Question

Arrange the following clay minerals in terms of their increasing activity (measured through cation exchange capacity)
(A) Kaolinite
(B) Vermiculite
(C) Gibbsite
(D) Illite
Choose the correct answer from the options given below:

• A. (A), (C), (B), (D).
• B. (A), (B), (D), (C).
• C. (B), (A), (D), (C).
• D. (C), (A), (D), (B).

Hint:

Standard CEC range order (\(\text{cmol}(p^+)\text{ kg}^{-1}\)):
Sesquioxides (Gibbsite) [$0-4$] $<$ Kaolinite [$3-15$] $<$ Illite [$20-40$] $<$ Smectite [$80-100$] $<$ Vermiculite [$100-150$].

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Cation Exchange Capacity (CEC) is a measure of the soil's ability to hold and exchange cations, serving as a primary indicator of chemical activity, nutrient-holding capacity, and buffer capacity.
The CEC of a soil is primarily determined by its clay mineral composition.
Different clay minerals have widely different CEC values (measured in \(\text{cmol}(p^+)\text{ kg}^{-1}\)), depending on their crystal structure, the extent of isomorphic substitution (which creates permanent negative charge), and their specific surface area.

Step 2: Detailed Explanation:
Let us evaluate and compare the typical CEC values of the listed minerals:
1. Gibbsite (C):
Gibbsite (\(\text{Al(OH)}_3\)) is an aluminum hydroxide mineral, representing a sesquioxide rather than a silicate clay.
It has no isomorphic substitution and possesses only pH-dependent charges at its broken edges.
Under typical soil pH conditions, its CEC is extremely low, virtually approaching zero (\( 0\text{ to }4\text{ cmol}(p^+)\text{ kg}^{-1} \)).
Thus, it is the least active mineral listed.
2. Kaolinite (A):
Kaolinite is a 1:1 type non-expanding silicate clay.
It has negligible isomorphic substitution, and its CEC is primarily due to pH-dependent charges at its broken edges.
Its typical CEC value is low, ranging from \( 3\text{ to }15\text{ cmol}(p^+)\text{ kg}^{-1} \).
3. Illite (D):
Illite (fine-grained mica) is a 2:1 type non-expanding silicate clay.
It has significant isomorphic substitution of \(\text{Al}^{3+}\) for \(\text{Si}^{4+}\) in the tetrahedral sheet, creating a high negative charge.
However, this charge is partially neutralized by potassium (\(\text{K}^+\)) ions locked securely in the interlayer space.
Its typical CEC value is moderate, ranging from \( 20\text{ to }40\text{ cmol}(p^+)\text{ kg}^{-1} \).
4. Vermiculite (B):
Vermiculite is a 2:1 type expanding silicate clay with extremely high isomorphic substitution occurring primarily in the tetrahedral sheets.
It has a very high specific surface area and a large amount of exchangeable cations in its hydrated interlayer space.
Its typical CEC value is exceptionally high, ranging from \( 100\text{ to }150\text{ cmol}(p^+)\text{ kg}^{-1} \).
Arranging these minerals in order of increasing CEC (activity):
Gibbsite (C) \(<\) Kaolinite (A) \(<\) Illite (D) \(<\) Vermiculite (B).
This sequence is represented as (C), (A), (D), (B).
This corresponds to option (D).

Step 3: Final Answer:
Therefore, the correct sequence of increasing activity is (C), (A), (D), (B).