Oriented Attachment and Nonclassical Formation in Iron Oxides

Iron oxide plays dynamic roles in environmental systems, and their ability to adsorb or release heavy metals as well as participate in redox reactions is highly dependent on factors such as mineral phase, crystal morphology, and accessible surface area. Understanding crystal nucleation and growth, therefore, is critical to improving predictions about the chemical and physical behavior of iron oxides in dynamic environments. Classical crystal growth models, aggregation, and recrystallization can adequately describe some, but not all, observed crystal growth kinetics, shapes, and textures. Nonclassical growth models, such as the relatively broad category of particle-mediated crystal growth, provide insight into how important microstructural features like edge dislocations, stacking faults, and twinned crystal planes form. The relative contributions to crystal growth by particle-mediated mechanisms and classical crystal growth are exquisitely sensitive to reaction conditions, including suspension pH and ionic strength, the presence of chemical additives, and the shapes of nanoparticles present at any given time. This review summarizes kinetic models that have been developed to describe nonclassical crystal growth mechanisms, highlights knowledge gaps, and discusses future directions for this important area of research.