T5 Visions of Minerals at the Nanoscale I: In Honor of Mineralogical Society of America Roebling Medalist Peter R. Buseck

Sunday, 22 September 2019
8:00 AM - 12:00 PM
Room 132ABC, North Building

STRUCTURAL EVOLUTION OF EXCEPTIONALLY IRON-DEFICIENT HEMATITE NANOCRYSTALS AS OBSERVED BY IN SITU SYNCHROTRON X-RAY DIFFRACTION

Paper number: T5-13
Program: Technical Sessions
Time: 11:10 AM – 11:25 AM

Authors

Abstract

Stoichiometric hematite (Fe2O3) is used in energy storage devices, catalysts, and contaminant ion removers because of its low-cost, nontoxicity, natural abundance, and, most important, structural capacity. The hematite structure can embed high concentrations of foreign ions to enhance its chemical performance. A surprisingly Fe-deficient hematite, which we describe as hydrohematite, has been successfully synthesized in our lab, and potentially allows higher ion adsorption capacity than stoichiometric hematite. We have investigated the structural transformation of ferrihydrite to (hydro)hematite and goethite using time-resolved synchrotron X-ray diffraction (TR-XRD) at temperatures ranging from 70-180oC and starting pH concentrations from 2-13. Our Rietveld structure refinements illustrate for the first time how the structure evolves as Fe occupancy increases during crystal growth.

The formation of hydrohematite always occurred in two stages. An initial rapid increase in Fe occupancy was followed by a stabilization with Feocc= 0.80-0.95, depending on temperature and pH. For example, the Fe occupancy at pH 11 and 90oC increased from 0.69 to 0.84 within the first 30 min and stabilized to ~0.84 over the next 4 hours. As Fe occupancy increased during crystal growth, the lattice parameters and unit-cell volume at first increased and then decreased. We interpret the initial expansion in lattice parameters as resulting from the introduction of crystal boundary defects as nanocrystals aggregate. The contraction of lattice parameters can be explained by crystal coarsening and deprotonation. Larger and more mature crystals contain fewer vacancies and exhibit smaller lattice parameters. Moreover, deprotonation results in a shortening of the longer Fe-O octahedral bond lengths. When Fe occupancies were less than 0.80, the hydrohematite structure exhibited extremely distorted octahedra, which evolved to more symmetrical octahedra with time. Our structural study of hydrohematite microcrystals highlights their unexpectedly high tolerance for Fe deficiencies, suggesting a promising strategy to enhance the assimilation capacity of foreign ions.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-337360

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.