Mineral Formation by Cluster Self-Assembly: Schwertmannite as a Partially Crystallized Nanomineral

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

Our ability to understand crystallization processes is a key to interpreting the past, present, and future evolution of our planet. Yet our scientific understanding of crystallization - the transformation of dissolved species into solids - is incomplete. Recent research is revealing a diversity of new "nonclassical" pathways and mechanisms by which crystals, particularly nanocrystals, form and grow through attachment of intermediate and precursor particles. However, immense gaps remain in our understanding of these processes. This project will investigate the assembly of clusters to form the ferric oxyhydroxide mineral schwertmannite in order to understand how it forms via nonclassical route. Schwertmannite is pervasive and of broad interest in acidic environments because it exhibits high surface area and an affinity for a variety of potential pollutants. Additionally, because schwertmannite and other poorly crystalline minerals are so abundant and important at and near Earth's surface, understanding their formation is the key to interpreting the geologic record and the interaction between organisms and geologic media. The overarching objective of this research is to document the nature of schwertmannite precursor clusters as a function of solution composition and growth conditions, and then to use that information to construct a general model that shows how dissolved iron and sulfate react in solution forming schwertmannite. Preliminary in situ synchrotron scattering data show the formation of clusters (~1.5 nm) having schwertmannite-like structural characteristics. These clusters are stable in solution as long as the solution remains turbulent (for up to 40 hours), but quickly aggregate and transform to schwertmannite under static fluid conditions. Based on preliminary data, the investigators hypothesize that schwertmannite forms via a nonclassical pathway involving self-assembly of schwertmannite-like precursor particles. They also hypothesize that the composition and properties of schwertmannite should reflect the structure and composition of the particles that aggregate. The investigators will apply a suite of in situ laboratory and synchrotron mineral characterization techniques to study the real-time formation of synthetic schwertmannite precursor clusters in solution and their aggregation into schwertmannite solids. The investigators will also collect and characterize water samples and sediments from an active AMD system and evaluate schwertmannite formation in nature. The structural, chemical, and physical data from these experiments will be used to develop a general crystallization model for schwertmannite that is consistent with thermodynamic and chemical principles, and with observations in the literature and this project of schwertmannite formation in nature.