The formation of the iron oxyhydroxide schwertmannite has been monitored by time-resolved transmission electron microscopy (TEM) studies. Schwertmannite aggregates are found to form initially as spherical agglomerates of ferrihydrite crystallites, which then begin to grow characteristic needles on their surfaces. High-resolution images of the needles show that they are initially comprised of aligned goethite nanocrystals, which subsequently coarsen to form crystallographically coherent needles of goethite. Thus, needle formation on schwertmannite aggregates can be considered as the first stage in the phase transformation from schwertmannite to goethite. The results of this study suggest that schwertmannite is not a distinct mineral phase, but may be a mixture of ferrihydrite and poorly crystalline goethite with a distinctive morphology directed by the presence of surface-adsorbed sulfate anions. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.