Question

The average cation exchange capacity (CEC) value of Montmorillonite clay mineral is [cmol(p$^{+}$)/kg].

• A. 3--10
• B. 10--40
• C. 40--50
• D. 80--150

Hint:

Montmorillonite has high CEC and high swelling capacity, making soils fertile but prone to waterlogging.

Answer 1

The Correct Option is D

Step 1: Understanding CEC.
Cation Exchange Capacity (CEC) is the ability of soil particles to hold and exchange positively charged ions (cations).
It depends on the type of clay and organic matter content.
Step 2: Montmorillonite properties.
Montmorillonite is a 2:1 type clay mineral (two silica sheets sandwiching one alumina sheet).
It has a high surface area and high negative charge due to isomorphous substitution.
This results in very high CEC compared to kaolinite or illite.
Step 3: CEC values of clays.
- Kaolinite: 3--10 cmol(p$^{+}$)/kg
- Illite: 10--40 cmol(p$^{+}$)/kg
- Montmorillonite: 80--150 cmol(p$^{+}$)/kg
- Organic matter: 200--400 cmol(p$^{+}$)/kg
Step 4: Analysis of options.
- (A) 3--10: Too low, this is kaolinite.
- (B) 10--40: This is typical of illite.
- (C) 40--50: Still much lower than Montmorillonite’s actual CEC.
- (D) 80--150: Correct, this is the CEC range of Montmorillonite.
Step 5: Conclusion.
Thus, Montmorillonite clay mineral has a very high CEC, ranging from 80--150 cmol(p$^{+}$)/kg.