Fundamentals of geology I. (lithosphere) 1 1. The formation of the Earth 1
Fig. 1.19. The places of metamorphic rock formation
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- Fig. 1.20. Classification of metamorphic rocks by Escola
Fig. 1.19. The places of metamorphic rock formation Metamorphic minerals are those that form only at the high temperatures and pressures associated with the process of metamorphism. These minerals, known as index minerals, include sillimanite, kyanite, staurolite, andalusite, and some garnet. Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may be found in metamorphic rocks, but are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. However, all minerals are stable only within certain limits, and the presence of some minerals in metamorphic rocks indicates the approximate temperatures and pressures at which they formed. The change in the particle size of the rock during the process of metamorphism is called recrystallization. For instance, the small calcite crystals in the sedimentary rock limestone change into larger crystals in the metamorphic rock marble, or in metamorphosed sandstone, recrystallization of the original quartz sand grains results in very compact quartzite, in which the often larger quartz crystals are interlocked. Both high temperatures and pressures contribute to recrystallization. High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact. The concept of metamorphic facies was first proposed by Eskola (1915) who gave the following definition: A metamorphic faciesis "a group of rocks characterised by a definite set of minerals which, under the conditions obtaining during their formation, were at perfect equilibrium with each other. The quantitative and qualitative mineral composition in the rocks of a given facies varies gradually in correspondence with variation in the chemical bulk composition of the rocks". It is one of the strengths of the metamorphic facies classification that it identifies the regularities and consistencies in mineral assemblage development, which may be related to P-T conditions, but does not attempt to define actual pressures and temperatures. Eskola (1920, 1939) distinguished eight facies, namely: greenschist, epidote-amphibolite, amphibolite, pyroxene-hornfels, sanidinite, granulite, glaucophane-schist and eclogite facies. Todays people use more metamorphic facies than Eskola, but his system usefull in the modern petrology also (Fig. 1.20.). 
Fig. 1.20. Classification of metamorphic rocks by Escola Relative terms such as high-temperature or low-pressure are often used to refer to the physical conditions of metamorphism but without precise designation of the temperatures and pressures involved. In order to maintain similarity of meaning it is proposed that the whole spectrum of temperature conditions encountered in metamorphism be divided into five parts, and the corresponding metamorphism may be designated as: very low-, low-, medium-, high-, very high-temperature metamorphism. Likewise the broad range of pressure conditions may be divided into five to give: very low-, low-, medium-, high-, very high-pressure metamorphism. In the highest part of the very high pressure ultra-high-pressure metamorphism may be distinguished. The whole range of P/T ratios encountered may be divided into three fields (radial sectors in a PT diagram) to give: low, medium, high, P/T metamorphism. The term metamorphic grade is widely used to indicate relative conditions of metamorphism, but it is used variably. Within a given metamorphic area, the terms lower and higher grade have been used to indicate the relative intensity of metamorphism, as related to either increasing temperature or increasing pressure conditions of metamorphism or often both. To avoid this it is recommended that metamorphic grade should refer only to temperature of metamorphism, following Winkler (1974, 1976). If the whole range of temperature conditions is again divided into four, then we may refer to very low, low, medium, high, grade of metamorphism (Fig. 1.21.). 
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