- © The Mineralogical Society Of America
The many tools with which one can probe the atomic-scale structures of surfaces include electron-, ion-, and X-ray based techniques (e.g., low energy electron diffraction, Rutherford ion backscattering, X-ray diffraction, photoelectron diffraction), as well as scanning probe microscopies (e.g., scanning tunneling microscopy and atomic force microscopy) (Somorjai 1981; Woodruff and Delchar 1986; Zangwill 1988; van Hove 1999). These tools have been extremely valuable for revealing surfaces structures and processes at ultra-high vacuum conditions. However, most of these surface-sensitive techniques suffer from the substantial shortcoming, from the perspective of mineral-fluid interface studies, that they cannot be applied to surfaces in contact with water. It is preferable to measure mineral-fluid interface structures in situ for direct insight into the geochemical phenomena of interest because there is no reason to assume that a mineral surface can be removed from an aqueous solution without substantially modifying the surface structure or properties.
X-rays are an ideal probe of mineral-water interfaces. X-rays readily penetrate macroscopic amounts of water and can therefore investigate the mineral-water interface directly, in situ. X-rays can measure atomic scale structures, such as the separation of individual atoms or molecules, because X-ray wavelengths are comparable to atomic sizes (Warren 1990). In fact, the interaction of X-rays with matter is known at a very fundamental level, and X-ray based techniques can provide truly quantitative data concerning the arrangements of atoms through a variety of approaches, such as crystallography and X-ray absorption spectroscopy (Als-Nielsen and McMorrow 2001). These characteristics can also be used to study the structure of the mineral-fluid interface (e.g., atomic locations, bond lengths) with sub-Ångstrom precision.
Of the many X-ray based techniques available, a very powerful approach for probing interfacial structures is based on the measurement of X-ray reflectivity. The X-ray reflectivity is simply defined as the ratio …