Between the 1:1 or 2:1 layers of a phyllosilicate, a bilayer is an arrangement within the interlayer of two planes of molecules. The planes are only approximate, but the spacing seem to suggest planar features. For a bilayer hydration shell, the H₂O molecules are in an octahedral coordination around a central cation, with three H₂O above and three below the cation, and the spacing appears as two H₂O planes or “layers”. The spacing is generally not affected by the cation because the cation is small and resides well within the hydration shell of H₂O. Thus, the bilayer only refers to the two planes of H₂O molecules.
Cf., alkylammonium organoclay bilayer