Formation Mechanism of Al13 Keggin Cluster in Hydrated Layered Polysilicates
Al13 ε-Keggin cluster AlO4Al12(OH)24(H2O)127+ is a predominant intermediate during 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 cage-like Al13 ɛ-keggin cluster is formed even though it could be readily synthesized by forced hydrolysis of Al3+. We found that Al13 ɛ-Keggin cluster was spontaneously formed not in monocrystalline octosilicate but in polycrystalline magadiite on the cation-exchange reaction with unhydrolyzed Al3+. Furthermore, Al13 ɛ-Keggin cluster was hardly detected in disaggregated magadiite crystals of which 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 simultaneous co-assembly of four planar trimers and one octahedral monomer. In addition, the spontaneous formation of Al13 in the heterogeneous systems could be a vital clue to its evaluation in soils and sediments.