Question

Arrange the minerals in the order of decreasing weathering index
(A) Kaolinite
(B) Montmorillonite
(C) Vermiculite
(D) Mica
Choose the correct answer from the options given below:

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

Hint:

In Jackson's weathering stages, the index number increases as weathering progresses (1 is least weathered, 13 is most weathered). Highly weathered 1:1 clay (Kaolinite, Stage 10) is more stable and has a higher index than 2:1 clays (Montmorillonite, Stage 9 $\rightarrow$ Vermiculite, Stage 8 $\rightarrow$ Mica, Stage 7).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
M.L. Jackson (1948) proposed a weathering index containing 13 stages to represent the weathering intensity of clay-sized minerals in soils.
Stage 1 represents the most easily weathered minerals (gypsum, halite), while Stage 13 represents the most resistant, highly weathered minerals (anatase, rutile).

Step 3: Detailed Explanation:
Let us identify the specific Jackson weathering stage index for each of the listed clay minerals:
- (A) Kaolinite: Belongs to Stage 10 of Jackson's weathering series. It is a highly weathered 1:1 non-expanding silicate clay mineral found in older, highly weathered soils.
- (B) Montmorillonite (Smectite): Belongs to Stage 9 of Jackson's series. It is an expanding 2:1 clay mineral typical of moderately weathered soils.
- (C) Vermiculite: Belongs to Stage 8 of Jackson's series. It is a semi-expanding 2:1 clay mineral formed from the moderate weathering of biotite or muscovite.
- (D) Illite Hydrous Mica (Mica): Belongs to Stage 7 of Jackson's series. It is a non-expanding 2:1 clay mineral representing early-stage weathering of primary mica minerals.
We are asked to arrange these minerals in decreasing order of weathering index (from highest index/most stable to lowest index/least stable):
\[ \text{Kaolinite (Stage 10)} > \text{Montmorillonite (Stage 9)} > \text{Vermiculite (Stage 8)} > \text{Mica (Stage 7)} \]
This corresponds to the sequence: (A), (B), (C), (D).
This matches Option (A).

Step 4: Final Answer:
The correct decreasing order of weathering index is (A), (B), (C), (D).