Roundabout Gypsum: Calcium sulfates are a common but perhaps underappreciated group of minerals used in a number of natural and industrial processes. In many ways, these crystals precipitate from solution in the same way that most other aqueous minerals form; however, mounting evidence suggests that different, unexplored mechanisms may be at work. Van Driessche et al. (p. 69 ; see the cover) performed high-resolution microscopy of the most common calcium sulfate mineral, gypsum, at various points along time-resolved, fast-quenching growth experiments. The images reveal that gypsum particles actually start out as crystalline nanoparticles of another mineral, bassanite, which then self-assemble into well-ordered nanorods. Finally, the nanorods transform into gypsum following a hydration reaction. The observation that the reaction pathway occurs below the solubility limit of the intermediate phase has wide-ranging implications for biomineralization processes and may provide ways to prevent fouling on the surfaces of desalination membranes.

Ferrihydrite was exposed to U(VI)-containing cement leachate (pH 10.5) and aged to induce crystallization of hematite. A combination of chemical extractions, TEM, and XAS techniques provided the first evidence that adsorbed U(VI) (≈3000 ppm) was incorporated into hematite during ferrihydrite aggregation and the early stages of crystallization, with continued uptake occurring during hematite ripening. Analysis of EXAFS and XANES data indicated that the U(VI) was incorporated into a distorted, octahedrally coordinated site replacing Fe(III). Fitting of the EXAFS showed the uranyl bonds lengthened from 1.81 to 1.87 Å, in contrast to previous studies that have suggested that the uranyl bond is lost altogether upon incorporation into hematite. The results of this study both provide a new mechanistic understanding of uranium incorporation into hematite and define the nature of the bonding environment of uranium within the mineral structure. Immobilization of U(VI) by incorporation into hematite has clear and important implications for limiting uranium migration in natural and engineered environments.