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

R. L. Parfitt, R. J. Furkert and Teruo Henmi
Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt, New Zealand
Ehime University, Matsuyama, Japan

Abstract: Samples containing allophane with molar Al/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 Al/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 Al in tetrahedral sites (AlIV), while pyridine adsorption results suggested that a small number of Bronsted acid sites (AlIVOH) are present in silica-rich allophanes. Lewis acid sites (AlH2O) are present in both silica-rich and alumina-rich allophanes. The results suggest that the framework for the allophane structure is an Al octahedral sheet.

Allophanes with Al/Si ratios close to 1.0 contain condensed silicate units either on the outside surface of the Al octahedral sheet, giving rise to a halloysite-like structure, or bonded on the inside surface of an imogolite-like structure. Allophanes with Al/Si ratios between 1.0 and 2.0 appear to be mixtures of the “proto-imogolite” structure and the allophane (Al/Si = 1.0) structure.

Key Words: Allophane • Deuterium exchange • Halloysite • Imogolite • Infrared spectroscopy

Clays and Clay Minerals; October 1980 v. 28; no. 5; p. 328-334; DOI: 10.1346/CCMN.1980.0280502
© 1980, The Clay Minerals Society
Clay Minerals Society (www.clays.org)