47-5 In Situ Neutron Scattering and First-Principles Insights into Hydroxyl Positions and Magnetic Structure of Superhydrous Hematite

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

Presenting Author:

Si Athena Chen

Authors:

Chen, Si Athena¹, Chakoumakos, Bryan C², Kubicki, James D³, Frontzek, Matthias D⁴, Cheng, Yongqiang⁵, Daemen, Luke L⁶, Zhang, Yuanpeng⁷, Zhang, Qiang⁸, Post, Jeffrey E⁹, Heaney, Peter J¹⁰

(1) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (2) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (3) Department of Earth, Environmental, and Resource Sciences, University of Texas at El Paso, El Paso, TX, USA, (4) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (5) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (6) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (7) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (8) Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA, (9) Department of Mineral Sciences, Smithsonian Institution, Washington, DC, USA, (10) Department of Geosciences, Penn State University, University Park, PA, USA,

Abstract:

The superhydrous variety of hematite (a-Fe₂O₃), named hydrohematite, can retain remarkable amounts of structural water by cation vacancies, thus significantly modifying its physical and chemical properties.  However, the structural hydroxyl positions in nominally anhydrous minerals (NAMs) are poorly constrained, most of studies focused on spectroscopic signatures of vibrational modes, while little is known about H structural positions. In this study, in situ neutron diffraction, neutron pair distribution function, inelastic neutron scattering, and density functional theory calculations are integrated to determine H and vacancy sites and their effect on magnetic structures.

The incorporation of hydroxyls and vacancies in hydrohematite expands the unit-cell volume, primarily via an increase in c, in comparison with stoichiometric hematite. Difference Fourier maps of neutron diffraction data suggest positions for H atoms. Local H motions were investigated combining inelastic neutron scattering spectroscopy and density functional theory calculations.  Modes at 800, 900, 1160, 1660, 2180 and 3450 cm^-1 are characteristic and relate to hydroxyl motions in hydrohematite from the inelastic neutron scattering spectra. As a result of Fe-deficiencies, hydrohematite exhibits a stronger net magnetization compared to stoichiometric hematite. Neutron diffraction reveals pronounced spin canting in Fe-deficient hydrohematite, with a tilt of 12.6(2)° from the ab plane, resulting in net magnetism along the c-axis. However, the Morin transition is completely suppressed in hydrohematite.

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

doi: 10.1130/abs/2025AM-5968

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