Bentonites are hydrated aluminosilicate minerals, comprised chiefly of montmorillonite. Structurally they consist of alumina sheets sandwiched between tetrahedral SiO4 units. Water is usually present between the triple layers.

Typical Chemical Analysis.
SiO2 57.1%
Na2O 3.27%
Al2O3 17.79%
K2O 0.9%
Fe2O3 4.64%
TiO2 0.77%
CaO 3.98%
Mn2O3 0.06%
MgO 3.68%
LoI 7.85%

Typical Mineralogy.
Montmorillonite 92%
Calcite 4%
Feldspars 2%
Quartz 1%
Dolomite 1%

Other Typical Properties.
Bulk Density 800 – 900 Kg/m3
Swelling Volume 29 ml/2g
Moisture Maximum 14% by weight
Cation Exchange Capacity 78 meq / 100g

Sieve Analysis.
Maximum 5% retained on a 150:m sieve