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

In celebration of the career of Dr. Peter Heaney, we invite submissions using in situ, synchrotron, X-ray, neutron, or electron techniques to study rates/mechanisms of mineral reactions, alongside contributions exploring new approaches to mineralogy education.

Sunday, 19 October 2025
1:30 PM - 5:30 PM
HBGCC, 217A

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

Paper number: 47-5
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

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

doi: 10.1130/abs/2025AM-412229

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