Short Essay on the Structure, Texture & Mineral Composition of Rocks

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Structural features of the rock affects the degree of weathering to a major extent. The presence of joints, cracks and fissures in the igneous rocks, the beddings of sedimentary rocks as well as foliations of some metamorphic rocks, facilitates maximum weathering.

These fea­tures help both mechanical and chemical weathering processes to operate effectively, for instance, water when freezes in the fissures and pores of rocks, exerts enormous pressure on the walls of fissures and make them disintegrate into fragments.

Texture of a rock determines its hardness and strength. It is commonly noticed that fine-grained rocks are most susceptible to weathering, in contrast to their coarse grained counterpart.

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Besides, it is also observed that harder the rocks less is the degree of weathering and that it is maximum in soft rocks.

The major factors affecting the degree of weathering is the mineralogical and chemical composition of the rocks. As we know, different silicate minerals of which most of the rocks are composed have different stability.

The stability order has been represented in the following way: of all the rock-forming minerals, quartz is the most inert oxide and persists under all conditions of temperature and pressure, below that of its formation. Muscovite is a close follower and feldspar comes next. Alkali-feldspars stay longer than the plagioclases.

Ferromagnesian silicates, as a group, are less stable than feldspars; biotite being more stable than the amphiboles which are stabler than the pyroxenes and the olivines.

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It is mostly due to the above fact that basic and ultrabasic rocks are easily weathered in a short duration of time, whereas granites need prolonged weathering; or in other words, basic rocks are more suscep­tible to chemical decomposition than the acid rocks. Thus, the nature of the pre-existing rocks also controls the degree of weathering.

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