Double-layer mineral (clay mineral) whose layer package comprising a combination of an octahedron and a tetrahedron layer. These layers are permanently bonded through bridging oxygen atoms.
Kaolinite is the the most frequently used double-layer mineral in engineering with a theoretical composition of 39.49 % SiO2, 46.55 % Al2O3 and 13.96 % H2O. The average particle diameter is 0.3 to 4.0 µm and particle thickness is 0.05 to 2.0 µm. Due to the low bonding capacity (low swelling capacity, see Swelling) of the double-layer minerals, in the foundry industry they are mainly used as refractory clays.