Allophane and its initial crystallization products as concretions in the south Limburg Chalk

Abstract

Silicon-alumina concretions with about equal amounts of both components and ranging in crystallinity between allophane and initial crystallization toward halloysite or gibbsite were found in and on Upper Cretaceous limestones. These concretions were probably the result of accumulation of Late Tertiary and Early Pleistocene weathering products, accumulated by stagnation of percolating waters. Crystallization might have been inhibited by the electrolyte concentrations in the limestone pore waters. pure allophane material was much like the allophane originally described in 1816.