|Weathering - Chemical Weathering|
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Chemical weathering involves the alteration of the chemical and mineralogical composition of the weathered material. A number of different processes can result in chemical weathering. The most common chemical weathering processes are hydrolysis, oxidation, reduction, hydration, carbonation, and solution.
Hydrolysis is the weathering reaction that occurs when the two surfaces of water and compound meet. It involves the reaction between mineral ions and the ions of water (OH- and H+), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution involved through the release of the hydroxide ions. Hydrolysis is especially effective in the weathering of common silicate and alumino-silicate minerals because of their electrically charged crystal surfaces.
Oxidation is the reaction that occurs between compounds and oxygen. The net result of this reaction is the removal of one or more electrons from a compound, which causes the structure to be less rigid and increasingly unstable. The most common oxides are those of iron and aluminum, and their respective red and yellow staining of soils is quite common in tropical regions which have high temperatures and precipitation. Reduction is simply the reverse of oxidation, and is thus caused by the addition of one or more electrons producing a more stable compound.
Hydration involves the rigid attachment of H+ and OH- ions to a reacted compound. In many situations the H and OH ions become a structural part of the crystal lattice of the mineral. Hydration also allows for the acceleration of other decompositional reactions by expanding the crystal lattice offering more surface area for reaction.
Carbonation is the reaction of carbonate and bicarbonate ions with minerals. The formation of carbonates usually takes place as a result of other chemical processes. Carbonation is especially active where the reaction environment is abundant with carbon dioxide. The formation of carbonic acid, a product of carbon dioxide and water, is important in the solution of carbonates and the decomposition of mineral surfaces because of its acidic nature.
Water and the ions it carries as it moves through and around rocks and minerals can further the weathering process. Geomorphologists call this phenomena solution. The effects of dissolved carbon dioxide and hydrogen ions in water have already been mentioned, but solution also entails the effects of a number of other dissolved compounds on a mineral or rock surface. Molecules can mix in solution to form a great variety of basic and acidic decompositional compounds. The extent, however, of rock being subjected to solution is determined primarily by climatic conditions. Solution tends to be most effective in areas that have humid and hot climates.