Proto-imogolite allophane in podzol concretions in Australia: possible relationship to aluminous ferrallitic (lateritic) cementation

Proto-imogolite allophane was found in amounts ranging from 1% to 5% in concretions and ortstein from several sandy podzols in Australia. Micromorphological observations on concretions in which allophane was the dominant clay component indicated that it had been deposited in layers during successive drying episodes to form a yellow, glassy, weakly anisotropic, fluorescent cement around sand grains. Iron oxides and organic matter were minor components of the cement. It is proposed that a proto-imogolite sol is the mobile phase that deposits Al and Si in these concretions, and that the proto-imogolite allophane could be a precursor of kaolinite in older concretions, and of gibbsite in leaching environments. Proto-imogolite sols and allophane could play a similar role in the pedogenesis of kaolinite and gibbsite in concretions in freely drained ferrallitic soils.