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Cristina Carbone, University of Genova (Italy)
Francesco Di Benedetto, Dipartimento di Scienze della Terra, (Italy)
Sangregorio Claudio, Dipartimento di Chimica (Italy)
Marescotti Pietro, DIP.TE.RIS (Italy)
Pardi Luca, CNR-IPCF (Italy)
Sorace Lorenzo, Dipartimento di Chimica (Italy)
Lucchetti Gabriella , DIP.TE.RIS (Italy)
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Hematite, α-Fe2O3, is an iron-oxide mineral belonging to the Oxides and Hydroxides group and it is, together with goethite, the most common iron oxide in Earth's crust and on Martian surface. Hematite is present in most magmatic, metamorphic, and sedimentary rocks, showing an extreme variability of mineral associations, textures and crystal size, ranging from a few nanometers up to several centimeters. Hematite weathering, when exposed to the atmosphere, allows iron to enter in a global cycle, where secondary Fe-oxyhydroxides play a major role, especially in the pedosphere (Cornell & Schwertmann, 1996). The release of iron in the environment affects also the cycling of many other elements (Zhang et al., 2001). A recent rise of interest involved the discovery of hematite on the Mars surface, due to the possible relationship between hematite and (former) water (Jakosky, 1999).
The present study reports the results of a combined approach using morphologic characterization, magnetic measurements, and multifrequency electron magnetic resonance (9-285 GHz) in order to identify and characterize both the bulk and the nanoscale magnetic effects on a sample of synthetic hematite, α-Fe2O3. We show here that the heterogeneous nature of the investigated sample is reflected in some peculiarities of the EMR spectra. The use of multifrequency EMR spectroscopy allowed us to assign, for the first time, the signals due to different magnetic modes of the bulk hematite on a powdered sample, indicating a large predominance of this behaviour in the considered sample. At the same time, however, we observed a large decrease of the Morin transition temperature for a part of the sample, that we have attributed to the presence of a fraction of smaller particles. Moreover, the presence of a single domain superparamagnetic (SP) phase was also evidenced, due to a fraction of nanometric particles in the sample. These results correlated well with results obtained by morphological and dimensional characterization carried ou by TEM and HR-TEM analysis thus confirming the good complementarity of the different techniques.
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