Mineral having maximum cation exchange capacity is
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To remember the CEC hierarchy of clay minerals:
\[ \text{Organic Matter (Humus)} > \text{Vermiculite} > \text{Smectite} > \text{Illite} > \text{Kaolinite} \]
Among pure mineral clays, Vermiculite and Smectite always have the highest CEC due to their expanding 2:1 structure.
Step 1: Understanding the Concept:
Cation Exchange Capacity (CEC) is the total capacity of a soil or mineral to hold and exchange exchangeable cations.
It is expressed in centimoles of positive charge per kilogram of dry soil or clay (\( \text{cmol}(+)/\text{kg} \)).
The CEC of a clay mineral depends on its crystal structure, surface area, and the amount of isomorphous substitution occurring within its lattice layers. Step 2: Detailed Explanation:
Let us compare the structure and CEC values of the clay minerals listed in the options:
- Kaolinite: This is a 1:1 type non-expanding clay mineral.
It is held together tightly by hydrogen bonding, preventing water and cations from entering between the layers.
As a result, adsorption occurs only on its outer surfaces, resulting in a very low CEC of \( 3 \text{ to } 15 \text{ cmol}(+)/\text{kg} \).
- Illite: This is a 2:1 type non-expanding clay mineral.
The layers are held together securely by potassium (\( \text{K}^+ \)) ions in the interlayer spaces, preventing expansion.
Its CEC ranges from \( 20 \text{ to } 40 \text{ cmol}(+)/\text{kg} \).
- Mica: This is a primary mineral with a similar structure to Illite, but with very little weathering or exposed surface area, resulting in a low CEC.
- Smectite (Montmorillonite): This is a 2:1 expanding-lattice clay mineral.
The bonding between its silica-alumina sheets is weak, allowing water molecules and hydrated cations to easily enter and expand the interlayer spaces.
This expanding structure provides a very high internal and external surface area.
Combined with extensive isomorphous substitution in its octahedral layer, smectite has a very high CEC, typically ranging from \( 80 \text{ to } 150 \text{ cmol}(+)/\text{kg} \).
Therefore, Smectite has the highest cation exchange capacity among the options. Step 3: Final Answer:
The mineral having the maximum cation exchange capacity is Smectite.
