Non-Expanding 2:1 Silicate Clays
A. Mica Group (Illite, Glauconite, Muscovite, Biotite)
a) Charge Development:
- Al³⁺ substitutes for about 20% of Si⁴⁺ in tetrahedral sheets, creating a strong negative charge.
- Potassium (K⁺) fits tightly between layers, neutralizing the charge.
b) Bonding:
- Strong K⁺ bonding prevents expansion.
c) Properties:
- Non-expanding, similar to kaolinite in water adsorption and plasticity.
- Less plastic and sticky than smectite.
B. Chlorite Group (2:1:1 Structure)
a) Charge Development:
- Iron (Fe) or Magnesium (Mg) often replace Aluminum (Al) in octahedral sites.
b) Bonding:
- An extra Mg-dominated trioctahedral hydroxide sheet is sandwiched between adjacent 2:1 layers → 2:1:1 structure.
- Hydrogen bonding strongly binds the layers together, preventing expansion.
c) Properties:
- Non-expanding, similar to fine-grained micas.
- Low plasticity and stickiness.
Summary of Silicate Clays
|
Clay Type |
Structure |
Isomorphous Substitution? |
Expansion? |
Plasticity & Stickiness |
Surface Area (m²/g) |
CEC (cmol/kg) |
|
Kaolinite (1:1) |
1 tetrahedral + 1 octahedral |
Low |
|
Low |
10–20 |
3–15 |
|
Montmorillonite (Smectite, 2:1) |
2 tetrahedral + 1 octahedral |
High (Mg²⁺ in Al³⁺ sites) |
|
High |
600–800 |
80–120 |
|
Vermiculite (2:1) |
2 tetrahedral + 1 octahedral |
Very High (Al³⁺ in Si⁴⁺ sites) |
|
Moderate |
700 |
100–150 |
|
Illite (Fine-Grained Mica, 2:1) |
2 tetrahedral + 1 octahedral |
High (Al³⁺ in Si⁴⁺ sites) |
|
Low |
100–200 |
20–40 |
|
Chlorite (2:1:1) |
2 tetrahedral + 1 octahedral + hydroxide layer |
Moderate |
|
Low |
100–150 |
10–40 |
No
Yes (High)