Structure and microstructure evolution of hercynite spinel (Fe2+Al2O4) after annealing treatment

Structure and microstructure evolution of hercynite spinel (Fe 2+ Al 2 O 4 ) after annealing treatment ILONA JASTRZĘBSKA 1,* ,JACEK SZCZERBA 1 ,ARTUR BŁACHOWSKI 2 and PAWEŁ STOCH 1 1 AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Department of Ceramics and Refractories, al. Mickiewicza 30, 30-059 Kraków, Poland *Corresponding author, e-mail: [email protected] 2 Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, ul. Podchorązych 2, 30-084 Kraków, Poland Abstract: The Fe–Al end-member spinel hercynite was prepared by the arc plasma synthesis (APS) technique and subjected to annealing at 1273 K for 1, 24 and 36 h in air atmosphere. The X-ray diffraction (XRD) analyses conducted before and after the thermal treatment revealed an evident alteration of the original hercynite. About half of the original spinel is preserved after the longest annealing experiment (36 h), the reminder of the Fe–Al spinel underwent decomposition into iron and aluminum oxide phases, among which non-stoichiometric magnetite and corundum were the most important. Mössbauer spectroscopy revealed that the initial hercynite was a normal spinel with a slight inversion degree of 11 %, which was increased to 34 % after the 36-h annealing process. An overlapping of reflections for spinel-like phases gFe 2 O 3 and gAl 2 O 3 with Fe–Al spinel impeded their detection by XRD. Mössbauer spectroscopy (MS) confirmed the presence of magnetite in the annealed samples. The SEM-EPMA observations of the oxidized surfaces of hercynite showed well-developed, scalenohedral crystals with different Fe/O ratios, and well defined compositions corresponding to Al-bearing Fe 2 O 3 as measured by WDS spectroscopy. The results of XRD, MS and SEM-EPMA permit to establish a general oxidation mechanism of hercynite decomposition. Key-words: hercynite; spinel; FeAl 2 O 4 ; oxidation; structure; arc plasma synthesis; Mössbauer spectroscopy; SEM; EDS; XRD. 1. Introduction Hercynite is a spinel mineral with the end-member formula Fe 2þ Al 2 O 4 that forms under Earth's conditions of relatively high temperature and pressure (e.g. Shulters & Bohlen, 1989). Hercynite solid solutions are encountered in many geological environments, both terrestrial and extraterrestial. Regarding the former occurrence, hercynite was found in high-grade metamorphic rocks (Bernier, 1990; Anthony et al., 1997; Allaby, 2013) (like granulites, alumina-rich xenoliths) as well as igneous mafic and ultramafic rocks like eclogites (Anthony et al., 1997; Bowles et al., 2011; Gargiulo et al., 2013). ‘Pleonaste’ is a variety of spinel forming as a solid solution between hercynite and spinel sensu stricto (FeAl 2 O 4 –MgAl 2 O 4 ) (Allaby, 2013). Inclusions of ‘pleonaste’ were reported to occur in magmatic olivine, pyroxene, plagioclase, native iron (Ryabov et al., 2000) or metaperidotites (Gargiulo et al., 2013). Furthermore, they were found in a few types of meteorites (Fahey et al., 1994; Treiman, 2005). Hence, the occurrence of hercynite does not relate to economically effective deposits, entailing today many industrial challenges for the development of its synthesis, which is often problematic by the traditional solid-state reaction (Gill, 2010; Chen et al., 2011). The spinel ternary oxides with a general formula AB 2 O 4 (so-called 2–3 spinels), where A and B represent metal cations, were extensively studied for their multiple properties like magnetic (Harrison & Putnis, 1999), electric (Lagg et al., 2004), multiferroic (Cótica et al., 2008), refractory (Szczerba et al., 2014) or ceramic pigment (Di Benedetto et al., 2006). Furthermore, they are valued gemstones being used in the jewellery industry (Verma, 2014). Moreover, the possibility of cation sublattice to exchange position with increasing temperature contributes to the potential usage of hercynite as geothermometer. Additionally, due to the possible presence of Fe 3þ in the lattice and to partitioning of Fe 2þ and Fe 3þ among tetrahedral (T d ) and octahedral (O h ) sites, the iron incorporation can be used as an indicator of oxygen fugacity f O 2 in rocks (Dyar et al., 1989). Hercynite belonging to the spinel sensu lato oxide group with Fd3m symmetry (space group no. 227) is a mineralogical name for a compound with the chemical formula FeAl 2 O 4 adopting a normal cation distribution at low temperatures (Hill, 1984; Harrison et al., 1998; Andreozzi & Lucchesi, 2002; Bowles et al., 2011). The normal cation distribution means that Fe 2þ ions occupy 8 tetrahedral sites (out of 64 T d sites), while Al 3þ ions fill up 16 octahedral sites (out of 32 O h sites) in the spinel unit Eur. J. Mineral. 2017, 29, 63–72 Published online 14 October 2016 DOI: 10.1127/ejm/2017/0029-2579 0935-1221/17/0029-2579 $ 4.50 © 2017 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart eschweizerbart_xxx

Structure and microstructure evolution of hercynite spinel (Fe2+Al2O4) after annealing treatment

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