X-ray absorption spectroscopy at the iron K-edge indicates that the iron cores of human andyeast frataxin polymers assembled in vitro are identical to each other and are similar but not identical toferritin cores. Both frataxin polymers contain ferrihydrite, a biomineral composed of ferric oxide/hydroxideoctahedra. The ferrihydrite in frataxin is less ordered than iron cores of horse spleen ferritin, havingfewer face-sharing Fe−Fe interactions but similar double corner-sharing interactions. The extended X-rayabsorption fine structure (EXAFS) analysis agrees with previous electron microscopy data showing thatfrataxin cores are composed of very small ferrihydrite crystallites.