Abstract: Feroxyhite (δ′-FeOOH) in association with goethite and lepidocrocite was found as a dominant mineral in some rusty precipitates from Finland. These precipitates formed in the interstices of sand grains from rapidly flowing, Fe(II)-containing water which was very quickly oxidized as it flowed through the sediment. The mineral is distinguished from other FeOOH forms and from ferrihydrite mainly by its X-ray powder diffractogram. Further characteristics are an acicular morphology (possibly thin, rolled plates), an internal magnetic field at 4°K of ∼510 kOe, Fe-OH stretching bands at ∼2900 cm−1 and Fe-OH bending bands at 1110, 920, 790, and 670 cm−1, and an oxalate solubility between ferrihydrite and goethite or lepidocrocite. Feroxyhites with very similar properties were synthesized by oxidation of an Fe(II) solution with H2O2 at a pH between 5 and 8.