Curie Temperature Enhancement and Cation Ordering in Titanomagnetites: Evidence From Magnetic Properties, XMCD, and Mössbauer Spectroscopy

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2019-05, Vol.20 (5), p.2272-2289
• Main Authors: Bowles, J. A., Lappe, S.‐C. L. L., Jackson, M. J., Arenholz, E., Laan, G.
• Format: Article
• Language: English
• Subjects: Analytical methods; cation ordering; Cations; Crystal structure; Curie temperature; Earth; Geomagnetic field; Iron; Low temperature; Magnetic field; Magnetic fields; Magnetic properties; Magnetic variations; Magnetism; Magnetization; Metal ions; Metals; mineral magnetism; Mossbauer; Oxidation; Oxide minerals; Procedures; Saturation; Spectroscopy; Spectrum analysis; titanomagnetite; Volcanic rocks; XMCD
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2019GC008217

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. Plain Language Summary Magnetization acquired by the iron‐titanium oxide mineral titanomagnetite (frequently found in volcanic rocks) provides a vital source of information about geomagnetic field history and tectonic plate motions. Yet, there are aspects of titanomagnetite's magnetism that remain poorly understood, particularly concerning the arrangement of metal cations (Fe2+, Fe3+, Ti4+, etc.) in the crystal structure, how the arrangement changes with temperature, and resulting changes in magnetic properties. Recent findings demonstrate that naturally occurring titanomagnetites exhibit dramatic changes in certain magnetic properties when subjected to moderate temperatures (300–500 °C). These changes can influence the outcome of laboratory procedures designed to recover valuable information about Earth's magnetic field. Here, we created synthetic titanomagnetite and used techniques that provide information on the arrangement of cations within the crystal structure. Titanomagnetite has two distinct types of “sites” in which metal cations can be situated. One way to produce the observed magnetic variations is to cha