The transformation of aqueous species into solids is perhaps the least understood process in low-temperature mineral formation under conditions typical of Earth’s surface. This is particularly true for solids that are nanosized and partially crystallized. Well-known examples include ferrihydrite, goethite, and schwertmannite, many aluminum and manganese-(oxy)hydroxides, and certain clays. Evidence suggests that many, if not most, of these phases do not grow by the classical terrace-ledge-kink (TLK) process where crystallization occurs via attachment of monomer species (atoms, ions or molecules). Instead, they form by the aggregation and assembly of larger precursors. This “nonclassical” crystallization process relies on polymers and the smallest nanoparticles as the primary crystal growth units. Information on the structural, chemical, and physical properties of these early formed products, as well as how they aggregate and self-assemble in solution, is limited due to experimental challenges associated with their size, transient nature, and heterogeneity. This has hampered efforts to develop models that describe the growth of these minerals via nonclassical routes. We are investigating the assembly of clusters to form the ferric oxyhydroxide nanomineral schwertmannite. Schwertmannite is a common mineral in acid mine drainage (AMD) environments and acid sulfate soils where it plays a role in elemental cycling and sequestering contaminants; however, we do not understand how it forms under environmentally relevant conditions. Based on in situ X-ray total scattering data collected in real time and X-ray diffraction and bulk thermal and chemical analyses of samples prepared ex situ, we present: 1) the effects of solution conditions (specifically sulfate/iron and sulfate/hydroxide ratio) on schwertmannite structure and composition, and 2) an evaluation of synthetic schwertmannite formation via a nonclassical route. From these data on laboratory schwertmannite we are starting to build a general model of schwertmannite formation under a range of starting compositions. We are also using these results to try and understand the important question of why schwertmannite solubility changes with sulfate/hydroxide ratio (J.-Y.Yu et al., Geochemical Journal, 2002, 36, 119-132; M.A. Caraballo et al., Chemical Geology, 2013, 360-361, 22-31).

Mineral formation by cluster self-assembly: Schwertmannite as a partially crystallized nanomineral

249th National Meeting (2015)

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