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Quartz

  Quartz   Quartz is hexagonal and commonly occurs as crystals ranging in size form microscopic to crystals weighing several tons. Where it crystallizes unhindered by other crystals, such as in cavities in rock or in a liquid containing few other crystals, it shows well-developed hexagonal prisms and sometimes showing apparent hexagonal pyramids or dipyramid. When it crystallizes in an environment where growth is inhibited by the surroundings, it rarely show crystal faces. It is also found as microcrystalline masses, such as in the rock chert, and as  fibrous masses, such as in chalcedony. Vitreous lusture.   As visible crystals, Quartz is one of the more common rock forming minerals. It occurs in siliceous igneous rocks such as volcanic rhyolite and plutonic granitic rocks. It is common in metamorphic rocks at all grades of metamorphism, and is the chief constituent of sand. Because it is highly resistant to chemical weathering, it is found in a wide variety of sedimentary r
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METAMORPHISM

                                                                 METAMORPHISM


                                                             figure 1.1 ( Metamorphosed Quartzite)

Definition :-

                The word "Metamorphism" comes from the Greek (Meta = after, morph = form,).So                 metamorphism means the after form. In geology this refers to the changes in mineral                    assemblage and texture that result from subjecting a rock to pressures and temperatures different from those under which the rock originally formed.

  •  The original rock that has undergone metamorphism is called the protolith . Protolith can be any type of rock and sometimes the changes in texture and mineralogy are so dramatic that is difficult to distinguish what the protolith was.

  •  Note that diagenesis and weathering are also a changes in form that occur in rocks. In geology however, we restrict diagenetic processes to those which occur at temperatures below 200° c and pressures below about 300 MPa (MPa stands for Mega Pascals pressure unit ), this is equivalent to about 3,000 atmospheres of pressure .                     

  •   Metamorphism therefore occurs at temperatures and pressures higher than 200° c and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they become buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction .

  •  The upper limit of metamorphism occurs at the pressure and temperature of wet partial melting of the rock in question. Once melting begins, the process changes to an igneous process rather than a metamorphic process .

  • During metamorphism the protolith undergoes changes in texture of the rock and the mineral make up of the rock. These changes take place mostly in the solid state and are caused by changes in physical or chemical conditions, which in turn can be caused by such things as burial, tectonic stress, heating by magma or interactions with fluids .
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