Clay mineralogy (illite vs. kaolinite vs. montmorillonite): choose the correct statements about bonding strength, swelling, and layer structure.

Introduction / Context:
Engineering behavior of fine-grained soils is governed by their clay mineralogy. Kaolinite, illite, and montmorillonite form a spectrum of bonding strength, surface activity, and swelling. Recognizing these differences helps predict compressibility, permeability, and volume change upon wetting and drying.

Given Data / Assumptions:

• Kaolinite: 1:1 layered structure; relatively strong hydrogen bonding; low swelling.
• Illite: 2:1 layered structure with potassium bridges; moderate bonding; negligible to slight swelling.
• Montmorillonite (smectite): 2:1 layers; weak interlayer bonding; high swelling.

Concept / Approach:

Relative bond strengths follow kaolinite > illite > montmorillonite. Illite’s interlayer K⁺ provides moderate bonding but far less than kaolinite’s hydrogen bonds and far more than the expansive smectite interlayers. Illite is classed as non-expanding to slightly expanding, unlike montmorillonite. Structurally, illite is a 2:1 clay (T–O–T) as stated.

Step-by-Step Solution:

Verification / Alternative check:

Classification tables in soil mechanics texts consistently show illite intermediate in plasticity and activity, with 2:1 structure and minimal swelling compared to smectites.

Why Other Options Are Wrong:

Since (a)–(d) are correct, the best choice is “All the above”.

Common Pitfalls:

Believing all 2:1 clays swell; illite is the non-expanding (or slightly expanding) 2:1 member.

Final Answer:

All the above