Question

Exhaustive hydrolysis and leaching in desert soils lead to the formation of _______ clay minerals

• A. Illitic
• B. Kaolinitic
• C. Smectites
• D. Allophanes

Hint:

Intense leaching and high rainfall wash away silica and basic cations, leaving behind 1:1 Kaolinitic clays or iron/aluminum oxides (Gibbsite). Moderate weathering retains more silica, leading to 2:1 clays like Smectite.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Chemical weathering of primary minerals under different environmental conditions yields different types of secondary clay minerals.
The intensity of hydrolysis and leaching determines whether 2:1 clays (like smectite or illite) or 1:1 clays (like kaolinite) are formed.

Step 2: Detailed Explanation:
Hydrolysis is the chemical reaction where water molecules react with mineral structures to break them down.
Under exhaustive (intense) hydrolysis and leaching conditions:
1. Soluble basic cations (such as $\text{Ca}^{2+}$, $\text{Mg}^{2+}$, $\text{Na}^+$, and $\text{K}^+$) are completely washed out of the soil profile.
2. A large portion of the silica ($\text{SiO}_2$) is also removed, a process known as desilication.
3. This intensive weathering breaks down 2:1 expanding clays (like smectite) into simpler 1:1 non-expanding clay minerals, predominantly Kaolinite.
Although desert soils naturally experience very little weathering and leaching (typically retaining 2:1 smectites), the process of "exhaustive hydrolysis and leaching" chemically leads to the formation of Kaolinitic clays.
Under moderate weathering, illitic or smectite clays persist, while amorphous allophanes are common in volcanic soils.
Thus, Kaolinitic clay is the correct product of exhaustive hydrolysis.

Step 3: Final Answer:
Exhaustive hydrolysis and leaching lead to the formation of Kaolinitic clay minerals, which corresponds to option (B).