"Imaging Geochemical Interfaces (With Phase-Sensitive X-ray Scattering and Interfacial Microscopy*)"

Paul Fenter, Argonne National Laboratory

The mineral-fluid interface is the principal site of low-temperature geochemical processes at and near Earth’s surface.  Interfacial processes therefore exert a powerful influence on the natural geochemical and biogeochemical cycles in our environment. Our ability to obtain a fundamental understanding of these processes has been limited, however, by a range of factors, including: the need to penetrate mm’s of fluid for in-situ measurements, the complexity of many rock-forming minerals, the need for element- and interfacial specificity, and the potential heterogeneity of these systems.  Specific phenomena that we have investigated include: hydration layers at mineral-water interfaces, the adsorption of ions to charged mineral-water interfaces, and real-time growth/dissolution reactions.  This work makes full use of recent advances to “image” interfacial structures and processes using phase-sensitive interfacial X-ray scattering techniques and analytical approaches.  These include the use of error-correction algorithms to image interfacial structures from X-ray reflectivity data (i.e., bypassing the “phase problem”), the use of anomalous dispersion as an element-specific and phase-sensitive approach to image elemental sub-structures at interfaces, and the use of interfacial X-ray microscopy to image elementary steps and lateral heterogeneities associated with interfacial reactions.

*This work was supported by the DOE’s Geoscience Research Program, Office of Basic Energy Sciences.