Chemical weathering, or chemical decomposition, involves several important reactions between the elements in the atmosphere and those in the mineral of the earth’s crust. In these processes, the internal structures of the original minerals are destroyed, and new minerals are formed, with new crystal structures that are stable under conditions at the earth’s surface.
Water is of prime importance in chemical weathering. It takes part directly in the chemical reaction. It acts as a medium to transport elements of the atmosphere to the minerals of the rocks where reactions can occur, and it removes the products of weathering to exposed fresh rock. The chemical reactions involved in the decomposition consist of the following three main groups i.e. Hydrolysis, Dissolution, and Oxidation.
1. Hydrolysis
The chemical union of water and a mineral is known as hydrolysis. The process involves not merely absorption of water, as in sponge, but a specific chemical change in which a new mineral is produced. In hydrolysis, ions derived from one mineral reach with the H+ or OH– ions to produce a new mineral.
A good example of hydrolysis is the chemical weathering of k-feldspar. Two substances are essential: carbon dioxide and water. The atmosphere and the soil contain carbon dioxide, which unites with rain water to form carbonic acid. If k-feldspar comes in contact with carbonic acid, the following chemical reaction occurs:
2KAlSi3O8+ H2CO3 + H2O → K2CO3 + Al2Si2O5(OH)4 + 4SiO2
Hydrolysis is an extremely important weathering process because it acts on the feldspars and ferromagnesium minerals, the dominant minerals in most rocks.
2. Dissolution
Water is one of the most effective and universal solvent known. Because of the polarity of the water molecule, practically all minerals are soluble in water to some extent. The water molecule is polar and behaves like a tiny magnet. It acts to loosen the bonds of ions at the surface of minerals with which it comes in contract.
Some rock types can be completely dissolved and leached away by water. Rock-salt is perhaps the best known example. It is extremely soluble, surviving at the earth’s surface only in the most arid regions. Gypsum is less soluble than rock salt, but is also easily dissolved by surface water. Few out crops of these rocks occur in humid regions. Limestone is also soluble in water, especially if the water contains carbon dioxide.
The chemical analysis of river water illustrates the effectiveness of dissolution in the weathering of rocks. Fresh rain water contain relatively little dissolved mineral matter, but running water soon dissolves the more soluble minerals in the rock and transport them in solution. Each year the rivers of the world carry about 3.9 million metric tons of dissolved minerals to the oceans, so it is not surprising that sea water contains 3.5% (by weight) dissolved salts, all of which were dissolved from the continents by pure rain water.
3. Oxidation
Oxidation is the combination of atmospheric oxygen with a mineral to produce an oxide. The process is especially important in the weathering of minerals that have high iron content, such as olivine, pyroxene, and amphibole. The iron in silicate minerals unites with oxygen to form hematite or limonite. Hematite is deep red, and if it is dispersed in sandstone or shale, it imparts a red color to entire rock.
