Identification and Structure of Two Types of Allophane from Volcanic Ash Soils and Tephra

Abstract

Samples containing allophane with molar AI/Si ratios from 1.0 to 2.0 have been examined by infrared spectroscopy, X-ray fluorescence, and phosphate adsorption methods. The infrared spectra of allophane with AI/Si ratios close to 2.0 showed that the wall of the allophane spherules is made up of imogolite structural units similar to "proto-imogolite." X-ray fluorescence gave no clear evidence of A1 in tetrahedral sites (ApV), while pyridine adsorption results suggested that a small number of Bronsted acid sites (AIIVOH) are present in silica-rich allophanes. Lewis acid sites (AIH20) are present in both silica-rich and alumina-rich allophanes. The results suggest that the framework for the allophane structure is an A1 octahedral sheet. Allophanes with A1/Si ratios close to 1.0 contain condensed silicate units either on the outside surface of the A1 octahedral sheet, giving rise to a halloysite-like structure, or bonded on the inside surface of an imogolite-like structure. Allophanes with A1/Si ratios between 1.0 and 2.0 appear to be mixtures of the "proto-imogolite" structure and the allophane (A1/Si = 1.0) structure.