Recent work has demonstrated increased levels of redox-active iron biominerals in Alzheimer's disease (AD) tissue. However, the origin, nature, and role of iron in AD pathology remains unclear. Using X-ray absorption, X-ray microspectroscopy, and electron microscopy techniques, we examined interactions between the AD peptide beta-amyloid (A beta) and ferrihydrite, which is the ferric form taken when iron is stored in humans. We report that A beta is capable of reducing ferrihydrite to a pure iron(II) mineral where antiferromagnetically ordered Fe2+ cations occupy two nonequivalent crystal symmetry sites. Examination of these iron(II) phases following air exposure revealed a material consistent with the iron(II)-rich mineral magnetite. These results demonstrate the capability of A beta to induce the redox-active biominerals reported in AD tissue from natural iron precursors. Such interactions between A beta and fenihydrite shed light upon the processes of AD pathogenesis, while providing potential targets for future therapies.