The nature of rocks is not permanent; it keeps on changing constantly. Rocks are formed by the movement in the interior of the earth and produced by various processes. The changes in the nature of the rocks are brought about by weathering and erosion. These processes mainly depend on the climate of the respective region, as these processes operate on the surface of the earth, they are called exogenous processes. There are various types of weathering, it may be mechanical weathering due to rise and fall in temperature, it may be biological weathering or it may be chemical weathering. Chemical weathering happens after physical weather.
Decomposition of constituent minerals in a rock is called a chemical weathering. These chemical reactions take place through the medium of water; therefore chemical weathering is faster in areas of high rainfall. Further it depends upon the properties of the rock. As the rocks get decomposed its properties also change. The 3 major types of chemical reactions are: Hydrolysis, Carbonation and Oxidation.
This is one of the most important processes in chemical weathering. When water disassociates into H and OH ions and combines chemically with minerals, they bring about changes such as decomposition of crystalline structure and form new compounds. Take for example Silicate minerals. Water acts as a weak acid on them and form Silicic acid, bases and clay which are required for plants to grow. Water also contains carbon dioxide that is absorbed from the atmosphere. It reacts with the minerals directly to produce insoluble clay minerals for plants to grow. Rocks also contain certain substances that are directly soluble in water. This results in the decomposition of rocks.
Carbonic acid is formed when carbon dioxide dissolves in water. This acid attacks many minerals and rocks transforming them into solution. Certain rocks are much affected by carbonated water specifically limestone. Cements that hold sand particles together when removed, leads to their disintegration.
This process adds and combines oxygen to minerals. The absorption is usually from Oxygen which is dissolved in soil water and also that which is present in the atmosphere. The oxidation is more active in the presence of moisture and results in hydrated oxides such as minerals containing magnesium and iron. When the reversals processes of oxidation takes place, the color of the soil changes to green, blue or grey as iron is converted to ferrous iron compounds. Reduction takes place under the conditions of excess water. When chemical weathering takes place, metamorphic rocks and igneous rocks can be considered to be destroying the primary minerals and producing secondary minerals. Weathering initially destroys relatively weak bonding agents in sedimentary rocks, which are made up of primary and secondary minerals. The particles are freed and can be individually subjected to weathering. There are soils forming minerals in rocks that have to undergo hydration when exposed to humid conditions as they do not contain any water. There is an increase in volumes of minerals as they begin to swell when they are hydrating. They lose their luster and become soft. This is one of the most common processes in nature that works with secondary minerals.