Clay mineral composition — the clay fraction of natural soils commonly includes which of the following minerals?

Introduction / Context:
Clay mineralogy governs plasticity, swelling, permeability, and strength behavior of fine-grained soils. Identifying the common mineral groups helps anticipate engineering responses such as shrink–swell potential and sensitivity to pore-water chemistry.

Given Data / Assumptions:

• Major clay mineral families include kaolinite, illite (mica), smectite (montmorillonite), and vermiculite.
• Natural soils are mixtures; a single deposit can contain multiple clay minerals.
• Engineering behavior reflects the dominant mineralogy and fabric.

Concept / Approach:

Kaolinite (1:1 layers) typically shows low plasticity and minimal swelling. Illite (mica-like, 2:1 with K+ interlayers) exhibits moderate plasticity and limited swelling. Montmorillonite (smectite, 2:1) has high plasticity, strong swelling, and low permeability when saturated. Vermiculite shows intermediate to high swelling depending on exchangeable cations. Because all listed minerals occur commonly in clays, the most inclusive answer is correct.

Step-by-Step Solution:

Verification / Alternative check:

X-ray diffraction and differential thermal analysis from site investigations routinely identify all these minerals in various proportions across different formations.

Why Other Options Are Wrong:

Choosing a single mineral would exclude other common constituents, misrepresenting typical natural clays.

Common Pitfalls:

Assuming one mineral dominates all properties; ignoring cation exchange and water chemistry effects on swelling and strength.

Final Answer:

All the above