Y-index value for illitic type clay is

An illite is a group of closely related non-expanding clay minerals. It is a secondary mineral precipitate, and an example of phyllosilicate, or layered alumino-silicate. Its structure is a 2 : 1 sandwich of silica tetrahedron-alumina octahedron-silica tetrahedron.
The y-index value for illitic clay is between 5 - 25.