New Data and a Revised Structural Model for Ferrihydrite

Richard A. Eggleton1 and Robert W. Fitzpatrick2
1 Geology Department, Australian National University GPO Box 4, Canberra, ACT 2601, Australia
2 CSIRO, Division of Soils, Glen Osmond South Australia 5064, Australia

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,H2O)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 Fe3+, a conclusion supported by heating experiments which show the development of maghemite after heating to 300°C 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°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.

Key Words: Crystal structure • Ferrihydrite • Iron • Magnetic susceptibility • Surface area • Synthesis • Thermal analysis • Transmission electron microscopy • X-ray absorption edge spectra

Clays and Clay Minerals; April 1988 v. 36; no. 2; p. 111-124; DOI: 10.1346/CCMN.1988.0360203
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)