A new interpretation of the structure of disordered α-cristobalite

Abstract

Disordered α-cristobalite which occurs extensively in bentonites, opals and particularly deep-sea cherts, has been previously interpreted in terms of a unidimensionally disordered structure in which cristobalite is interstratified with two-layer tridymite-like sheets. An alternative interpretation is that the structure is essentially tridymitic but that the sheets are stacked with random transverse displacement normal to the c axis, an arrangement similar to the translational, turbostratic stacking postulated for smectites. This interpretation was arrived at after a comparitive study of a silica phase in an Italian bentonite and a deep-sea chert, material which yielded an X-ray powder pattern almost identical with that of disordered α-cristobalite, but electron diffraction patterns and infrared spectra more consistent with tridymite. It is suggested that this type of silica, which has been described almost universally as cristobalite, is more appropriately referred to as disordered α-tridymite.