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Metamorphic rocks compose about 15% of the Earth's crust. They are rocks that have been altered by heat and/or pressure beneath the Earth's surface. Table below shows the classification of Metamorphic rocks
Foliated rocks
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| Slate | Shale, tuff | Mica, quartz - excellent foliation, very fine-grained |
| Chlorite schist | Basalt, felsite, tuff | Chlorite, plagioclase - good foliation, coarse-grained |
| Mica schist | Shale, felsite, tuff | Muscovite, quartz, biotite - very good foliation, coarse-grained |
| Hornblende schist | Basalt, gabbro, felsite | Hornblende, plagioclase - good foliation, coarse-grained |
| Gneiss | Granite, felsite, shale, mica schist | Feldspar, quartz, plagioclase, hornblende, etc. - rough foliation, coarse-grained |
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| Hornfels | Any fine-grained rock especially shale | Variable composition - fine-grained, hard |
| Quartzite | Sandstone | Quartz - fine- to coarse-grained |
| Marble | Limestone, dolomite | Calcite - coarse grained |
| Anthracite | Bituminous coal | Carbon - conchoidal fracture |
Two kinds of changes occur during metamorphism. Mechanical, which involves fracturing, crushing, and change in shape due to pressure, and chemical, which consists of recrystallization (growth of mineral crystals either by reorganization of original minerals or production of an entirely new set of minerals) and loss of water.
Thermal metamorphism also called (not accurately) as contact metamorphism is brought about by heat with little pressure. It occurs in shallow bedrock by heat of molten magma from below. The rock adjacent to this magma is initially coarse-grained, but it grades into dark fine-grained rock containing recrystallized minerals with random orientation and is known as hornfels.
Dynamic metamorphism occurs when rocks are changed more by pressure than temperature, and is most common along fractures and shear planes when one rock unit slides past another. Mechanical deformation shatters the grains or changes their shape plastically.
Regional metamorphism is so called due to the extremely large areas it affects, and consists of metamorphic rocks affected by both temperature and pressure, and is the case for most metamorphic rocks.
Hydrothermal metamorphism occurs when there are profound chemical changes, releasing enormous quantities of fluids carrying sodium, potassium, and other metals. This leads not just to recrystallization of its existing components, but also in its overall chemical composition.
Foliation is the ability of a metamorphic rock to split along a smooth plane. Foliated rocks contain a large number of flat, platy or sheetlike minerals, all oriented in the same direction. They contain minerals such as mica, chlorite, and hornblende. Foliation develops in response to confining pressure exerted equally in all directions and in the presence of an additional directed pressure, that accompanies mountain building.
Shale after mild metamophorphism is transformed into slate, a fine-grained rock similar to shale but has excellent slaty cleavage. Under intense heat and pressure, slate is converted into schist, with visible grains, but with foliation not as good as that of slate. Very intense metamorphism produces gneiss (pronounced as "nice"), an even coarser-grained rock with rough foliation and distinct banding. Higher grades of metamorphism produce larger grains and rougher foliation.
Metamorphic rocks that lack foliation are called massive. Marble, extensively used in the construction of fine buildings (such as the Taj Mahal) is formed from limestone and quartzite, which is metamorphosed sandstone.
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