Total scattering experiments using synchrotron X-rays and spallation neutrons are providing new insights into the structural characteristics of synthetic and natural ferrihydrite, a poorly crystalline nanomineral that has industrial applications and is pervasive in the environment. Modern synchrotron and neutron facilities with dedicated total scattering beamlines now enable the collection of scattering data for ferrihydrite to large scattering angles and with superior signal/noise ratio. The pair distribution function, or PDF, derived from these scattering data is a real-space depiction of atomic structure with exceptional resolution. A number of recent high-energy X-ray total scattering studies have focused on as-precipitated, pure synthetic ferrihydrite of different particle sizes, as well as ferrihydrite co-precipitated with environmentally relevant impurities, such as aluminum, arsenic, chromium, and silicon. The findings from these investigations have contributed to our understanding of the extended framework of iron and oxygen in ferrihydrite, and the specific associations of these various impurities over a range of impurity content. In situ and ex situ synchrotron total scattering methods are being applied in studies of the structural, physical, chemical, and magnetic changes taking place during ferrihydrite transformations. Neutron total scattering is shedding new light on the association of water with the surfaces and in the bulk structure of ferrihydrite. In several of these studies, real-space fitting of PDFs derived from both neutrons and X-rays are being used to test both new and previously proposed structural models for synthetic ferrihydrite. Total scattering and PDF analysis is also revealing new insights into the structural characteristics of different samples of natural ferrihydrite that were formed in a diverse range of geochemical settings. The results of these studies, combined with new PDF data for a suite of natural samples presented in this review, show that the structure of natural ferrihydrite can vary almost continuously between high- and low-crystallinity end members. The natural samples with the greatest degree of structural order are virtually identical to pure laboratory ‘six-line’ ferrihydrite. Interestingly, total scattering shows that the majority of the natural ferrihydrite samples in the suite presented here are less crystalline than ‘two-line’ ferrihydrite, the variety of synthetic ferrihydrite that is often assumed in laboratory studies to be equivalent to natural ferrihydrite. The implications of this decreased crystallinity are not yet fully understood in terms of evaluating and predicting the chemical behavior of natural ferrihydrite in the environment.
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Advanced Applications of Synchrotron Radiation in Clay Science
This volume presents the majority of topics in synchrotron science that are of use to the clay science community. The chapters presented in this volume serve not only as significant statements on the state of these applications, but also as useful primers to potential new users of synchrotron facilities.