In situ synchrotron X-ray diffraction of Fe oxide transformations in aqueous solutions

The mineral hematite (Fe2O3) is of critical economic importance, as it serves as the primary mineral reserve for the extraction of iron, which accounts for 90% of worldwide metal consumption. In addition, hematite formation in soils is of the highest environmental significance, because hematite nanoparticles can attach to the surfaces of soil particles and modify their ability to sequester harmful metals. Hematite also is used technologically in applications ranging from magnetic data storage to pigmentation, and it is being tested as an agent for water-splitting in solar cells. However, despite the ubiquity of hematite at Earth's surface, the chemical reactions by which hematite crystallizes are still poorly understood. This study will explore a novel hypothesis that the first crystals of hematite to nucleate from aqueous solutions are chemically and physically distinct from ideal hematite, not only because of their nanometer-scale particle size but because up to one-third of the iron atoms are missing from the crystal structure. Previous studies have shown that these iron defects can dramatically alter the magnetic properties, color saturation, and chemical reactivity of hematite. In this study, the investigators will explore the relation between iron defects and the physical properties of hematite, and they will determine the connection between the abundance of defects and the temperature, pH, and oxygen content of the groundwaters from which natural hematite crystallizes.

The investigators hypothesize that when hematite (Fe2O3) forms from ferrihydrite and goethite in aqueous solutions, the hematite first nucleates as highly iron-deficient, hydrous nanocrystals. These defective precursor phases are metastable and may anneal to endmember hematite with time. The investigators base these inferences on their recent observations of crystallization sequences in ferric chloride solutions. That work revealed incipient phases of 'hydrohematite' with a chemical formula close to FeOOH forming from precursor akaganéite. The investigators will determine whether the more common crystallization pathways for natural hematite - from antecedent ferrihydrite and goethite - experience an analogous multi-step reaction route. In contrast to approaches based on dry heating of powders or on 'before-and-after' characterization of batch-reacted mixtures, the investigators will exploit novel synchrotron-based techniques that allow them to capture X-ray diffraction patterns of mineral-fluid mixtures from 25 to 200 degree C with very high time resolution. The diffraction data are amenable to Rietveld analysis and will yield high-quality cell parameters, atom positions, and occupancies. In addition to pinpointing the evolution of hematite growth at the atomic scale, the experiments will yield kinetic models from which they can extract robust activation energies for the nucleation and growth of iron (hydr)oxides as a function of pH. The investigators will couple these experiments with detailed microscopic and spectroscopic examinations of natural and historic specimens of 'hydrohematite', and they will perform density functional calculations to gauge the relative stabilities of hydrous hematite phases and to relate chemical properties to particle size.