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What is a Crystalline Rock?

A crystalline rock is a rock composed entirely of crystallized minerals without any glassy matter. Intrusive igneous rocks, especially the ones that turn semi-solid on cooling at the depth are always observed to be crystalline whereas extrusive igneous rocks are mostly non-crystalline rocks as they can be partly or fully glassy. Another type of rock that is mostly crystalline in nature is a metamorphic rock. The metamorphic rocks are known to be subjected to high temperatures and pressures which aids in the crystallisation processes of their mineral content. Thus, in general, two types of rocks form crystalline rocks which are: crystalline igneous rocks and crystalline metamorphic rocks. 

Brief Description of Rocks

There are many different types of rocks found in the crust of the Earth. Typically a rock is formed under different conditions that occur due to geological processes and is a mixture of one or more minerals. These conditions and the contents provide a rock with soft or hard physical properties. Such variation is observed in-between granite and soapstone as the granite is hard and the soapstone is soft. Other differences in properties or rocks are shown by gabbro which is black in colour and quartzite which is white in colour. Also, these properties are sometimes defined by the mineral contents. Although rocks might not contain minerals in a definite composition the most common minerals are feldspar and quartz. 

As said previously there are many different types of rocks present in the Earth’s crust. They are mainly classified into three types on the basis of their mode of formation.

These Three Rocks along with their Formation Process are Listed below:

  1. Igneous Rocks: These types of rocks are formed by the solidification of magma and lava. Examples include granite, gabbro, pegmatite, basalt, etc. 

  2. Sedimentary Rocks: These are formed by the deposition of fragments of rocks that were subjected to exogenous processes. Examples of these types are limestone, coal, halite, potash, etc. 

  3. Metamorphic Rocks: These are the types of rocks that are formed from the existing rocks which are undergoing recrystallisation. Granite, gneiss, slate, schist, marble, etc are examples of metamorphic rocks. 

Out of the above three mentioned types of rocks, the rocks that fulfill crystalline rock meaning are igneous rock and metamorphic rock and the non-crystalline rocks include the sedimentary rocks. This is mainly because both these types of rocks undergo processes that involve high temperature, high pressure, and higher stress with the changing geological conditions that help in the process of crystallization especially of the mineral content contained within these rocks. As mentioned the prime examples of crystalline igneous rocks include granite, and gabbro whereas the examples of crystalline metamorphic rocks include the gneiss and the schist which is a black crystalline rock. Although limestones are primarily sedimentary rocks, there are also crystalline limestone rock/limestone crystalline rocks that are part of certain metamorphic rocks. Another significant example of crystalline rocks includes the Precambrian crystalline rocks that are a hard crystalline rock which along with high-grade metamorphic rocks formed tectonically stable areas. 

Salient Features of Crystalline Rocks

The crystalline rock meaning is a salient feature of the physical characteristics of igneous rock and a metamorphic rock mainly because of its mineral composition. Crystalline rocks are formed because of the polymerisation of minerals. Crystalline rocks are so-called because of the excess of crystals that are present in them and also because of the highly organised microscopic structures of these crystals for which the highly tensed geological processes are responsible. The environment of crystalline igneous rocks or crystalline metamorphic rocks is evidently composed of many igneous and metamorphic rocks. The compositions and the physical properties of igneous rocks are known to be mostly controlled by the crystallization history of the rock. Some types of igneous rocks like the ultramafic rocks contain mineral assemblages that typically crystallize at higher temperatures than felsic (another classification of igneous rocks) rocks. They contain mineral phases of different groups such as iron-magnesium-calcium silicates, etc. indicating that any interaction between the fluids found in this crystalline environment is an interaction between minerals and the fluid is the same as fluid and rock.

A crystalline rock can either be classified as batholiths which have a dominant composition of granite rock with somewhat metamorphic characteristics and metamorphic rocks with very few granitic instructions. Images, as described, Joseph A. Dipietro in Geology and Landscape Evolution (Second Edition), 2018,  of such a classification of rocks is shown below:

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FAQs (Frequently Asked Questions)

1. How are Crystalline Rocks Formed?

Ans: Crystalline features are mostly observed in two different types of rocks, the igneous and the metamorphic rocks. In igneous rocks, the solidification of the magma and the lava occurs either below the surface which is known as the intrusive rocks, or above the surface which is known as extrusive rocks. The intrusive igneous rocks may undergo crystallization to form structured and granular crystalline rocks while the extrusive rocks form natural glasses as they do not undergo crystallization. The metamorphic rocks on the other hand are formed by the subjection of the existing rock to high temperatures and high pressures which are the primary conditions that can help in the formation of crystalline rock.

2. Why are Sedimentary Rocks not Crystalline?

Ans: Sedimentary rocks are one of the three common types of rocks, the other two being igneous and metamorphic rocks. The igneous and metamorphic rocks are often subjected to higher temperatures or higher pressure or both which are primary conditions for crystallization. On the other hand, sedimentary rocks are formed because of the transportation and deposition of sediments or fragments of rocks that are present in the river beds. As there is no thermal process required for the formation of sedimentary rocks, which can lead to high temperatures, which as mentioned earlier is the primary requirement for crystallization, the sedimentary rocks are incapable of forming crystals unlike the igneous rocks and the metamorphic rocks. 

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