Controlled time–temperature oxidation reaction in a synthetic Mg-hercynite

The oxidation of a synthetic hercynite with composition Fe2+0.699Mg0.301Al1.941Fe3+0.059O4 was investigated by X-ray single-crystal diffraction. Heating runs at 500°C up to 212 h did not produce detectable oxidation, but only a small variation in oxygen coordinate u, consistent with very limited reo...

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Bibliographic Details
Published in:Physics and chemistry of minerals 2005-06, Vol.32 (2), p.83-88
Main Authors: Lavina, B., Princivalle, F., Della Giusta, A.
Format: Article
Language:English
Subjects:
Aluminum
Atomic properties
Displacement
Hercynite
Magnesium
Mathematical analysis
Oxidation
Oxygen
Parameters
Point defects
Single crystals
Vacancies
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Summary:The oxidation of a synthetic hercynite with composition Fe2+0.699Mg0.301Al1.941Fe3+0.059O4 was investigated by X-ray single-crystal diffraction. Heating runs at 500°C up to 212 h did not produce detectable oxidation, but only a small variation in oxygen coordinate u, consistent with very limited reordering of Mg and Al in tetrahedral (T) and octahedral (M) sites, respectively. Oxidation began after the first run at 600°C, producing progressive decreases in u, cell parameter a and the mean atomic number in T site. After 1,842 h at 600°C, the transformation was close to equilibrium, with about 70% of ferrous iron transformed into ferric. This produced about 0.17 vacancies per formula unit, and caused a great increase in the displacement parameters of oxygen and M sites. Vacancies were strongly ordered in M sites, and the oxygen displacement parameter becames anisotropic, unlike stoichiometric spinels—because some of the oxygen coordination polyhedra have a vacancy instead of a cation in one vertex. The behaviour of displacement parameters in this case supports the calculated point defect distribution.
ISSN:0342-1791
1432-2021
DOI:10.1007/s00269-004-0438-3