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: Define Cation Exchange Capacity (CEC).
CEC is the soil's capacity to retain and exchange positively charged ions (cations).
It is influenced by clay type and organic matter content.
Step 2: Describe Montmorillonite's characteristics.
Montmorillonite is a 2:1 clay mineral, composed of two silica sheets and one alumina sheet.
It possesses a large surface area and a significant negative charge due to isomorphous substitution, resulting in a substantially higher CEC than kaolinite or illite.
Step 3: List CEC values for various soil components.
- 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: Evaluate potential CEC ranges.
- (A) 3--10: This range is too low and corresponds to kaolinite.
- (B) 10--40: This range is typical for illite.
- (C) 40--50: This range remains significantly below Montmorillonite's actual CEC.
- (D) 80--150: This range accurately reflects Montmorillonite's CEC.
Step 5: Conclude Montmorillonite's CEC.
Montmorillonite clay mineral exhibits a very high CEC, falling within the 80--150 cmol(p$^{+}$)/kg range.