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Title: Iron site occupancies in magnetite-ulvospinel solid solution: A new approach using XMCD

Abstract

Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was > 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrentmore » with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.« less

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
Advanced Light Source Division
OSTI Identifier:
983186
Report Number(s):
LBNL-3328E
Journal ID: ISSN 0003-004X; AMMIAY; TRN: US1004573
DOE Contract Number:  
DE-AC02-05CH11231
Resource Type:
Journal Article
Journal Name:
American Mineralogist
Additional Journal Information:
Journal Name: American Mineralogist; Journal ID: ISSN 0003-004X
Country of Publication:
United States
Language:
English
Subject:
75; BUFFERS; CHEMICAL ANALYSIS; ELECTRONS; IRON; MAGNETIC CIRCULAR DICHROISM; MAGNETIC MOMENTS; MAGNETIC PROPERTIES; MAGNETITE; MOESSBAUER EFFECT; OXIDATION; OXIDE MINERALS; SOLID SOLUTIONS; SPECTRA; SPECTROSCOPY; SPINELS; VALENCE

Citation Formats

Pearce, C I, Henderson, C M. B., Telling, N D, Pattrick, R A D, Vaughan, D J, Charnock, J M, Arenholz, E, Tuna, F, Coker, V S, and Laan, G van der. Iron site occupancies in magnetite-ulvospinel solid solution: A new approach using XMCD. United States: N. p., 2009. Web.
Pearce, C I, Henderson, C M. B., Telling, N D, Pattrick, R A D, Vaughan, D J, Charnock, J M, Arenholz, E, Tuna, F, Coker, V S, & Laan, G van der. Iron site occupancies in magnetite-ulvospinel solid solution: A new approach using XMCD. United States.
Pearce, C I, Henderson, C M. B., Telling, N D, Pattrick, R A D, Vaughan, D J, Charnock, J M, Arenholz, E, Tuna, F, Coker, V S, and Laan, G van der. Mon . "Iron site occupancies in magnetite-ulvospinel solid solution: A new approach using XMCD". United States. https://www.osti.gov/servlets/purl/983186.
@article{osti_983186,
title = {Iron site occupancies in magnetite-ulvospinel solid solution: A new approach using XMCD},
author = {Pearce, C I and Henderson, C M. B. and Telling, N D and Pattrick, R A D and Vaughan, D J and Charnock, J M and Arenholz, E and Tuna, F and Coker, V S and Laan, G van der},
abstractNote = {Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was > 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrent with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.},
doi = {},
journal = {American Mineralogist},
issn = {0003-004X},
number = ,
volume = ,
place = {United States},
year = {2009},
month = {6}
}