The surface area and roughness of natural iron oxide precipitations were quantified by 3D optical microscopy in order to get information about fluid-rock interface topography in high-permeability zones. Converged surface roughness data of microscale to submicroscale topography show the predominance of macroporous half-pores (> 500 nm) and the occurrence of smaller half-pores (<500 nm) that dominate the BET surface area of iron oxides. A relationship was found between the occurrence of macroporous surface structures (micrometer range) and the uranium content of iron oxide encrustations. Iron-normalized uranium concentrations of an X-ray amorphous iron oxide encrustation correlate linearly with maximum topography heights of 1 to 2 mu m on hand specimen subsamples. Our study shows the potential importance of micrometer- to submicrometer-size surface features, whose environmental impact is often ignored.