Weathering of rocks is due to the processes of mechanical disintegration and chemical decomposition. It can happen due to different physical, chemical and biological factors of nature.
Chemical weathering is the process by which rocks are decomposed by chemical processes like Hydration, reduction, oxidation, carbonation, etc. All these can be discussed under chemical factors.
1. Dissolution: Simple Solution = (Solid + Acid or Water = Ion in Solution)
Simple solution (dissolution) is the chemical rection of solid rocks and minerals with water or acid. Bonds between ions present in rigid crystalline lattice are broken. Freed ions are disseminated in solution.
Example: The mineral Quartz is not very soluble. Only less than 6 ppm is dissolved in fresh water.
Silica in solution as hydrosilicic acid.
Calcite is much more soluble than Quartz. Chemical weathering of limestone –
2. Hydration and Dehydration = (Solid Mineral + Water = New Hydrated Mineral, Dehydration is reverse)
Example: Dehydration of Gypsum to form Anhydride.
Hydration of Iron Oxide (hematite) to form Limonite.
3. Hydrolysis (Hydrogen Ion + Mineral With mobile cations = Entirely dissolved Mineral or Partially altered Mineral in which Hydrogen Ion replace mobile Ions that are put into solution)
Hydrolysis is defined as replacement of cations in a mineral structure by hydrogen ions either derived from water or acid.
4. Oxidation-Reduction (Atmospheric oxygen Gains electrons and is reduced as Mineral constituents lose electrons and are oxidized, producing new Rusted Minerals)
Oxidation is process by which an atom loses electron. Reduction is the process by which an atom or ion gains electrons.
Best oxidizing agent is atmospheric Oxygen i.e., O2. Oxygen combines with other existing ion and gain electron to become anion of Oxygen (O2-).
Example: Oxidation of ferrous iron (Fe2+) to Ferric iron (Fe3+).
5. Carbonation (Chemical combination of water with carbon dioxide is called carbonation)
Carbonation is important in the dissolution of limestone and the production of clay from feldspars. Indirectly it supports vegetation which produces humus and thereby brings about chemical weathering.