Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

/pdf/crystallization-by-particle-attachment-in-synthetic-biogenic-2voi5gdwxm.pdf

Crystallization by particle attachment in synthetic, biogenic, and geologic environments

Crystallisation is at the heart of various scientific disciplines, but still the understanding of the molecular mechanisms underlying phase separation and the formation of the first solid particles in aqueous solution is rather limited. In this review, classical nucleation theory, as well as established concepts of spinodal decomposition and liquid–liquid demixing, is introduced together with a description of the recently proposed pre-nucleation cluster pathway. The features of pre-nucleation clusters are presented and discussed in relation to recent modifications of the classical and established models for phase separation, together with a review of experimental work and computer simulations on the characteristics of pre-nucleation clusters of calcium phosphate, calcium carbonate, iron(oxy)(hydr)oxide, silica, and also amino acids as an example of small organic molecules. The role of pre-nucleation clusters as solute precursors in the emergence of a new phase is summarized, and the link between the chemical speciation of homogeneous solutions and the process of phase separation via pre-nucleation clusters is highlighted.

/pdf/pre-nucleation-clusters-as-solute-precursors-in-2bz3nzlo56.pdf

Pre-nucleation clusters as solute precursors in crystallisation

Mechanisms of nucleation from electrolyte solutions have been debated for more than a century. Recent discoveries of amorphous precursors and evidence for cluster aggregation and liquid-liquid separation contradict common assumptions of classical nucleation theory. Using in situ transmission electron microscopy (TEM) to explore calcium carbonate (CaCO3) nucleation in a cell that enables reagent mixing, we demonstrate that multiple nucleation pathways are simultaneously operative, including formation both directly from solution and indirectly through transformation of amorphous and crystalline precursors. However, an amorphous-to-calcite transformation is not observed. The behavior of amorphous calcium carbonate upon dissolution suggests that it encompasses a spectrum of structures, including liquids and solids. These observations of competing direct and indirect pathways are consistent with classical predictions, whereas the behavior of amorphous particles hints at an underlying commonality among recently proposed precursor-based mechanisms.

http://science.sciencemag.org/content/sci/345/6201/1158.full.pdf

In situ TEM imaging of CaCO3 nucleation reveals coexistence of direct and indirect pathways

The formation of crystalline materials from solution is usually described by the nucleation and growth theory, where atoms or molecules are assumed to assemble directly from solution. For numerous systems, the formation of the thermodynamically stable crystalline phase is additionally preceded by metastable intermediates . More complex pathways have recently been proposed, such as aggregational processes of nanoparticle precursors or pre-nucleation clusters, which seem to contradict the classical theory. Here we show by cryogenic transmission electron microscopy that the nucleation and growth of magnetite-a magnetic iron oxide with numerous bio- and nanotechnological applications-proceed through rapid agglomeration of nanometric primary particles and that in contrast to the nucleation of other minerals, no intermediate amorphous bulk precursor phase is involved. We also demonstrate that these observations can be described within the framework of classical nucleation theory.

Nucleation and growth of magnetite from solution.

Nucleation, the initial process in vapor condensation, crystal nucleation, melting, and boiling, is the localized emergence of a distinct thermodynamic phase at the nanoscale that macroscopically grows in size with the attachment of growth units. These phase changes are the result of atomistic events driven by thermal fluctuations. The occurrence of atomistic level events with the length scales on the order of 10–10 m and time scales of 10–13 S equivalent to the vibrational frequency of atoms makes the nucleation a very complicated phenomenon to study. Even though abundant literature is available about fundamental aspects of nucleation, the knowledge on these phenomena is far from complete. The classical pathway to nucleation which was once considered to have general applicability to all nucleating systems is gradually giving way to a nonclassical pathway which is now considered as a dominating mechanism in solution crystallization and other systems. In this review, an attempt is made to compare underlyin...

A Review of Classical and Nonclassical Nucleation Theories

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][1]; 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.

 [1]: /lookup/doi/10.1126/science.1215648

http://science.sciencemag.org/content/sci/336/6077/69.full.pdf

The role and implications of bassanite as a stable precursor phase to gypsum precipitation.

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The role and implications of bassanite as a stable precursor phase to gypsum precipitation.