Transmission electron microscopy of synthetic 2-and 6-line ferrihydrite

High-resolution transmission electron microscopy (HRTEM), selected-area ele ctron diffraction (SAED), annular dark-field scanning transmission electron microscope (STEM) images, and electron nanodiffraction were used to examin e structures of synthetic 2- and 6-line ferrihydrite specimens. HRTEM image s of 2-line ferrihydrite (2LFh) show scattered small (similar to 1-3 nm) ar eas with lattice fringes surrounded by areas free of fringes. All SAED patt erns show two bright rings corresponding to d-values of similar to 0.15 and 0.25 nm; each ring has a conspicuous shoulder on each side. Faint rings co rresponding to d-values of 0.08, 0.095, 0.100, 0.106-0.114 (very broad ring ), and 0.122 nm are visible in strongly exposed SAED patterns. Nanodiffract ion patterns show conspicuous streaks and a lack of superlattice formation. HRTEM images of 6-line ferrihydrite (6LFh) display larger crystallites (typ ically similar to 5-6 nm) with lattice fringes visible in many thin areas. SAED patterns show rings corresponding to d-values of 0.148, 0.156, 0.176, 0.202, 0.227, and 0.25-0.26 nm and a shoulder extending between d-values of similar to 0.25 and 0.32 nm. Faint rings corresponding to d-values of 0.08 6, 0.093, 0.107, 0.112, 0.119, 0.125, and 0.135 nm are visible in strongly exposed SAED patterns. Small quantities of hematite, magnetite or maghemite , and an acicular material tentatively identified as goethite were observed in the 6-line ferrihydrite, but these quantities do not contribute signifi cantly to the overall diffracted intensity from the sample.