sentences:  The Dawson structure, X2M18O62"n"−, is made up of two Keggin lacunary fragments with 3 missing octahedra.The cluster cation (Al13O4(OH)24(H2O)12)7+ has the Keggin structure with a tetrahedral Al atom in the centre of the cluster coordinated to 4 oxygen atoms.The formula can be expressed as (AlO4Al12(OH)24(H2O)12)7+.