Chemical Weathering and its type

Chemical weathering may be defined as the spontaneous and irreversible thermodynamic process that causes degradation of the mineral phases under the prevailing environmental conditions at the surface of the Earth. It is usually considered to be the solid/solution interactive process occurring in contact with the atmosphere. In other words chemical weathering may be defined as that set of processes which alter a rock into a layer of chemically and physically transformed superficial material (regolith) which is susceptible to evolve (soil or karst) with time following biogeochemical processes.

Among the conditions present near the Earth’s surface that are different from those deep within the Earth are :

• Lower Temperature (Near the surface T = -20 to 50^oC)
• Lower Pressure (Near the surface P = 1 to several hundred atmospheres)
• Higher free water (there is a lot of liquid water near the surface, compared with deep in the Earth)
• Higher free oxygen (although O₂ is the most abundant element in the crust, most of it is tied up in silicate and oxide minerals –  at the surface there is much more free oxygen, particularly in the atmosphere).

Because of these differing conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface. Minerals that are stable under P, T, H₂O, and O₂ conditions near the surface are, in order of most stable to least stable:

Iron oxides, Aluminum oxides – such as hematite Fe₂O₃, and gibbsite Al(OH)₃ > Quartz > Clay Minerals > Muscovite > Alkali Feldspar > Biotite > Amphiboles > Pyroxenes > Ca-rich plagioclase > Olivine

The main agent responsible for chemical weathering reactions is water and weak acids formed in water

• An acid is solution that has abundant free H⁺ ions.
• The most common weak acid that occurs in surface waters is carbonic acid.
• Carbonic acid is produced in rainwater by reaction of the water with carbon dioxide (CO₂) gas in the atmosphere.
  • H₂O + CO₂ = H₂CO₃ = H⁺ + HCO₃^–
  • H⁺ is a small ion and can easily enter crystal structures, releasing other ions into the water

Types of Chemical Weathering :

1. Hydrolysis : H⁺ or OH^– replaces an ion in the mineral. 
   • 4KAlSi₃O₈ + 4H⁺ + 2H₂O = 4K⁺ + Al₄Si₄O₁₀(OH)₈ +8SiO₂
2. Leaching : Ions are removed by dissolution into water. In the above example, we say that the K⁺ ion was leached
3. Oxidation : Since free oxygen (O₂) is more common near the Earth’s surface, it may react with minerals to change the oxidation state of an ion. This is more common in Fe (iron) bearing minerals, since Fe can have several oxidation states, Fe, Fe⁺², Fe⁺³. Deep in the Earth the most common oxidation state of Fe is Fe⁺².
   • 3Fe⁺²SiO₃ + 1/2 O₂ = Fe₃O₄ + 3SiO₂
4. Dehydration : Removal of H₂O or OH^– ion from a mineral
   • 2FeO OH = Fe₂O₃ + H₂O
5. Complete Dissolution : All of the mineral is completely dissolved by the water.
   • CaCO₃ + H₂CO₃ = Ca⁺² + 2(HCO₃)^–
6. Living Organisms : Organisms like plant, fungi, lichen and bacteria can secrete organic acids that can cause dissolution of minerals to extract nutrients. The role of microorganisms like bacteria has only recent been discovered.