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The detailed mineralogical study revealed that the initial products of Fe oxidation aggregate in colloids or stick to cell polyers or cell stalks. The product are typically 2-3 nm diameter ferrihydrite. These nanocrystals orient with resepect to each other to form larger single crystals and defective single crystals. Thus, the crystals grow by aggregation, primarily not atom by atom. The microstructures may play a role in nucleating the subsequent transformation to goethite.
Low-magnification TEM image showing typical aggregates of fine particles. The true particle size is difficult to determine from such low-magnification images, but it is ~ 3nm.
Intermediate magnification TEM image of ferrihydrite showing aggregate structure. Inset is a higher magnification image showing lattice fringes (these are 0.25 nm fringes - particles are ~ 2.5 nm diameter). These data verify that the material consists of aggregates of small particles, some of which are oriented with respect to each other (as indicated by parallelism of lattice fringes).
Low and inset intermediate magnification TEM image of ferrihydrite showing its construction from small, orietned particles. Particle size is given by lattice fringes, which are 0.25 nm appart.
Intermediate magnification TEM image of ferrihydrite. Individual particles (areas containing lattice fringes) are separated by space. Low contrast regions indicate absence of material (the support film produces minor contrast).
High-resolution TEM image of an chain-like arrangement of ferrihydrite nanoparticles. Note the hexagonal pattern of 0.3 nm fringes in teh top right hand side and center and lower right hand sides indicate view down [001]. Most particles in this chain share a common orientation. The inset Fourier transform (FT) verifies the common orientation of most particles in this area.
High-resolution TEM image of an array or highly oriented nanoparticles. The inset has been filtered to improve visibility. The fringe spacing is 0.25 nm (rounded 0.3 nm).
High-resolution TEM image of an array of somewhat oriented nanoparticles.
High-resolution TEM image of an array of oriented nanoparticles.
High-resolution TEM image of a largely "annealed" area of nanoparticles. The result is a defective single crystal. Low contrast regions result from space or gaps (in 3D) between particles.
High-resolution TEM image of a largely "annealed" area. The previous existence of individual particles is evident in the external margin shape and as the result of contrast variations. Lattice fringe spacing is ~ 0.25 nm. Dislocations occur within regions such as this (due to the growth mechanism, as described).
Structure of ferrihydrite, as reported by Drits. Note the holes (with circles representing water) give the material a structure reminiscent of an array of very small (neuclei). Compare the texture with that evident in images above.
Figures showing origin of dislocations in nanoparticles that grow via oriented aggregation. From Penn and Banfield 1998 (Sci.)
Figure showing the origin of new structural slabs in ferrihydrite growing via oriented aggregation. The individual particles (3) are colored differently, but all are viewed down [001] ferrihydrite. The transformation mechanism involves displacement of Fe from some filled octahedra to adjacent face-sharing sites below or above the plane of this page. Note the chains of octahedra at particle boundaries. These are found in the goethite structure but not in ferrihydrite. A full description of this complex process will appear elsewhere.
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