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Hexagonal platy halloysite in an altered tuff bed, Komaki City, Aichi Prefecture, Central Japan

Abstract XRD analysis and electron microscopy show that hexagonal platy halloysite is the main component of an altered tuff (Ueno tuff bed) in the Pliocene Seto group, Aichi Prefecture, Central Japan. In the natural state it shows a single basal peak at 10·1 Å, which collapses to 7·2 Å, by dehydration through a segregate-type interstratification. The (02,11) non-basal band consists of slightly separated peaks which indicates moderate ordering of the crystal structure. The b-dimension is 8·936–8·939 Å. The stability of the interlayer water is intermediate with respect to halloysites of different morphologies. Between 3·5 and 4% Fe2O3 is present in the deferrated sample and the calculated chemical formula can not explain the anomalously high CEC of 21·9 mEq/100 g. Because the curvature radius and b-dimension of halloysite increase with increase in Fe2O3 content, the platy morphology is ascribed to replacement of Al3+ by Fe3+ in the octahedral sheet. Based on the geological and chemical data, the hexagonal platy halloysite is considered to have been formed from volcanic glass after deposition in a freshwater lake, where conditions were oxidizing and weakly acidic.