No abstract

We introduce giant liposomes to investigate phase transformations in picoliter volumes. Precipitation of calcium carbonate in the confinement of DPPC liposomes leads to dramatic stabilization of amorphous calcium carbonate (ACC). In contrast, amorphous strontium carbonate (ASC) is a transient species, and BaCO3 precipitation leads directly to the formation of crystalline witherite.

Clay swelling is a colloidal phenomenon that has a large influence on flow and solute migration in soils and sediments. While models for clay swelling have been proposed over many years, debate remains as to the interaction forces that combine to produce the observed swelling behavior. Using cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering, we study the influence of salinity, in combination with layer charge, interlayer cation, and particle size, on montmorillonite swelling. We observe a decrease in swelling with increased layer charge, increased cation charge, and decreased cation hydration, each indicative of the critical influence of Coulombic attraction between the negatively charged layers and interlayer cations. Cryo-TEM images of individual montmorillonite particles also reveal that swelling is dependent upon the number of layers in a particle. Calculations of the van der Waals (vdW) interaction based on new measurements of Hamaker coefficients confirm that long-range vdW interactions extend beyond near-neighbor layer interactions and result in a decrease in layer spacing with a larger number of layers. This work clarifies the short- and long-range attractive interactions that govern clay structure and ultimately the stability and permeability of hydrated clays in the environment.

Use of amorphous precursors is a widespread strategy in biomineralization. In sea urchin embryos, controlled transformation of amorphous calcium carbonate (ACC) to calcite results in smoothly curving and branching single crystals. However, the mechanism of the disorder‐to‐order transformation remains poorly understood. Here, the use of strontium as a probe in X‐ray absorption spectroscopy (XAS) greatly facilitates investigation of the evolution of order. In pulse‐chase experiments, embryos incorporate Sr2+ from Sr‐enriched seawater into small volumes of the growing endoskeleton. During the chase, the Sr‐labeled mineral matures under physiological conditions. Based on Sr K‐edge spectra of cryo‐frozen whole embryos, it is proposed that the transformation occurs in three stages. The initially deposited calcium carbonate has short‐range order resembling synthetic hydrated ACC. Within 3 h, the short‐range order of calcite is established. Between 3 h and 24 h, the short‐range order does not change, while long‐range order increases. These results refute the notion that organisms imprint the local order of the final crystal on ACC. Furthermore, it is proposed that the intermediate is more similar to disordered calcite than to anhydrous ACC. Pulse‐chase experiments in conjunction with heavy element labeling have great potential to improve understanding of phase transformations during biomineralization.