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

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 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.

Curie Temperatures and Emplacement Conditions of Pyroclastic Deposits From Popocatépetl Volcano, Mexico

“…Cation vacancies can be due to oxygen fugacity at titanomagnetite crystallization temperatures (e.g., Lattard et al, 2006) or due to low-temperature oxidation. This issue could be solved by the chemical analysis, high-resolution transmission electron microscopy, or X-ray magnetic circular dichroism (Bowles et al, 2019).…”

“…Curie temperature irreversibility can be either due to vacancy‐enhanced nanoscale chemical clustering (Bowles et al., 2019) or maghemitization (e.g., Lied et al., 2020; Oliva‐Urcia et al., 2011). Their distinction is difficult because cation vacancies (□) follow independent of process the paired substitution 3Fe 2+ = 2Fe 3+ + □ (Jackson & Bowles, 2018).…”

“…St. Helens volcano, USA, and first argued that this nonreversibility is related to cation ordering that might be affected by the emplacement temperature and cooling rate of volcanic sequences containing homogeneous titanomagnetite. The same group of researchers has studied the atomic-scale processes responsible for the T C variations and suggested that vacancy-enhanced nanoscale chemical clustering on the octahedral sites within the titanomagnetite sublattice is more consistent with their X-ray magnetic circular dichroism and Mössbauer spectroscopy study (Bowles et al, 2019) than a reordering of cations. Their data also suggest that Mg substitution mainly occurs on octahedral sites, while Al substitution occurs on both octahedral and tetrahedral sites.…”

“…Alva-Valdivia et al (2019) have reported three emplacement temperature ranges (320-370°C, 400-460°C, and >500°C) for the phreatomagmatic Cerro Colorado tephra deposits, Mexico, and many of their κ(T) curves show significant nonreversible behavior, which may indicate different thermal profiles due to stratigraphic variation of titanomagnetite within the PDC deposit (Bowles et al, 2018). Nonreversibility may be due to maghemitization (Oliva-Urcia et al, 2011), high-temperature nonstoichiometry (Brachfeld & Hammer, 2006), cation ordering processes below the titanomagnetite binary solvus (Jackson & Bowles, 2014), or vacancy-enhanced nanoscale chemical clustering in the octahedral sublattice (Bowles et al, 2019).…”

Cooling rates of pyroclastic deposits inferred from mineral magnetic investigations: a case study from the Pleistocene Mýtina Maar (Czech Republic)

“…It is also conspicuous that this T C 1 is shifted either to higher (∆T C 1: −12 to −57 °C) or lower (∆T C 1: 15-59 °C) values in the cooling curve (∆T C equals T C from heating curve minus T C from cooling curve). This phenomenon indicates cation ordering effects as suggested by Bowles et al (2018Bowles et al ( , 2019, Jackson and Bowles (2014), and Harrison and Putnis (1999). While a second transition temperature (T2 between 362 and 480 °C) is often not reversible (e.g., Fig.…”