47-14 Reflections on a Career in the Crystallography of Recalcitrant Minerals

Session: Minerals in Motion: Tracking Mineral Reactions Using In Situ and Synchrotron Techniques, A Celebration of the Career of Peter Heaney

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I welcome this opportunity to review my adventures with minerals in motion, and I am grateful to the many collaborators who made them possible.

My Ph.D. advisor, David Veblen, introduced me to high-temperature electron microscopy of quartz and leucite, and my first graduate student, Hongwu Xu, extended this work to the feldspar system.  When Poul Norby, Jon Hanson, and John Parise developed environmental reaction cells for time-resolved synchrotron X-ray diffraction (TRXRD) in the 1990s, Jeffrey Post and I were excited by the technique’s potential for exploring soil reactions involving poorly crystalline Mn and Fe hydroxides. Graduate students Christina Lopano, and later, Claire Fleeger, demonstrated the ability of TRXRD to capture pathways and kinetics for cation exchange in birnessite-like phases with unprecedented resolution. Timothy Fischer showed that biologically mediated reduction and dissolution of birnessite can be captured by the innovative use of enzymes and chelators.

The addition of an automated fraction collector for the eluate, by APS beamline scientist Peter Eng, allowed Andrew Wall to connect phase transitions discerned with TRXRD to isotopic fractionation. Phil Kong employed this protocol in combination with XANES, thanks to APS beamline scientist Joanne Stubbs, to show that the oxidation of Cr(III) to Cr(VI) by triclinic birnessite is accompanied by a reductive reaction to hexagonal birnessite, thereby inhibiting further Cr oxidation. The link between these laboratory studies of Mn oxides to natural systems was and is being made by Florence Ling, particularly for passive treatment systems of acid mine waste.

In parallel with this work, our group developed methods for exploring aqueous crystallization of transition metal (hydr)oxides by TRXRD. Controls on the formation of Ti oxides were determined by Daniel Hummer, who built a cell with small- and wide-angle scattering capabilities to capture particle size and mineral phase evolution. Kristina Peterson focused on hematite formation in Cl-rich solutions at low pH, and Matt Oxman analyzed the kinetics of goethite precipitation at very high pH. Si Athena Chen extended this work to goethite and hematite crystallization from ferrihydrite across a range of pH and temperatures. Dongyoun Chung has documented disparate mechanisms of atomic breakdown in hematite and goethite, and he has broadened our scope to carbonates. These outstanding mentors, colleagues, and students inspired a most joyful and gratifying career.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9196

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