Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

Amorphous calcium carbonate (ACC) is an important phase involved in calcification by a wide variety of invertebrate organisms and is of technological interest in the development of functional materials. Despite widespread scientific interest in this phase a full characterization of structure is lacking. This is mainly due to its metastability and difficulties in evaluating structure using conventional structure determination methods. Here we present new findings from the application of two techniques, pair distribution function analysis and nuclear magnetic resonance spectroscopy, which provide new insight to structural aspects of synthetic ACC. Several important results have emerged from this study of ACC formed in vitro using two common preparation methods: (1) ACC exhibits no structural coherence over distances > 15 Å and is truly amorphous; (2) most of the hydrogen in ACC is present as structural H 2 O, about half of which undergoes restricted motion on the millisecond time scale near room temperature;(3) the short-and intermediate-range structure of ACC shows no distinct match to any known structure in the calcium carbonate system; and (4) most of the carbonate in ACC is monodentate making it distinctly different from monohydrocalcite. Although the structure of synthetic ACC is still not fully understood, the results presented provide an important baseline for future experiments evaluating biogenic ACC and samples containing certain additives that may play a role in stabilization of ACC, crystallization kinetics, and final polymorph selection.

Iron(III) hydroxides are abundant in near-surface natural environments and play an important role in geochemical processes and the fate of contaminants. The issue of the structure of the common nanophase material ferrihydrite (ferric hydroxide) is controversial and has been debated in the literature for many years without definitive resolution. At least two types of ferrihydrite, the 2-line and 6-line forms, are conventionally recognized. It has been suggested that these forms possess different structures built up by different mixtures of distinct nanophase components. However, traditional crystallographic methods provide depictions of structure that are most sensitive either to short-range order (X-ray absorption) or long-range periodicity (X-ray diffraction or electron diffraction). We used high-energy X-ray total scattering for pair distribution function analysis to observe both the short- and intermediate-range ordering (exceeding ∼15 Å) of synthetic ferrihydrite with three distinct average domain sizes of 2, 3, and 6 nm. We show that there are no significant differences in the underlying structures of these materials and that the differences in the diffraction patterns can be entirely interpreted by variations in the average size of the coherent scattering domains. The average crystallite sizes inferred from the PDF analysis are in good agreement with direct observation by high-resolution transmission electron microscopy.