MINERAL, the COMBINED silica-alumina sheets are held together by HYDROGEN (H) bonding.Halloysite: Halloysite has a similar composition as that of Kaolinite except that it contains ADDITIONAL water molecules between the layers and most commonly has a tubular morphology. Halloysite loses its interlayer water very easily so it is often observed in a partly dehydrated state.Illite: It is the combination of 2 silica and 1 alumina sheets which are held together by an ionic bond, which is stronger than Vander wall forces but weaker than H bond.Smectite: It was formerly known as montmorillonite. The combination of 2 silica and 1 alumina sheets, held together by weak Vander wall forces.∴ The clay mineral, whose structural units are held together by potassium bond is Illite.