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Fe site occupancy in magnetite-ulvospinel solid solutions: A new approach using X-ray magnetic circular dichroism

Pearce, C. I.; Henderson, C. M. B.; Telling, N. D.; Pattrick, R. A.D.; Charnock, J. M.; Coker, V. S.; Arenholz, E.; Tuna, F.; van der Laan, G.

Authors

C. I. Pearce

C. M. B. Henderson

R. A.D. Pattrick

J. M. Charnock

V. S. Coker

E. Arenholz

F. Tuna

G. van der Laan



Abstract

Ordering of Fe3+ and Fe2+ cations between octahedral and tetrahedral sites in synthetic members of the magnetite-ulvöspinel (Fe3O4-Fe2TiO4) solid-solution series was determined using Fe L2,3-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L2,3-edge and Fe K-edge X-ray absorption spectroscopy (XAS), and unit-cell parameters. Microprobe analyses, cell edges, and chemical FeO determinations showed that bulk compositions were stoichiometric magnetite-ulvöspinel solid solutions. XMCD showed that the surface was sensitive to redox conditions, and samples required re-equilibration with solid-solid buffers. Detailed site-occupancy analysis gave Fe2+/Fe3+ XMCD-intensity ratios close to stoichiometric values.

L2,3-edge XAS confirmed that Ti4+ was restricted to octahedral sites. XMCD showed that significant Fe2+ only entered the tetrahedral sites when Ti content was >0.40 atoms per formula unit (apfu), whereas Fe2+ in octahedral sites increased from 1 apfu in magnetite to a maximum of ~1.4 apfu when Ti content was 0.45 apfu. As Ti content increased, a steady increase in Fe2+ in tetrahedral sites was observable in the XMCD spectra, concurrent with a slow decrease in Fe2+ in octahedral sites. Calculated magnetic moments decreased rapidly from magnetite (4.06 μB) to USP45 (1.5 μB), then more slowly toward ulvöspinel (0 μB). Two synthesized samples were maghemitized by re-equilibrating with an oxidizing buffer. XMCD showed that Fe2+ oxidation, with concomitant vacancy formation, was restricted to octahedral sites. Through the direct measurement of Fe oxidation states, XMCD results can be used to rationalize the magnetic properties of titanomagnetites, along with oxidized titanomaghemitized analogs, in Earth’s crustal rocks.

Citation

Pearce, C. I., Henderson, C. M. B., Telling, N. D., Pattrick, R. A., Charnock, J. M., Coker, V. S., …van der Laan, G. (2010). Fe site occupancy in magnetite-ulvospinel solid solutions: A new approach using X-ray magnetic circular dichroism. American Mineralogist, 95(4), 425-439. https://doi.org/10.2138/am.2010.3343

Journal Article Type Article
Acceptance Date Dec 11, 2009
Online Publication Date Apr 2, 2015
Publication Date Apr 1, 2010
Deposit Date May 22, 2024
Journal American Mineralogist
Print ISSN 0003-004X
Publisher Mineralogical Society of America
Peer Reviewed Peer Reviewed
Volume 95
Issue 4
Pages 425-439
DOI https://doi.org/10.2138/am.2010.3343
Keywords Titanomagnetite; titanomaghemite; cation site ordering; vacancy ordering; X-ray absorption spectroscopy; X-ray magnetic circular dichroism; magnetic moment
Public URL https://keele-repository.worktribe.com/output/831251
Publisher URL https://www.degruyter.com/document/doi/10.2138/am.2010.3343/html?lang=en