Speciation of Iron In Nano-Crystalline Aluminosilicates.

Nano-crystalline aluminosilicates such as allophane and imogolite are formed by weathering of glassy volcanic materials, and may be the major secondary mineral phases present in many environments.  In natural weathering environments, incorporation of Fe in poorly crystalline aluminosilicates is common.  However, Fe speciation and its effects on the properties of these minerals are not well understood because they are difficult to characterize using traditional techniques such as X-ray diffraction.  In this project Fe-allophanes and imogolites were synthesized with one to ten percent of the structural Al substituted with Fe.  Synchrotron-based X-ray absorption spectroscopic (XAS) methods were used to examine the immediate environment surrounding the Fe atoms.  Data showed that molecular coordination of Fe is primarily octahedral.  The occurrence of Fe next neighbor scattering suggests that within the mineral structure Fe segregates into Fe-rich domains because a random substitution would result in no Fe-Fe backscattering with the low Fe substitution.  Varying Si:Al ratio and synthesis conditions resulted in minerals with different composition and morphology.  Complete XAS data analysis will provide further details on the factors that affect the formation of Fe-substituted aluminosilicates and will provide insights into the effect of Fe clustering on the evolution of these nanoparticles into more crystalline clay minerals.