Ochre sediments from acid mine drainage in Cornwall have been investigated using X-ray diffraction, thermal methods, infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and chemical methods in order to determine their mineralogical and chemical composition. Fresh sediments consist of ferrihydrite and goethite. Large fractions of these minerals are dissolved by the ammonium oxalate treatment reflecting their poorly crystalline structure. Fresh sediments contain large amounts of surface-adsorbed SO 4 (up to 9.3%) which is readily desorbed by the PO 4 treatment. Goethite is the only mineral present in relatively older sediments and the mineral is well crystallized with rod-shaped morphology. Environmental conditions, such as pH and SO 4 content, are not favourable for the presence of schwertmannite at the site. Iron minerals appear to be precipitating around filamentous algae and the shape of algae is preserved in the Fe oxide matrix. The ubiquitous presence of algae in close association with Fe minerals indicate their possible role in the crystallization of Fe oxides.