Nanoscale characterization of so-called iron oxide coatings on aquifer sands is vital for understanding the fate and transport of naturally occurring and anthropogenic chemical species. These coatings, which typically have a strong reddish color, are commonly assumed to consist primarily of iron oxides and oxyhydroxides. This work shows that the yellowish red to strong brown coatings on sediments from an Atlantic coastal plain aquifer in Virginia are predominantly a mixture of Si- and Al-rich nanophases of variable crystallinity with interspersed smectite and agglomerates of goethite nanoparticles. Clusters of bacterial cells, diatom fossils, sponge spicules, and other trace minerals are also observed.

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