Aggregation-Based Crystal Growth and Microstructure Development in Natural Iron Oxyhydroxide Biomineralization Products
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A number of experimental studies describe growth via oriented aggregation. For example this has been reported in anatase (TiO 2 ) under hydrothermal conditions [
]. Also see (2–7).
Aggregation-based grown has been demonstrated in hematite (α-Fe 2 O 3 ) formed by aging of FeCl 3 in the presence of KH 2 PO 4 [
A mine-trained SCUBA diving team led by T. T. Ebert collected samples of water and biofilms into sterile syringes. Samples were kept on ice until returned to the laboratory where they were stored at 4°C before examination by optical and electron microscopy.
Cells were detected by staining their DNA with 4′ 6-diamidino-2-phenylindole (DAPI). Stained cells were visualized with epifluorescence microscopy. Population analyses with 16 S ribosomal RNA gene sequencing are underway. Results will be reported separately.
High-resolution TEM (HRTEM) images show cross-fringes (periodicities not detected by SAED) in orientations that indicate 3D order in two-line ferrihydrite (see Fig. 3). In some regions additional diffraction maxima indicate feroxyhite (12) and less commonly six-line ferrihydrite. Results are consistent with previous studies (13 14) which provide structural details.
H. L. Zhu and R. S. Averback [ Philos. Mag. 73 27 (1996)] demonstrated rotation of copper particles to achieve parallel orientations with molecular dynamics simulation. The homogeneous sizes of the initial nanoparticles and their clear separation into distinct 2- to 3-nm diameter entities rule out an alternative interpretation that oriented aggregates arise from sequential nucleation of seeds on earlier formed particles. No damage (such as creation of porosity) was detected during examination with the HRTEM operated at 200 kV. No microstructures suggestive of contact epitaxy were observed.
Oriented aggregation requires coherency in the 2D plane of the interface. In addition to attachment that generates perfect 3D structure coherent {100} and {110} interfaces with structural characteristics intermediate between ferrihydrite and goethite can be created. These regions may nucleate transformation reactions. We reported analogous phenomena in titania; see (4 5).
For example see
Pykhteev O. Y., Efimov A. A., Moskvin L. N., Russ. J. Appl. Chem. 72, 9 (1999);
. Although some data supporting the formation of polynuclear hydrolysis products have been reported Schwertmann et al. (21) maintain that there is no good evidence for intermediates between trimers and solids.
Subnanometer-scale porosity within ferrihydrite (7) may be generated if ferrihydrite itself forms by aggregation of protocrystals. Protocrystals could consist of three oxygen planes that sandwich pairs of face-sharing octahedra (to our knowledge no small aqueous clusters containing face-sharing units are known so face-sharing must be introduced by aggregation of smaller corner- and edge-linked molecules). Formation of ferrihydrite could proceed via sequential elimination of n(OH) following aggregation of pairs of {∼n[O 0.25 (OH) 0.75 (Fe 2/3 ) O 0.5 (OH) 0.5 (Fe 2/3 ) O 0.25 (OH 0.75 )or n(FeOOH) 4 ·H 2 O] units}. If some factor favors joining of alternate nuclei that are rotated by 180° ferrihydrite ABAC oxygen closest packing would result.
Web figures 1 through 3 are available at Science Online at www.sciencemag.org/feature/data/1050910.shl.
We thank V. Dritts and A. Manceau for their comments on a longer report describing structural aspects of iron oxyhydroxides and W. H. Casey for discussion of multinuclear clusters. Supported by NSF grants EAR 9317082 and EAR 9814333 and the NASA Astrobiology Institute (JPL-Caltech). We also thank T. O'Connor for permission to reproduce Fig. 1.