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 P 6 ₃ mc ; a = ∼5.95 angstroms, c = ∼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 δ-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.

A phenomenological model of the reconstructive transformation between the NaCl and CsCl structure types is described. It consists of two consecutive displacive mechanisms, coupled to tensile and shear strains. These mechanisms give rise to intermediate orthorhombic structures, including the B16 and B33 structure types. The model is shown to explain consistently the experimental observations in different groups of binary compounds as monochalcogenides, monopnictides, and monohalides. The isostructural transitions found in rare-earth chalcogenides are interpreted by the coupling of the displacive order parameters to the change in compressibility related to the valence transitions. The previous models proposed for the NaCl-CsCl transformation are critically analyzed.

We present the results of a combined study of shocked labradorite from the Lonar crater, India, using optical microscopy, micro‐Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, high‐energy X‐ray total scattering experiments, and micro‐Fourier transform infrared (micro‐FTIR) spectroscopy. We show that maskelynite of shock class 2 is structurally more similar to fused glass than to crystalline plagioclase. However, there are slight but significant differences—preservation of original preimpact igneous zoning, anisotropy at infrared wavelengths, X‐ray anisotropy, and preservation of some intermediate range order—which are all consistent with a solid‐state transformation from plagioclase to maskelynite.