of hills that are formed by the metamorphic hornfels. Granites are often massive lacking internal structures. They are tough, hard and it has gained widespread use for constructive purposes. 2.75g/cm3 is the average density of granite. If you look at the word ‘granite’, it comes from the Latin word ‘granum’ which is a grain, in reference to its coarse grained constitution.

According to a scientific diagram, Granites are classified for coarse grained plutonic rocks – granitoids and it is named based upon the percentage of quartz, alkali feldspar - sanidine, orthoclase or microcline. Granite like rocks which are silica under saturated might have a feldspathoid such as nepheline. According to modern petrologic, true granite contains both alkali feldspars and plagioclase. When a granitoid lacks or almost lacks plagioclase the rock is said to be alkali granite. If the granitoid carries <10% of orthoclase, then it is called tonalite. Amphibole and Pyroxene are common in tonalite. Granites that have both biotite micas and muscovite are called two-mica or binary granite. Typically two-mica granites are low in plagioclase and higher in potassium; A-type or S-type granites. Rhyolite is a volcanic equivalent of plutonic granite.

Presently Earth is the only place Granites are known to form a main part of continental crust. Often granites occur as relatively small, less than 100 km² of stock masses and also in batholiths which are associated with orogenic mountain ranges. Smaller dikes of granite compositions are known as aplites and are often linked with margins of granite intrusions. In some places along with granites, very coarse grained pegmatite masses occur. Granites have been intruded into Earth’s crust during all geologic periods like earthquakes, although most of it has happened in the Precambrian age. Granite rocks are distributed widely throughout the Earth’s continental crust and it is the most abundant basement rocks which underlies the relatively thin sedimentary layer of the continents.

In spite of being fairly common across the globe, the places with the most commercial granite quarries are located in Norway, Sweden - Bohuslän, Finland, Northern Portugal in Chaves, Spain mostly in Galicia and Extremadura, India Brazil and several other countries in southern Africa, such as Angola, Namibia, South Africa and Zimbabwe. Today granites are widely used in making luxury floors, kitchen tables, office tables, bath rooms and tubs