Granite Rocks

Granite is a type of igneous rock which forms when the magma cools down slowly underground. It comprises mainly the minerals like quartz, feldspar and mica. Some granites contain hornblende and augite, although in small amounts, along with the main ingredients. Accessory ingredients of apatite, hematite, rutile, tourmaline and zircon are also constituents of granite rock. Biotite and other ferromagnesian minerals are also present sometimes in granite. Granite igneous rock usually is pale grey, white or pink in colour which depends on the colour of their constituent feldspars. For example, in the presence of pink microcline feldspar, the granite looks pink. Granite mainly occurs in the form of huge batholiths, stock and sometimes laccoliths.

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Constituents of Granite

Granite igneous rock consists of minerals like K-feldspar (orthoclase, microcline and perthite) about 50 to 80 per cent, quartz about 20 to 40 per cent, Na-plagioclase and mica(s) such as biotite and muscovite. The intergrowth of albite or oligoclase with a microcline or orthoclase host results in the formation of mineral perthite.

The feldspars are the dominant components of granite. They can easily be identified on the basis of the appearance, colour and cleavage properties of granite. Quartz is generally anhedral and is usually present as fillings at interstices between the other mineral constituents while overriding its own characteristic crystal shape. In general, quartz is colourless or smoky. It is identified by its hardness, glass-like appearance, conchoidal fracture and lack of cleavage property. The constituents like biotite, hornblende and muscovite in granite can be distinguished by flaky black or silver colour, black dark green grains or prism characteristics.

Various igneous rocks especially the granodiorite, gabbro and diorite are often wrongly designated as granites by architects and builders. However, they are petrologically defined and cannot be considered commonly as granites. Granites are used commonly in buildings, as decorative stones, for kitchen stall surfaces, tiles, modern sculptures, engineering, rock climbing etc.

Granite Composition

Granite consists of minerals and rocks, dominantly quartz, potassium feldspar, mica, amphiboles and small traces of some other minerals. Granite comprises 20-60 per cent quartz, 10-65 percent feldspar and 5-15 per cent mica(s). The presence of different minerals in different proportions gives the granite different colours and textures.

The variation in proportions of differently coloured minerals in granite comes from the original source of molten rock that was cooled to form the granite. For instance, if we consider a molten rock that was abundant in potassium feldspar, it will be more likely to set into granite with salmon pink colour. While, if it was rich in quartz and minerals that make up amphibole, it would be a black and white speckled granite which we often see on the kitchen stalls and countertops.

Following are the typical effects of different constituents on the granite colour and appearance:

• Quartz - milky white colour

• Feldspar - off-white colour

• Potassium Feldspar - salmon pink colour

• Biotite - black or dark brown colour

• Muscovite - metallic gold or yellow colour

• Amphibole - black or dark green colour

The minerals listed above are responsible for the colours we see in granites generally. In the next section, we will talk about the structural features of granite.

Granite Structure

The term Granite is derived from the Latin word ‘granum’ which means grain. This is related to the coarse-grain-like structure of the rock. As the granite rocks consist mainly of feldspar, quartz, mica and amphibole minerals, they form interlocking between them. The equigranular matrix of feldspar and quartz usually form the lighter portions in the granite structure while minerals like biotite and amphibole (or/and hornblende) form the scattered darker portions. Sometimes, few individual crystals become larger than the groundmass. The texture formed in this case is known as porphyritic and such rocks with the porphyritic structure are known as a granite porphyry. Light-coloured and coarse-grained igneous rocks are often called Granitoid in general and descriptive fields. Granitoids are classified for their specific types through petrographic examination. The mineralogy of the granitic rocks is the deciding factor for their colours which can be predominantly white, pink or grey typically. 

Uses of Granite

Being one of the world’s toughest substances, granite has been in use in various ways for a long time. Apart from its use in the construction of monuments and buildings, granite is also useful in curling balls and gym walls for the training of mountain climbing.

Let’s see the different uses of granite in detail:

• Building Monuments: In order to live long lives and create beautiful designs to the structures like temples, gravestones and monuments, they are usually made from granite. Although, in early times, the unavailability of advanced equipment made the carving process of granite quite difficult and laborious. Hence, such stones were used only in important structures.

• Jewellery: Some granites are quite rare and exceptionally beautiful. Hence, they are used as gemstones in jewellery. An example of such granite is the K2 Azurite which is found in the Himalayas, has a blue tint and is a rare stone.

• Fireplace Mantle and Floor: Granite makes an ordinary fireplace mantle more attractive and suitable for being used as a place of gathering with friends and family. Also, granite tiles are preferably better than other options for perfect, smooth and nice-looking flooring. The availability of granites in different colours and textures is an add-on for suiting the different preferences of people. As it is really hard, it is exceptionally wear-resistant. Also, it is bacteria resistant and easy to clean. One could easily keep such flooring neat and clean with regular sweeping and mopping.

• Bathrooms, Shelves, Tabletops, Basins: Kitchen tops, tabletops and shelves look more elegant while maintaining strength when made up of granite. They are also cleaned very easily and look really stylish in appearance. Also, granite is completely water-resistant. Granite sinks, angular basins and modern or pedestal sinks are some more applications of granite in kitchens and bathrooms.

Do You Know?

• At ambient pressure, dry granite melts at around 1215 to 1260°C. However, in the presence of water content, it gets melted around 650°C at a few hundred megapascals of pressure.

• The primary permeability of granite is poor overall. However, the secondary permeability due to the presence of cracks and fractures is very strong.

Conclusion

We get to know all the information related to granite rock such as what is granite and its usage. Granite is an igneous rock that has grains large enough to be easily visible to naked eyes. The two main constituents of granite are feldspar and quartz. Apart from these, granite also consists of other compounds such as amphiboles, mica, etc. in small and different proportions. Granite is mainly used in flooring, decorating purposes and in jewellery also.