Igneous Rocks Essay

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Igneous rocks show great variations both in chemical and mineralogical composition as well as textural characteristics. There is no general agreement among the penologists as to the classification of igneous rocks. Different classifications have been proposed or the basis of: 1. Mineralogical composition. 2. Chemical composition. 3. Textural characteristics. 4. Genesis. 5. Association. 1. On the basis of mineralogical composition: Since the relative amounts of the various minerals in a rock can be measured or esti­mated with a fair degree of accuracy, this criteria is given more emphasis for the classification of igneous rocks. The minerals occurring in igneous rocks may be classed as (a) Essential, (b) Accessory, and (c) Secondary. The first two are the products of magmatic crystallisation whereas the third one is formed by the alteration of the primary minerals, i.e. (the 1st two), through the processes of weathering, metamorphism etc. (a) Essential minerals: These are the major constituents of the rock which help in the diagnosis of the rocks types. The disappear­ance of these minerals would cause the relegation of the rock to another type. (b) Accessory minerals: These are the minor constituents of a rock and their presence has no bearing on the nomenclature of the lock. The minerals commonly occurring in igneous rocks may be broadly classified into felsic and mafic varieties. Felsic is a term derived from feldspar, felspathoid and silica and mafic is similarly derived from ferromagnesian minerals, like biotite, pyroxene, amphiboles etc. Felsic minerals are light in colour, low melting point, low specific gravity, comparatively of late crystallisation. The mafic minerals are dark coloured, higher specific gravity, higher melting point and are early crystallised minerals. Terms like leucocratic and melanocratic
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