Evidence and importance of intermediate nanostructures in the journey from molecular precursors to allophane and imogolite nanocrystals

•Cryo-TEM pictures reveal non-tubular nanostructures even at the end of synthesis.•Proto-imogolites are formed by reorganization of aluminum hydroxide precipitates.•The average size of protoimogolites depends on the concentration of precursors.•The size of proto-imogolites can span above and below the sphere/tube transition.