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)
