New Zealand allophanes: a structural study

Abstract

Three natural allophane samples from New Zealand, the Egmont, Waxy-Pan and iron-rich Ruapehu, were investigated by infrared spectroscopy, X-ray diffraction and Mössbauer spectroscopy. The specific surface area and the pore sizes of these samples increase as the pretreatment temperature is raised, up to a temperature where a decrease of the surface area and a narrowing of the pores is observed. This can be explained by two competing phenomena: the release of water from the pores and the collapse of other pores. In comparison with the Egmont and Waxy-Pan allophanes, the iron-rich Ruapehu sample contains considerably less pores of sufficient size to accommodate water. Its exceptional chemical composition and the infrared spectra, X-ray diffraction patterns and Mössbauer spectra suggest the presence of a separate, amorphous iron (III) oxide phase in the pores of this allophane.