The different contacting organic and inorganic components in a soil define a complex, hierarchically structured, and extremely large biogeochemical in-terface to the soil's liquid and gaseous phases (Young and Crawford, 2004). The characterization of biogeochemical interfaces in soils is essential to gain a mecha-nistic understanding of their role as transformers, buffers, accumulators, and filters of energy, water, and dissolved as well as dispersed particles. The physical, chemical, and biological heterogeneity of these interfaces produces a multitude of reactive sites, that control the fate and effect of chemicals in soils (Totsche et al., 2010). An important step toward the structural and functional characterization of the soil biogeochemical interface is the study of the type of particle surfaces present. Due to their large SSA and high reactivity, pedogenic oxides, next to clay minerals, are among the most important mineral phases providing interfaces in soil. Although several types of oxides (Fe, Al, Mn, and others) occur in soil, iron oxides are expected to play an important role in the stabilization of organic matter and formation of soil aggregates and have therefore received specific attention. Iron oxides are often present in soil as the weakly crystalline mineral ferrihydrite that consists of nanoparticles with ill-defined structure and composition (Qafoku, 2010). Its high surface area and reactivity make it an important interface component in soils, even at low concentrations, but due to its low structural arrangement it remains difficult to identify. Ferrihydrite was successfully identified with X-ray diffraction (XRD) in soils only at concentrations >5 to 10% mass (Childs, 1992; Schwertmann et al., 1982). Some information on the crystallinity and the presence