Between 2:1 layers of a phyllosilicate, a monolayer involves an arrangement within the interlayer of one single plane of H₂O molecules around interlayer cations (= hydration shell). Although the plane is only approximate, the spacing seems to suggest a planar feature. Because the H₂O molecules are in planar coordination around the cation, the spacing appears as an H₂O plane or “layer”.
Cf., bilayer; bilayer, phyllosilicate