Formation mechanism of an Al13 Keggin cluster in hydrated layered polysilicates

Abstract

An Al₁₃ ε-Keggin cluster, AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺, is a predominant intermediate during the hydrolysis and polymerization of aluminum as well as a highly toxic substance to plants and fishes. However, no one could clearly explain why and how a cage-like Al₁₃ ε-Keggin cluster is formed even though it could be readily synthesized by the forced hydrolysis of Al³⁺. We found that the Al₁₃ ε-Keggin cluster was spontaneously formed not in monocrystalline octosilicate but in polycrystalline magadiite by the cation-exchange reaction with unhydrolyzed Al³⁺. Furthermore, the Al₁₃ ε-Keggin cluster was hardly detected in disaggregated magadiite crystals whose morphology was changed into monocrystalline crystals like octosilicate. Our findings prove that Al₁₃ formation is necessary to relieve localized inhomogeneity and rationalize that Al₁₃ is formed by the simultaneous co-assembly of four planar trimers and one octahedral monomer. In addition, the spontaneous formation of Al₁₃ in heterogeneous systems could be a vital clue to its evaluation in soils and sediments.