Myneni, SCB, MB Hay, and B Mishra. 2013. “Applications of scanning transmission X-ray microscopy in studying clays and their chemical interactions.” Special Volume, Advanced Applications of Synchrotron Radiation in Clay Science. Vol. 19. Clay Mineral Society, 19.
Scanning transmission X-ray microscopy (STXM), or X-ray microscopy in general, is an ideal molecular probe for investigating the chemistry of clay minerals, clay-sized mineral particles, and mineral-organic aggregates in heterogeneous aqueous matrices. This technique allows mapping of different elements, elements in different valence states and bonding environments, and mineral phases at a spatial resolution of a few nanometers. In addition, high-resolution XANES spectra can be obtained at the same spatial resolution, and the characteristic XANES features of individual phases can also be used in mapping. While STXM studies can be conducted using X-ray transmission, X-ray fluorescence, or electron-yield, the type of detection used can limit the sample thickness, bulk vs. surface sensitivity, and sensitivity to detecting dilute samples. Using STXM and associated Al K-edge XANES spectra, researchers have examined successfully mixtures of clay suspensions. These studies indicate that STXM can be used to identify specific clay-mineral assemblages in natural samples. STXM has also been used successfully in studying Fe- and Mn-biominerals, their characteristics, and rates of production under different biogeochemical conditions. More recently this technique has been applied successfully to sstudy aquatic colloids from both marine and terrestrial systems. A detailed discussion of the method and its applications are summarized here.