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 Fe4(O,OH,H20)12 and Fe4.6(O,OH,H20)lZ, respectively. The composition depends on crystal size, which averages 20/ ~ for 2-line and 35/k 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 ~ in the presence of N2, 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 ~ Transmission electron microscopy shows that 2-line ferrihydrite has no Z-axis regularity but does show hexagonal 2.54-/ ~ lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-/ ~ c-parameter only detectable in dark field. In bright field, 2.54-/ ~ 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 suscep-tibility are consistent with the recorded properties of ferrihydrite