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Curie temperature enhancement and cation ordering in titanomagnetites: Evidence from magnetic properties, XMCD, and Mössbauer spectroscopy

DOI: 10.1029/2019GC008217 DOI Help

Authors: J. A. Bowles (University of Wisconsin – Milwaukee) , S.‐c. L. L. Lappe (University of Wisconsin – Milwaukee) , M. J. Jackson (University of Minnesota) , Elke Arenholz (Advanced Light Source) , G. Van Der Laan (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Geochemistry, Geophysics, Geosystems

State: Published (Approved)
Published: April 2019

Abstract: 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 325‐400 °C for up to 1000 h 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.

Journal Keywords: titanomagnetite; Curie temperature; cation ordering; XMCD; Mossbauer; mineral magnetism

Subject Areas: Earth Science

Facility: Advanced Light Source (ALS); Advanced Photon (APS)