New Data and a Revised Structural Model for Ferrihydrite

Abstract Synthetic 2-line and 6-line ferrihydrite samples prepared from ferric nitrate solutions have the bulk compositions Fe 4 (O,OH,H 2 O) 12 and Fe 4,6 (O,OH,H 2 O) 12 , respectively. The composition depends on crystal size, which averages 20 Å for 2-line and 35 Å for 6-line ferrihydrite. X-ray absorption edge spectra indicate the presence of tetrahedral Fe 3+ , a conclusion supported by heating experiments which show the development of maghemite after heating to 300°C in the presence of N 2 , followed by the formation of hematite at higher temperatures. These two reactions are recorded on differential thermal analysis traces by exotherms at 350° and 450°C. Transmission electron microscopy shows that 2-line ferrihydrite has no Z -axis regularity but does show hexagonal 2.54-Å lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-Å c -parameter only detectable in dark field. In bright field, 2.54-Å lattice fringes indicate greater atomic regularity than in 2-line ferrihydrite. Analysis of the X-ray powder diffraction pattern of 6-line ferrihydrite suggests a structure based on double-hexagonal close-packed oxygens, containing 36% Fe in tetrahedral sites. Selective chemical dissolution, surface area measurements, and magnetic susceptibility are consistent with the recorded properties of ferrihydrite.