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After reading this article you will learn about:- 1. Classification of Metamorphic Rocks 2. Structures in Metamorphic Rocks 3. Engineering Uses.
Classification of Metamorphic Rocks:
1. Metamorphic rocks may be classified on the basis of foliation into two types.
They are:
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(a) Foliated rocks:
This group includes the rock that can split into thin sheets.
Examples:
Slate, Schist, Gneiss, etc.
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(b) Non-Foliated rocks:
This group includes the rock that cannot be split into thin sheets.
Example:
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Marble.
2. Metamorphic rocks are classified on the basis of metamorphism into three types.
They are:
(a) Contact (thermal) metamorphism:
It is a process in which temperature is the dominating factor.
Example:
(b) Dynamic Metamorphism:
It is a process in which pressure is the dominating factor.
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Example:
(c) Dynamo thermal or thermodynamic metamorphism:
It is a process in which both temperature and pressure are dominating factors.
Structures in Metamorphic Rocks:
The structures of the metamorphic rocks are:
(i) Slaty structure,
(ii) Schistose structure,
(iii) Gneissose structure, and
(iv) Granulose structure.
(i) Slaty structure:
The slaty structure is also called slaty cleavage.
The rock possessing slaty cleavage has a unique property of splitting into thin sheets. The slaty cleavage may form at any angle to the bedding planes of the shale from which the slaty rock has been derived as shown in 3.19.
(ii) Schistose structure:
It is formed by the parallel arrangement of flat, tabular, elongated or flaky minerals, such as Muscovite, Biotite, Chlorite, talc and Hornblende as shown in Fig. 3.20.
The rock having Schistose structure has a tendency to split readily into flakes, leaves or thin slabs.
(iii) Gneissose structure:
A rock possessing gneissose structure exhibits a pronounced appearance in which light and dark coloured band alternate as in Fig. 3.21.
The light coloured bands are due to quartz and Feldspar, while the dark coloured bands are due to the presence of Ferro-magnesium minerals.
(iv) Granulose structure:
Granulose (even grained) structure is produced due to the predominance of equigranular minerals such as quartz, feldspar, pyroxenes and calcite as shown in Fig. 3.22.
The flaky minerals are absent or present only in small amount on breaking a granulose rock producing a rough fracture surface.
Engineering Uses of Metamorphic Rocks:
The metamorphic rocks are extensively used as building stones. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc.
The most important non-foliated rock is marble. It is considered to be an excellent building material for important monumental, historical and architectural buildings. Marble is extensively used in modern buildings also for the decorative purpose in columns, staircases and floors.