Amorphous iron sulfide minerals containing either 500 or 3000 iron atoms in each cluster have been synthesized in situ within the nanodimensional cavity of horse spleen ferritin. Iron-57 Mossbauer spectroscopy indicated that most of the iron atoms in the 3000-iron atom cores are trivalent, whereas in the 500-iron atom clusters, approximately 50 percent of the iron atoms are Fe(III), with the remaining atoms having an effective oxidation state of about +2.5. Iron K-edge extended x-ray absorption fine structure data for the 500-iron atom nanocomposite are consistent with a disordered array of edge-shared FeS4 tetrahedra, connected by Fe(S)(2)Fe bridges with bond lengths similar to those of the cubane-type motif of iron-sulfur clusters. The approach used here for the controlled synthesis of bioinorganic nanocomposites could be useful for the nanoscale engineering of dispersed materials with biocompatible and bioactive properties.