Previous work has documented time- and temperature-dependent variations in the Curie temperature (Tc) of natural titanomagnetites, independent of any changes in sample composition. To better understand the atomic-scale processes responsible for these variations, we have generated a set of synthetic titanomagnetites with a range of Ti, Mg, and Al substitution; a subset of samples was additionally oxidized at low temperature (150 °C). Samples were annealed at temperatures between 325 and 400 °C for up to 1,000 hr and characterized in terms of magnetic properties; Fe valence and site occupancy were constrained by X-ray magnetic circular dichroism (XMCD) and Mössbauer spectroscopy. Annealing results in large (up to ~100 °C) changes in Tc, but Mössbauer, XMCD, and saturation magnetization data all demonstrate that intersite reordering of Fe2+/Fe3+ does not play a role in the observed Tc changes. Rather, the data are consistent with vacancy-enhanced nanoscale chemical clustering within the octahedral sublattice. This clustering may be a precursor to chemical unmixing at temperatures below the titanomagnetite binary solvus. Additionally, the data strongly support a model where cation vacancies are predominantly situated on octahedral sites, Mg substitution is largely accommodated on octahedral sites, and Al substitution is split between the two sites.
Bibliographical noteFunding Information:
We thank Richard Pattrick and Carolyn Pearce for advice and assistance with our initial XMCD studies, and David Keavney for beamline support and advice at APS. We thank Bruce Moskowitz and Peat Sølheid for helpful discussions on Mössbauer analysis, Lindsay McHenry for assistance with XRD measurements, and John Fournelle for microprobe preparation and measurements. Many thanks to reviewers Richard Harrison and Dominique Lattard for their thoughtful and helpful comments that improved the manuscript. This research used resources of the Advanced Light Source (ALS) and the Advanced Photon Source (APS). ALS is a U.S. Department of Energy (DOE) Office of Science User Facility under contract DE‐AC02‐ 05CH11231. APS is a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract DE‐AC02‐06CH11357. This research was made possible by NSF grants EAR1315971 to J. A. B. and EAR1315845 to M. J. J., as well as access to the Institute for Rock Magnetism, which is supported by the NSF Instruments and Facilities program and by the University of Minnesota. This is IRM contribution #1815. Magnetic data associated with this manuscript are available from the Magnetics Information Consortium (MagIC) database at the website (https:// earthref.org/MagIC/doi/10.1029/ 2019GC008217).
- Curie temperature
- cation ordering
- mineral magnetism