Clay Minerals (Phyllosilicates)

Groups of phyllosilicates: Clay minerals, Serpentine minerals, Pyrophyllites, Talc, Micas, Chlorite group minerals. Formation of clay minerals:

The type of clay mineral that forms from weathering will depend on:

Fluid rock ratio||pH of fluid||Priginal composition of weathered rock||Reaction rates

Kaolinites (1:1) – Non expanding

Smectites (2:1) – Expanding Illites (2:1) – Non-Expanding Chlorites (2:1:1) – Non-Expanding

1 silica tetrahedron : 1 alumina octahedron

No interlayer activity

no shrink-swell

little isomorphous substitution (replacement of cation of same size while crystal is growing)

H-bonding holds sheets tightly together

most common clay mineral

Examples are Kaolinite, Dickite, Nacrite, and Halloysite

2 layers of Si sheets (tetrahedral) on both sides of Al sheet (octaahedral).

‘Di’ (In case of Al3+ i.e 2 Al ions in

octahedral layer) or ‘Tri’ (incase of Mg2+ i.e 3 Mg ions in octahedral layer) octahedral.

Montmorillonite, nontronite, and beidellite (all dioctahedral).

Saponite and palygorskite (trioctahedral)

Water can also move in and out therfore large shrink/expansion.

Large internal surface.

Ca and Mg can move into the octahedral layer.

Net negative charge but neutralized due to movable (attraction) cations e.g Ca2+, Mg2+.

They absorb Oil.

2 silica sheets : 1 alumina sheet

Mostly dioctahedral minerals with interlayer K.

Nonexpanding

Common in shales.

High net negative charge balanced by K+.

Al3+ and K+ substitute for Si4+ (tetrahedral sheet).

Some substitution of Mg for Al in octahedral layer leads to permanent negative (-) charge.

weathering at edges = release of K+.

Can form from smectites by the addition of K+.

Interlayer is Mg6(OH)12 (brucite).

Brucite allows for strong H-bonding between layers.

Possible substitution in brucite layer can by Al3+, Fe3+, Fe2+ for Mg2+.

Nonexpanding.

Iron rich.

Brucite

Jahanzeb Ahsan