Classification of Silicate Clays
(Based on the arrangement of tetrahedral and octahedral sheets)
Overview; Silicate clay minerals are classified into three major structural types based on the number and arrangement of tetrahedral (Si-based) and octahedral (Al/Mg-based) sheets:
|
Type |
Structure |
Example Minerals |
|
1:1 |
1 Tetrahedral : 1 Octahedral sheet |
Kaolinite, Halloysite |
|
2:1 |
2 Tetrahedral : 1 Octahedral sheet |
Montmorillonite, Illite, Vermiculite |
|
2:1:1 |
2:1 + 1 extra octahedral (brucite layer) |
Chlorite |
(a) 1:1 Type Clay Minerals
- Consist of 1 silica (tetrahedral) sheet and 1 alumina (octahedral) sheet.
- Example: Kaolinite (most common), Halloysite, Dickite, Nacrite
🔍 Key Features:
- Sheets joined by H-bonding → strong, rigid structure
- No swelling on wetting
- Low CEC: 3–10 me/100g soil
- Low plasticity, shrink–swell potential
- Kaolinite: Larger particles (0.1–50 μm), pseudohexagonal
- Halloysite: Contains water between layers → slightly higher plasticity
Specific surface area: ~15 m²/g
Non-expanding clay
(b) 2:1 Type Clay Minerals
- Consist of two tetrahedral sheets sandwiching one octahedral sheet
- These are expanding or non-expanding, depending on interlayer space
(b1) Expanding 2:1 Clays
|
Group |
Members |
Notes |
|
Smectite |
Montmorillonite, Beidellite, Saponite, Nontronite |
High swelling, high CEC (80–150 cmol/kg), large surface area (700–800 m²/g) |
|
Vermiculite |
— |
Limited swelling, very high CEC (100–150 cmol/kg) |
Montmorillonite:
- High swelling potential
- Isomorphic substitution: Mg²⁺ replaces Al³⁺ (octahedral)
- Water and cations enter layers → swelling
- Dominant in black cotton soils
Vermiculite:
- Less expansion than montmorillonite
- Substitution: Al³⁺ replaces Si⁴⁺ (tetrahedral)
- CEC > all other clays, but moderate swelling
(b2) Non-Expanding 2:1 Clays
|
Mineral |
Notes |
|
Illite (Fine Mica) |
K⁺ ions in interlayers → prevent swelling |
|
Biotite/Muscovite |
Found in sand/silt → non-expanding, low CEC (10–40 cmol/kg) |
Illite has 20% Al³⁺ substitution for Si⁴⁺ in tetrahedral sheet → high negative charge
→ K⁺ binds sheets tightly → no expansion
📌 CEC: ~15–40 cmol/kg
📌 Surface area: 100–120 m²/g
(c) 2:1:1 (or 2:2) Type Clay Minerals
- Example: Chlorite
- Structure: 2:1 sheet + brucite layer [Mg(OH)₂]
- Non-expanding mineral
- CEC: ~10–40 cmol/kg (similar to Illite)
Rich in Mg²⁺, Fe²⁺, Al³⁺
Found in highly weathered soils
Limited nutrient holding capacity
Cation Exchange Capacity (CEC) Comparison
|
Clay Mineral |
CEC (cmol(P⁺)/kg or me/100g soil) |
|
Kaolinite |
3–10 |
|
Illite |
10–40 |
|
Chlorite |
10–40 |
|
Montmorillonite |
80–150 |
|
Vermiculite |
100–150 |
|
Organic colloids (humus) |
>200 |
Order of CEC: Kaolinite < Illite ≈ Chlorite < Montmorillonite < Vermiculite < Humus
⚡ Sources of Negative Charges in Clay Minerals
- Isomorphic Substitution (Permanent Charge)
- Replacement of one ion by another of lower charge within the clay structure.
- Examples:
- Al³⁺ replaces Si⁴⁺ in tetrahedral sheet
- Mg²⁺ replaces Al³⁺ in octahedral sheet
- Results in permanent negative charge, not affected by pH
Common in 2:1 clays
- Exposed Crystal Edges (pH-Dependent Charge)
- Hydroxyl groups on broken edges of minerals (like Kaolinite)
- Ionization at higher pH releases H⁺ → surface becomes negatively charged
- Negative charge increases with pH rise
Common in 1:1 clays and organic matter
Comparison Table: Kaolinite vs Illite vs Montmorillonite
|
Property |
Montmorillonite |
Illite |
Kaolinite |
|
Size (µm) |
0.01–1.0 |
0.1–2.0 |
0.2–2.0 |
|
Shape |
Irregular flakes |
Irregular |
Hexagonal |
|
Surface Area (m²/g) |
700–800 |
100–120 |
5–20 |
|
External Surface |
High |
Medium |
Low |
|
Internal Surface |
Very High |
Low |
None |
|
Plasticity |
High |
Medium |
Low |
|
CEC (me/100g) |
80–150 |
10–40 |
3–10 |
Key Points for Competitive Exams
- 1:1 Clays: Kaolinite → low plasticity, low CEC, no swelling
- 2:1 Clays: Smectite (montmorillonite) → high plasticity & CEC
- Illite: Non-expanding due to K⁺ ions
- Vermiculite: Highest CEC of all clays
- Chlorite: 2:2 structure, non-expanding
- Isomorphic substitution causes permanent charge
- Edge ionization causes pH-dependent charge
- Organic matter has variable charge, very high CEC (>200 cmol/kg)
