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Epsilon iron oxide: Origin of the high coercivity stable low Curie temperature magnetic phase found in heated archeological materials

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López Sánchez, Jesús and McIntosh, G. and Osete López, María Luisa and Campo García, A. del and Villalain, J.J. and Pérez García, Lucas and Kovacheva, M. and Rodríguez de la Fuente, Óscar (2017) Epsilon iron oxide: Origin of the high coercivity stable low Curie temperature magnetic phase found in heated archeological materials. Geochemistry geophysics geosystems, 18 (7). pp. 2646-2656. ISSN 1525-2027

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Official URL: http://dx.doi.org/10.1002/2017GC006929


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Abstract

The identification of epsilon iron oxide (-Fe2O3) as the low Curie temperature high coercivity stable phase (HCSLT) carrying the remanence in heated archeological samples has been achieved in samples from two archeological sites that exhibited the clearest evidence of the presence of the HCSLT. This uncommon iron oxide has been detected by Confocal Raman Spectroscopy (CRS) and characterized by rock magnetic measurements. Large numbers of -Fe2O3 microaggregates (in CO) or isolated clusters (in HEL) could be recognized, distributed over the whole sample, and embedded within the ceramic matrix, along with hematite and pseudobrookite and with minor amounts of anatase, rutile, and maghemite. Curie temperature estimates of around 170 degrees C for CO and 190 degrees C for HEL are lower than for pure, synthetic -Fe2O3 (227 degrees C). This, together with structural differences between the Raman spectra of the archeologically derived and synthetic samples, is likely due to Ti substitution in the -Fe2O3 crystal lattice. The -Fe2O3--Fe2O3--Fe2O3 transformation series has been recognized in heated archeological samples, which may have implications in terms of their thermal history and in the factors that govern the formation of -Fe2O3.


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© 2017. American Geophysical Union.
This work has been supported by the MICINN through projects MAT2012-38045-C04-03, MAT2013-48009-C04-01, and CGL2014-54112-R. J.L.S. thanks the FPI fellowship for predoctoral fellowship. Constructive comments and suggestions from anonymous reviewers have highly improved the manuscript and are therefore gratefully acknowledged. Readers can find in the supporting information the low temperature magnetic behavior and the X-ray diffraction pattern corresponding to the HEL sample. A Raman spectra comparison of maghemite and hematite microparticles obtained from these specific samples, with pure maghemite and hematite microparticles coming from synthetic samples is also provided. Raman and rockmag data from HEL and CO samples used in this paper are available from the Web site: http://earthref.org/ERDA/2214/ as well as Raman data from synthetic epsilon, maghemite, and hematite.

Uncontrolled Keywords:Micro-raman; West-Africa; Thin-films; Spectroscopy; Maghemite; Ceramics; Rock; Archeointensity; Nontronite; Insights; Epsilon iron oxide; Archaeomagnetism; Rock magnetism
Subjects:Sciences > Physics > Materials
Sciences > Physics > Solid state physics
Sciences > Physics > Geophysics
Sciences > Physics > Meteorology
ID Code:45273
Deposited On:31 Oct 2017 16:52
Last Modified:23 Jan 2018 00:01

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