Which type of clay mineral lllite is?

IIIite is a group of clay minerals in a 2 : 1 ratio and is non-expanding. It is a secondary mineral precipitate, like a phyllosilicate or layered alumino-silicate. Its structure is a 2 : 1 sandwich of silica tetrahedron (T) - alumina octahedron (O) - silica tetrahedron (T) layers.