DIFFRACTING X-RAYS WITH TIME: ADVENTURES IN THE DYNAMIC CRYSTALLOGRAPHY OF FE OXIDES

In 1989, David L. Bish and Jeffrey E. Post organized a Mineralogical Society of America short course entitled “Modern Powder Diffraction.” The workshop, and the RIM volume that accompanied it, provided a remarkably prescient roadmap for the next three decades of X-ray crystallography in the Earth sciences. A chapter by RC Reynolds on diffraction by small and disordered crystals anticipated the forthcoming spotlight on poorly crystalline biomaterials and soil minerals. Post and Bish co-authored the first geologically oriented review of Rietveld refinement, which quickly became foundational for the next generation of crystallographers. Chapters on synchrotron (L Finger) and neutron (R Von Dreele) powder diffraction advertised technologies that once seemed exotic but have assumed central roles in mineral and rock interrogation.

This talk will illustrate the impact of the novel approaches outlined in RIM Vol 20 through time-resolved synchrotron XRD studies of Fe (hydr)oxide growth in aqueous solutions. Over the past decade, our group (including JEP) has investigated the aqueous crystallization of hematite from precursor akageneite at low pH (1-2) and moderately high temperatures (150 to 200 ^oC), and we reproducibly documented a non-classical crystallization pathway that involved the initial formation of a highly defective hydrous hematite that evolved to a stoichiometric phase by the continuous exchange of Fe³⁺ for 3H⁺ as nanocrystals grew (Peterson et al. 2018). Subsequent experiments by Chen et al. (2021 and in press) demonstrated that H-rich hematite is not merely a result of initially high H concentrations. Ferrihydrite gels heated to 70-170 ^oC at moderately alkaline pH (8-13) similarly generated superhydrous hematite, sometimes terminating with an Fe occupancy of only ~0.85. Curiously, this Fe-infilling crystallization mechanism was not reversed during dissolution: When Fe-deficient hematite was dissolved in 3 M HCl solutions at 70 ^oC, Fe occupancy of the residual hematite steadily increased (Chung et al., in prep.). We are grateful that Bish and Post (1989) laid the groundwork for atomic-scale explorations of unexpected behaviors in environmentally important mineral systems, and we hope their retirements are long, rich, and fulfilling.