Mineralogical Organization of Crystalline Silicate Clays

• Crystalline silicate clays are classified based on the number and arrangement of tetrahedral (Si) and octahedral (Al, Mg, Fe) sheets in their crystal structure.
• The two main groups are:

1. 1:1 Silicate Clays (Kaolinite Group)

a) Structure: Each layer consists of one tetrahedral sheet and one octahedral sheet.

• Example Minerals: Kaolinite, Halloysite, Nacrite, Dickite.

b) Bonding:

• The tetrahedral and octahedral sheets are held together tightly by shared oxygen atoms.
• Hydrogen bonding holds these layers together, making them non-expanding.

c) Properties:

• No swelling when wetted (fixed structure).
• Low cation exchange capacity (CEC) due to little isomorphous substitution.
• Effective surface area is low (only external surfaces are active).
• Large particle size (0.10 to 5 µm, mostly 0.2 to 2 µm).
• Poor plasticity, cohesion, shrinkage, and swelling.

1. 2:1 Silicate Clays

a) Structure:

• Each layer consists of one octahedral sheet sandwiched between two tetrahedral sheets.

b) Subgroups:

• Expanding clays – Smectite, Vermiculite.
• Non-expanding clays – Fine-grained mica (Illite), Chlorite (2:1:1 type).