Chemical weathering is the breakdown of rocks through chemical reactions, leading to changes in their mineral composition. It is most effective in warm, humid climates where water and atmospheric gases react with minerals.

Processes of Chemical Weathering:

A. Hydration

o Water molecules combine with minerals, forming new minerals.

o Causes expansion, making rocks softer and more prone to weathering.

Example: 

Anhydrite (CaSO₄) + Water → Gypsum(CaSO₄·2H₂O)

Hematite (Fe₂O₃) + Water → Limonite (Fe₂O₃·3H₂O)

 

B. Hydrolysis

o Water molecules break down minerals by replacing ions.

o Important for the formation of clay minerals from feldspar.

Example: 

Orthoclase (KAlSi₃O₈) + Water → Kaolinite (Clay) + KOH

 

C. Solution (Dissolution)

o Some minerals dissolve directly in water, leaving rock hollow and fragile.

o Common in limestone and salt-rich rocks.

Example: 

NaCl (Halite) + Water → Na⁺ + Cl⁻ (Dissolved ions)

 

D. Carbonation

o Carbon dioxide (CO₂) dissolves in water to form carbonic acid, which reacts with minerals.

o Major cause of cave formation in limestone regions.

Example:

CO₂ + H₂O → H₂CO₃ (Carbonic acid)

CaCO₃ (Limestone) + H₂CO₃ → Ca(HCO₃)₂ (Soluble calcium bicarbonate)

 

F. Oxidation (Rusting of Rocks)

o Oxygen reacts with minerals, especially those containing iron, forming oxides.

o Weakens the rock structure and leads to reddish-brown discoloration.

Example: 
x

4FeO (Iron oxide) + O₂ → 2Fe₂O₃ (Hematite)


o RG. Reduction

o Occurs in waterlogged environments where oxygen is scarce.

o Iron and other minerals lose oxygen, leading to color changes in rocks.

Example: 

Fe₂O₃ (Hematite) → FeO (Iron oxide)