Abstract
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.
Similar content being viewed by others
References
Andreozzi G, Lucchesi S (2002) Intersite distribution of Fe2+ and Mg in the spinel (sensu strictu)-hercynite series by single-crystal X-ray diffraction. Am Mineral 87:1113–1120
Brown ID (2002) The chemical bond in inorganic chemistry: the bond valence model. In: Series: international union of crystallography monographs on crystallography, no. 12. Oxford University Press, pp 288
Chassagneux F, Rousset A, Redoules JP (1985) Elaboration et caracterisation de cromites de fer(III) a structure spinelle lacunaire. J Sol State Chem 56:74–83
Colombo U, Fagherazzi G, Gazzarini F, Lanzavecchia G, Siboni G (1964) Mechanisms in the first stage of oxidation of magnetites. Nature 202:175–176
Ferguson GAJ, Hass M (1958) Magnetic structure and vacancy distribution in γ Fe2O3 by neutron diffraction. Phys Rev 112(4):1130–1131
Figueiras J, Waerenborgh JC (1997) Fully oxidised chromite in the Serra Alta (South Portugal) quarzites: chemical and structural characterization and geological implications. Miner Mag 61:627–638
Goss CJ (1988) Saturation magnetisation, coercivity and lattice parameter changes in the system Fe3O4-γ-Fe2O3 and their relationship with structure. Phys Chem Min 16:164–171
Kullerud G, Donnay G (1969) Omission solid solution in magnetite: kenotetrahedral magnetite. Zeit Krist B128:1–17
Lavina B, Salviulo G, Della Giusta A (2002) Cation distribution and structure modelling of spinel solid solutions. Phys Chem Min 29:10–18
Lindsley DH (1976) The crystal chemistry and structure of oxide minerals as esemplified by the Fe-Ti oxides. In: Rumble D (ed) Oxide minerals. Min Soc Am L1–L24
Lucchesi S, Della Giusta A (1994) Crystal chemistry of non-stoichiometric Mg-Al synthetic spinels. Zeit Krist 209:714–719
Menegazzo G, Carbonin S (1998) Oxidation mechanisms in Al-Mg-Fe spinels. A second stage: α-Fe2O3 exolution. Phys Chem Min 25:541–547
Menegazzo G, Carbonin S, Della Giusta A (1997) Cation and vacancy distribution in an artificially oxidised natural spinel. Min Mag 61:411–421
North A, Phillips D, Scott-Mattews F (1968) A semi-empirical method of absorption correction. Acta Cryst A24:351–352
O’Reilly W, Banerjee K (1967) The mechanism of oxidation in titanomagnetites: a magnetic study. Min Mag 36:29–37
Sheldrick G (1993) Shelxl-93. Program for crystal structure refinement. University of Gottingen, Germany
Tilley RJD (1987) Defect crystal chemistry and its applications. Blackie, Glasgow
Acknowledgements
Thanks are due to the reviewers J.M. Hughes and S. Lucchesi. The sample was kindly supplied by G.B. Andreozzi . This work was supported by MIUR Grant (A. Della Giusta: Cofin 2001: intracrystalline ordering-disordering process in rock-forming minerals; F. Princivalle, Cofin 2001: Crystal chemistry and cation exchange reaction in spinels, olivines and pyroxenes). The English text was revised by Ms G. Walton.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Lavina, B., Princivalle, F. & Della Giusta, A. Controlled time–temperature oxidation reaction in a synthetic Mg-hercynite. Phys Chem Minerals 32, 83–88 (2005). https://doi.org/10.1007/s00269-004-0438-3
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00269-004-0438-3