Types and structure of clay minerals

Clay minerals are hydrous aluminium silicates formed by chemical weathering. They are built from two basic units, the silica tetrahedron and the alumina (gibbsite) octahedron, stacked in sheets.

Building units

• Silica sheet: tetrahedra with Si at the centre and four O at corners, joined at the base corners.
• Gibbsite sheet: octahedra with Al at the centre and six OH (or O) at corners.

1. Kaolinite (1:1 mineral)

One silica sheet bonded to one gibbsite sheet: a layer about 7.2 Å thick.

   Silica   [T T T T T]
   Gibbsite [O O O O O]
   ------ hydrogen bond ------
   Silica   [T T T T T]
   Gibbsite [O O O O O]

• Layers are held by strong hydrogen bonds and some van der Waals force, so they do not separate.
• Little isomorphous substitution; low swelling and shrinkage, low plasticity, low activity ($A \approx 0.4$), specific surface about 15 m²/g.

2. Illite (2:1 mineral)

A gibbsite sheet is sandwiched between two silica sheets (~10 Å thick); potassium ions between the layers hold them together.

   Silica   [T T T T T]
   Gibbsite [O O O O O]
   Silica   [T T T T T]
   ---- K+ ions (bond) ----
   Silica   [T T T T T]

• Some Si is replaced by Al (isomorphous substitution). Moderate swelling, activity about 0.9, specific surface about 80 m²/g.

3. Montmorillonite (smectite, 2:1 mineral)

Same layered structure as illite, but layers are held by weak van der Waals forces and water or exchangeable cations can enter between the layers.

   Silica   [T T T T T]
   Gibbsite [O O O O O]
   Silica   [T T T T T]
   ~~~ water + cations ~~~ (weak)
   Silica   [T T T T T]

• Large isomorphous substitution of Mg for Al; very high swelling and shrinkage, activity 1 - 7, specific surface up to 800 m²/g. Bentonite is mainly montmorillonite.