Curie Temperature Enhancement and Cation Ordering in Titanomagnetites: Evidence From Magnetic Properties, XMCD, and Mössbauer Spectroscopy. (15th May 2019)

• Record Type: Journal Article
• Authors: Bowles, J. A.; Lappe, S.‐C. L. L.; Jackson, M. J.; Arenholz, E.; van der Laan, G.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 20:Number 5(2019)
• Page Start: 2272
• Page End: 2289
• Publication Date: 2019-05-15
• Subjects: titanomagnetite -- Curie temperature -- cation ordering -- XMCD -- Mossbauer -- mineral magnetism; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2019GC008217
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Previous work has documented time‐ and temperature‐dependent variations in the Curie temperature ( T c ) 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 T c, but Mössbauer, XMCD, and saturation magnetization data all demonstrate that intersite reordering of Fe 2+ /Fe 3+ does not play a role in the observed T c 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 (Fe 2+, Fe 3+, Ti 4+, 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 change the distribution of iron cations between these two sites. However, this work demonstrates that the observed magnetic property variations are not related to this type of rearrangement of iron cations. Instead, it appears to be related to oxidation and may result from a rearrangement of cations within one of the crystal sites. Key Points: Synthetic titanomagnetite Curie temperatures increase when annealed at 325–400 degrees C and decrease when rapidly cooled from ~600 degrees C XMCD, Mossbauer, and magnetic data suggest the cause is vacancy‐enhanced nanoscale chemical clustering Vacancies and Mg cations are accommodated on octahedral sites; Al cations are split between octahedral and tetrahedral sites