Expanding 2:1 Silicate Clays
A. Smectite Group (Montmorillonite)
a) Charge Development:
- Isomorphous substitution of Mg²⁺ in Al³⁺ sites (octahedral layer).
- Some Al³⁺ may replace Si⁴⁺ in the tetrahedral sheet.
- Leads to high negative charge → very high CEC (cation exchange capacity).
b) Bonding:
- Weak bonding between layers allows water and cations to enter → high expansion upon wetting.
- Very high total surface area (internal > external).
c) Properties:
- High plasticity and stickiness.
- Very difficult to cultivate or excavate.
- Wide cracks when drying.
- Total surface area: 600–800 m²/g (vs. Kaolinite’s 10–20 m²/g).
- Common Minerals: Montmorillonite, Beidellite, Nontronite, Saponite.
B. Vermiculite Group
a) Charge Development:
- Extensive isomorphous substitution of Al³⁺ for Si⁴⁺ in the tetrahedral sheets.
- Higher CEC than smectites.
b) Bonding:
- Mg²⁺ bridges adjacent layers, limiting expansion.
- More expanding than kaolinite, but less than smectite.
c) Properties:
- Moderate swelling and shrinkage.
- Crystals are larger than smectite but smaller than kaolinite.