Question

The cation exchange capacity of clay minerals follows in the order of

• A. Montmorillonite $>$ Vermicullite $>$ Illite $>$ Kaolinite
• B. Montmorillonite $>$ Illite $>$ Vermicullite $>$ Kaolinite
• C. Vermicullite $>$ Montmorillonite $>$ Illite $>$ Kaolinite
• D. Vermicullite $>$ Illite $>$ Montmonillonte $>$ Kaolinite

Hint:

Cation Exchange Capacity (CEC) of common clay minerals:
Vermiculite ($100-150$) $>$ Montmorillonite ($80-120$) $>$ Illite ($20-40$) $>$ Kaolinite ($3-15$).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Cation Exchange Capacity (CEC) of a soil or mineral is the total quantity of exchangeable cations that it can retain per unit mass.
CEC depends on the mineral structure, the extent of isomorphous substitution, and the accessible surface area.

Step 2: Detailed Explanation:
Let us compare the typical CEC values and structures of the listed clay minerals:
1. Vermiculite: A 2:1 expanding-type silicate clay mineral with high levels of isomorphous substitution in both the tetrahedral and octahedral sheets.
Its CEC ranges from $100\text{ to }150\text{ cmol}(p^+)/\text{kg}$ because both its internal and external surface areas are highly accessible.
2. Montmorillonite: A 2:1 expanding-type silicate clay mineral (smectite group).
It exhibits moderate isomorphous substitution, primarily in the octahedral sheet.
Its CEC ranges from $80\text{ to }120\text{ cmol}(p^+)/\text{kg}$ due to its expanding interlayer spaces.
3. Illite: A 2:1 non-expanding silicate clay mineral.
Although it has significant isomorphous substitution in the tetrahedral sheet, the negative charge is partially satisfied by potassium ions ($\text{K}^+$) held within the interlayer spaces.
This prevents the layers from expanding, limiting the CEC to $20\text{ to }40\text{ cmol}(p^+)/\text{kg}$.
4. Kaolinite: A 1:1 non-expanding silicate clay mineral.
It shows minimal isomorphous substitution, and its charge is primarily pH-dependent at the broken crystal edges.
Its CEC is low, ranging from $3\text{ to }15\text{ cmol}(p^+)/\text{kg}$.
Arranging these in descending order of CEC gives: Vermiculite $>$ Montmorillonite $>$ Illite $>$ Kaolinite.

Step 3: Final Answer:
The correct order of cation exchange capacity is Vermicullite $>$ Montmorillonite $>$ Illite $>$ Kaolinite, which corresponds to Option (C).