Chemical Weathering

 

Chemical Weathering-https://edugeocul.blogspot.com

Chemical Weathering:

Definition: It includes all processes where solid rock is decomposed and weathered chemically.

This process involves in alteration and solution of minerals contents in the rock. Minerals are decomposed, loosened and dissolved by water and temperature. In some cases dissolved, in acids or oxygen of the atmosphere and soil water. Minerals are decomposed, by organisms and the product of their decay.

Characteristics:

i) More common in hot and humid regions.

ii) The gradual decomposition of rocks takes place due to exposure to air and Water. 

iii) Chemical changes occur in the rock and completely new rock characteristics are formed.

iv)Parent rock cannot be identified from the semi dissolve ingots. 

Classification- The main chemical changes that occur during weathering are 

a) Formation of Carbonates b) Oxidation c) Hydration d) Hydrolysis e) Solution

a) Carbonation:

Definition: Carbonation is the reaction of carbonic acid on carbonate minerals based rock like limestone.

Process: Rainwater, when mixed with carbon dioxide in the atmosphere forms a weak solution of carbonic acid (H₂ Co₃). It is easily reacted with carbonate rocks like limestone and chalk. The reaction results in the Calcium bicarbonate, which is highly soluble and gets washed away in solution and flows through below the surface. 

Resultant Landforms: carbonic acid water when sinks into the ground and forms huge caves.  ‘Grikes’ and ‘clints’ form in the upper surface.  

     H₂O + Co₂ → H₂ Co₃

     H₂ Co₂ +  CaCo₃ → Ca(HCo₃)₂

Carbonic acid + Calcium carbonate → Calcium Bicarbonate    

https://edugeocul.blogspot.com- weathering in the karst region

b) Oxidation:-

Definition: When oxygen in the air dissolved in water reacts with certain rock minerals, especially iron to form an oxide and ultimately decay the rock is called oxidation.

Process: In iron-rich rocks like Magnetite and Hematite are when comes to contact with air and water containing oxygen, change into iron oxide. Iron oxide crumbles easily and is far more easily eroded than original iron. The ferrous state changes to the oxidized ferric state with yellow brown-red colour. It leads to the formation of rust in iron-containing rocks. Ferric oxide weaves away easily.  

Resultant Landforms: Red Soil forms from oxidized rocks.

4FeO+3H₂O+O₂→2Fe₂O₃,3H₂O

Ferrous oxide+ water+ oxygen→ Hydrated Ferric oxide

https://edugeocul.blogspot.com-Chemical weathering in hot and humid region 

Hydration: 

Definition:  The pure rainwater enters into the rock, minerals absorb it and increase their volume and loosen the rock solidity then turns into powder.

Process: Chemical reaction of water with minerals in the rock, minerals are changed into different chemical constituent, for example, Feldspar is formed by aluminium silicate minerals when minerals absorb water to increase their volume cause stress and strains in the rock and become a mass of clay or kaolin and whatever is left behind is a residue of sand. This ultimately leads to rock decomposition. 

This process also accelerates granular disintegration and exfoliation. It also helps further alteration by oxidation and carbonation.

CaSO₄+2H₂O→ CaSO₄,2H₂O

Calcium sulfate + water→ gypsum

Hydrolysis: 

Definition: The water in the rock when attached to the minerals, changes it into new minerals by molecular recombination then it is called Hydrolysis. 

Formation: In hydrolysis, ions of the minerals (of a rock) and ions of water i.e. Hydroxyl ion and Hydrogen ion reacts in such a way that new mineral compounds are formed. In this process, we found the exchange of the H+ ion from the water to act with the minerals in the rock. With the increase of temperature the degree of separation of water and the hydrogen ion concentration increase. The hydronium ions are attached to the decomposable minerals and alter them completely. In the Kaolinization process, Feldspar mineral is converted to Kaolin by hydration.

Silicate minerals are most affected by hydrolysis. In this process hydroxides of potassium, magnesium, iron, calcium etc. are formed.

2KAlSi₃O₈+ HOH→ HAlSi₃O₈ + KOH

Feldspar + Ionized water→ Alumino Silicic acid + Potassium Hydroxide

Solution: 

Definition: The minerals which are dissolved in water, the process called Solution. This is the first step of chemical decomposition.

Process: The solution of rocks depends on the nature of rocks. It means how much solid and solvent materials are present in the rock or their ratio in the rock. Sometimes cementing materials which is present in sedimentary rock dissolve by the solution process loosens the solidity and physical breakdown occurs. 

Example: Highly soluble – Common salt

Moderately soluble – Limestone (only soluble in Carbonic acid)

Biochemical Weathering: 

Chemical weathering by the formation of acid :

 If rock is covered by moss, lichens, Sargasso due to the presence of the moisture (water) always present on the rock surface. In high temperature and humid conditions dead skin of mass, lichens, Sargasso turns into humus and produces humic acid. Humic acid can do chemical weathering.

 Different acids are also formed by the involvement of bacteria like acetic acid, lactic acid, oxalic, citric, tartaric etc. Acetic acid, Aspartic acids if are mixed with pure water then weathering rate increase up to 10% whereas the presence of the heavy acids weathering rate increases up to 100%.

Chelation: Plants used their biological organs (roots) to take metallic ions to absorb nutrients from the solid minerals of the rock. This process is called chelation. As a result of chelation metallic cations (+ ion) are left out from the crystalline structure, become unstable and disintegrated.  

https://edugeocul.blogspot.com-Biochemical Weathering

The Organisms living on the limestone rocks release Co₂ with their out-breath. When this Co₂ mixed with water of the atmosphere turns into Carbonic acid. This Carbonic acid easily decomposed limestone (CaCo₃) by forming Calcium Bicarbonate or Ca(HCo₃)₂