Soil Colloids: Silicate Clay Minerals (1:1, 2:1 Expanding, Non-expanding), Sesquioxides and Amorphous Clays - Physical Science | Agrigyan

Basal spacing ($d_{001}$) refers to the distance from a plane in one layer to the corresponding plane in the adjacent layer:

• Montmorillonite (expanding 2:1 mineral): basal spacing ranges from $14\text{ \AA}$ (air dry) to $18 - 21\text{ \AA}$ (glycol solvated or hydrated).
• Vermiculite (limited expanding 2:1 mineral): basal spacing is approximately $14.0 - 15.0\text{ \AA}$ (due to two layers of interlayer water molecules and $\text{Mg}^{2+}$).
• Illite (non-expanding 2:1 mineral): basal spacing is fixed at $10.0\text{ \AA}$ due to unhydrated potassium ions ($\text{K}^+$) locked in the hexagonal cavities.
• Kaolinite (1:1 mineral): basal spacing is fixed at $7.2\text{ \AA}$ held tightly by hydrogen bonds.
  Decreasing order of basal spacing: Montmorillonite ($18\text{ \AA}$) > Vermiculite ($14-15\text{ \AA}$) > Illite ($10\text{ \AA}$) > Kaolinite ($7.2\text{ \AA}$), which corresponds to (B), (D), (C), (A).

Statement (I) is correct: The fundamental crystal unit of a 2:1 layer silicate (e.g., smectite, illite, vermiculite) consists of a central aluminum/magnesium octahedral sheet sandwiched symmetrically between two silicon tetrahedral sheets (Tetrahedral - Octahedral - Tetrahedral, or T-O-T sandwich), where the apical oxygens of both tetrahedral sheets point inward toward the octahedral sheet.
Statement (II) is incorrect: The two tetrahedral sheets are situated on opposite sides of the central octahedral sheet (forming a sandwich), NEVER bound to the same side.

Imogolite is an aluminosilicate mineral ($ ext{Al}_2 ext{SiO}_3( ext{OH})_4$) characteristic of volcanic ash soils:

• It is predominantly formed and found in Andisols (volcanic ash soils) along with allophane.
• Imogolite and allophane exhibit non-plastic or very low plasticity behavior, not high plasticity.
• Potassium fixation is characteristic of 2:1 expanding clays with wedge zones (e.g., illite and vermiculite), not imogolite.
• It consists of hollow tubular structures with short-range order, giving it a paracrystalline or poorly crystalline nature.

The clay fraction ($\lt 0.002\text{ mm}$) constitutes the colloidal and active part of the mineral soil:
(A) Soil clay exerts a dominant control over soil physical (swelling, shrinkage, plasticity, water retention) and chemical behavior (cation exchange capacity, nutrient buffering).
(R) This disproportionate influence arises directly from its colloidal size, high specific surface area ($10$ to $800\text{ m}^2/\text{g}$), and presence of surface electrical charges making it highly reactive.