Fe K-edge XAFS spectra of phyllosilicates of varying crystallinity

Abstract

Fe K-edge X-ray absorption fine structure (XAFS) spectroscopy was used to examine local molecular structure and cation distribution in the natural nanosilicate hisingerite and synthetic nanoaluminosilicate allophane. Fitting procedures for XAFS spectra were established on well-characterized clay mineral standards for application to the poorly crystalline nanosilicate materials. Nanospherical models for these materials were tested against clay-like structures. From XAFS interpretations, the following structural information was concluded: (1) Fe in both hisingerite and allophane is present in well-defined octahedral sites, (2) less long-range structure is observed in hisingerite and allophane than in smectites, and (3) Fe in allophane is present in small clusters in the octahedral sheet. The best fit for hisingerite was obtained using a 1:1 model structure rather than a 2:1 model, supporting a description of it as a ferric halloysite-type structure. Allophane could not be fit using paths based on Fe-substituted proto-imogolite nanospherical models but was successfully fit using a montmorillonite structure. Published models of nanospherical particles suggest backscattering path lengths in allophane and hisingerite should be shortened compared to clay minerals; however, no such shortening was observed, suggesting the nanosphere model does not accurately describe the local atomic structure of these Fe-substituted minerals.