The classification of metamorphic rocks has been done generally in two ways.
Metamorphic rocks are defined as those rocks which have formed through the operations of various types of the metamorphic process such as the pre-existing igneous and sedimentary rocks involving changes in the texture, structure, and mineralogical compositions. The direction of change depends upon the type of the original rock and the type of the original rock and the types of the metamorphic process that operates on the rock.
Heat, Pressure and chemically active fluids that are the main agents involved in metamorphic processes. Plastic deformation, recrystallisation of mineral constitutes and development of the parallel orientation are typical characters of the metamorphic rocks.
They are megascopic ally distinguished from other types of rocks by the development of features like cleavage, foliation, schistosity and granulation and by the presence of the such minerals that are known as to be end products of the metamorphic recrystallisation.
- Metamorphic rocks classification based on their mineralogical composition.
- Metamorphic rocks classification based on their texture and structure.
In this article, we will discuss about the both the classification of metamorphic rocks.
Classification of Metamorphic rocks based on its mineralogical composition.
Metamorphic rocks generally show good variance in their original structure due to their mineralogical composition. The mineralogical composition of the metamorphic rocks generally depends upon the following two points.
- The composition of the parent rock, and
- The type of metamorphism undergone by the rock.
- The degree of metamorphism in which the parent rock has been undergone.
The hosts of the new minerals formed due to metamorphism are generally stable under the changed conditions of the temperature, pressure and chemical environment.
There are two types of minerals that have been formed during the metamorphism process depending upon the atomic structure. This type of classification usually depending upon the development process during the metamorphism process.
- Stress Minerals
The minerals produced in the metamorphic rocks generally due to the stress factor are known as the stress minerals. They are characterized by flaky, lamellar, flattened, and elongated forms. Kyanite, staurolite, muscovite, chlorite, and amphiboles. These minerals are formed mostly under the conditions of dynamo thermal metamorphism.
2. Anti- Stress Minerals.
These are metamorphic minerals that are produced under the influence of the temperature factor. Such minerals are generally of a regular equidimensional outline. Examples of such minerals are sillimanite, olivine, cordierite, and many other pyroxenes. These types of minerals are quite unstable under high-stress conditions. In general, it is shown that the plutonic metamorphism process plays a major role in the development of the following types of metamorphic minerals.
Now, we will move towards the second type of classification of the metamorphic rocks depending upon the texture and structure of the metamorphic forming minerals.
Classification of Metamorphic rocks based on texture and Structure.
Before we discuss the classification of the metamorphic rocks based on the texture and structure then you must have to keep in mind that this type of classification is just not based upon its structure and texture, the other factors that are also important in this type of classification are Origin of the rock, Degree of metamorphism and mineralogical composition of the metamorphic rocks.
Above all mentioned factors are really necessary for the classification of metamorphic rock based on its structure and structure. The most common type of the classification of the metamorphic rocks has been done whether the presence or absence of the layered structure or you may call it FOLIATION is defined as classified as under.
- Foliated Rocks
All metamorphic rocks showing the development of conspicuous parallelism in their mineralogical and structural constitution under the influence of the term which is very familiar with most of all the geologists are “Foliation”. The mineral which is formed due to this is grouped in one particular metamorphic type of the rocks. They are actually known as the “Foliated Rocks”. Parnellism actually includes the features of the cleavage, schistosity structures. The most common example of this type of rock slates, Genesis, and Schists.
2. Non-Foliated Rocks.
This type of rock are actually classified in all those metamorphic rocks characterized with a total or nearly total absence of the foliation or Parnellism of mineralogical constitutes. The most common examples of this type of rock are quartzites, hornfels, marble, and soapstone, etc.
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