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 is metamorphic rock. The metamorphic rocks are known to be subjected to high temperatures and pressures which aids in the crystallization processes of their mineral content. Thus, in general, two types of rocks form crystalline rocks which are: crystalline igneous rocks and crystalline metamorphic rocks.


Detail Description of Crystalline Rock:

Crystalline rock means any rock composed entirely of crystallized minerals without glassy matter. Intrusive igneous rocks are those that congeal at depth and are virtually always crystalline, whereas extrusive igneous rocks, or volcanic rocks, may be partly to entirely glassy. Many factors affect the crystallization capacity of magma, but the length of time it takes to cool is the decisive factor. Metamorphic rocks are mostly crystalline. The term crystalline shale has been used to describe all rocks of metamorphic origin, so the term crystalline rock can be understood to be of igneous origin. Sedimentary rocks are also  crystalline, like crystalline limestone that precipitates directly from solution. Debris deposits are formed primarily from the accumulation of crystalline material, but the term is not commonly used for debris deposits.


Salient Features of Crystalline Rock:

  • Crystalline rocks, particularly granitic rocks, and basalts, are one of the principal rock types under consideration as the potential host rocks for a high-level radioactive waste repository.


  • Rocks with crystalline texture are usually harder and more compact than those with a granular texture. When crystalline rocks are broken they tend to fracture along smooth, angular surfaces within individual crystals, rather than between crystals.


  • The crystalline rock contains angular grains held together by cement.


  • Crystalline textures include phaneritic, foliated, and porphyritic. Phaneritic textures are where interlocking crystals of igneous rock are visible to the unaided eye. The foliated texture is where metamorphic rock is made of layers of materials. Fragmental textures include clastic, bioclastic, and pyroclastic.


Formation:

Crystalline rocks are formed because of the polymerization of minerals. The Crystalline rocks are called so because of the excess of crystals that are present in them and also because of the highly organized microscopic structures of these crystals for which the highly tensed geological processes are responsible.


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 color and quartzite which is white. 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 based on 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.


  1. 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.


  1. Metamorphic Rocks: These are the types of rocks that are formed from the existing rocks which are undergoing recrystallization. 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 hard crystalline rocks which along with high-grade metamorphic rocks formed tectonically stable areas.


More about the 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 polymerization of minerals. Crystalline rocks are so-called because of the excess of crystals that are present in them and also because of the highly organized 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 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 of 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 on Crystalline Rock

1. How are Crystalline Rocks Formed?

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?

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.

3. How long does it take for a crystalline rock to form?

The manner can take as low as some days to perhaps 1000 years. Natural crystals that come from the Earth shape the identical way. These crystals had been fashioned over one million years in the past withinside the Earth's crust. They arise whilst the liquid withinside the Earth consolidates and the temperature chills. 

4. Why are Sedimentary Rocks now no longer Crystalline?

Sedimentary rocks are one of the 3 not unusual place forms of rocks, the opposite  being igneous and metamorphic rocks. The igneous and metamorphic rocks are frequently subjected to better temperatures or better strain or each which can be number one situations for crystallization. On the opposite hand, sedimentary rocks are fashioned due to the transportation and deposition of sediments or fragments of rocks which can be gift withinside the river beds. As there may be no thermal manner required for the formation of sedimentary rocks, that may result in excessive temperatures, which as cited in advance is the number one requirement for crystallization, the sedimentary rocks are incapable of forming crystals not like the igneous rocks and the metamorphic rocks.

5. How does crystallization form igneous rock?

Igneous rocks are rocks formed from the crystallization of a liquid (molten rock).  The texture of an igneous rock (fine-grained vs. coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals.

6. Which soil develops on the crystalline igneous rock?

The Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern parts of the Deccan plateau. These all soils develop a reddish color due to the diffusion of iron in crystalline and metamorphic rocks. Thus it looks yellow when it occurs in a hydrated form. This soil is found in parts of Odisha, Chhattisgarh, etc. Red soil is rich in iron content, which is responsible for its color and is deficient in nitrogen, humus, phosphoric acid, etc.

7. What type of rock is basalt?

Basalt is an aphanitic extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or the moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System.

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