Question

Arrange the following in decreasing order of Basal Spacing:
(A) Kaolinite
(B) Montmorillonite
(C) Illite
(D) Vermiculite
Choose the correct answer from the options given below:

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

Hint:

Basal spacing order: Montmorillonite > Vermiculite > Illite > Kaolinite.

Answer 1

The Correct Option is D

Montmorillonite has the largest basal spacing due to swelling properties, followed by Vermiculite, then Illite (moderate), and Kaolinite (lowest, non-expanding).

Answer 2

Alternate approach — Clay mineral structure classification:
Clay minerals are grouped by their layer structure and expansion behaviour, which directly controls basal spacing.
Montmorillonite is a 2:1 expanding clay with weak interlayer bonding, so water molecules freely enter between layers giving the largest basal spacing (~14–21 Å). Vermiculite is also 2:1 but has stronger interlayer bonding (hydrated cations), limiting expansion to a lesser degree than montmorillonite. Illite is a 2:1 clay with non-exchangeable potassium ions locking the layers together, restricting expansion further. Kaolinite is a 1:1 non-expanding clay held by strong hydrogen bonds between layers, giving the smallest basal spacing (~7.2 Å).
Ranking the layer-bonding strength from weakest to strongest interlayer attraction gives the decreasing basal spacing order: Montmorillonite > Vermiculite > Illite > Kaolinite, confirming option (B), (D), (C), (A).