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Preliminary 57Fe Mössbauer spectroscopy study of metamict allanite-(Ce) from granitic pegmatite, Fone, Aust-Agder, Norway

Allanite-(Ce) is classified as a member of the epidote group, allanite subgroup, which is derived from clinozoisite by homovalent substitutions and one coupled heterovalent substitution: A2(REE)3+ + M3Fe2+  A2Ca2+ + M3Fe3+ (Armbruster et al., 2006). There are eight structurally different cation sites (A1-A2, M1-M3, T1-T3) along with 10 anion sites (O1-O10) (Dollase, 1971). A1 and A2 are large, 10- and 11- coordinated sites, respectively, where large ions are located. A1 site usually accommodates Ca, while A2 accommodates REE+actinides ; M1-M3 are octahedrally coordinated, more or less distorted sites which accommodate smaller cations (Al, Fe3+, Mn3+, Cr3+, V3+, Fe2+, Mg2+, Mn2+) (Dollase, 1971 ; Armbruster et al., 2006). M1 and M2 are moderately distorted sites. M2 accommodates exclusively Al, while M1 accommodates divalent and trivalent cations (Dollase, 1971 ; Armbruster et al., 2006). M3 site is the most distorted and accommodates larger cations, e.g. Fe2+, (Kartashov et al., 2002 ; Armbruster et al., 2006). T site is tetrahedrally coordinated and usually accommodates Si4+ (Dollase, 1971 ; Armbruster et al., 2006). In this study, Mössbauer spectra were collected on one treated sample: natural (ALF), annealed for 24 h at 650°C in air (ALF_650), and hydrothermally treated for 2 h at 150°C (ALF_AK) in order to possibly determine site occupancy, iron valence and coordination number, and to observe processes connected with possible changes in the valence of iron due to treatment. Mössbauer spectra of allanite-(Ce) sample are complex and consist of several doublets (fig. 1), i.e. more than one position and/or oxidation state is present in the crystal structure. Unshaded doublets (fig. 1) represent Fe3+ cations in different positions ; light grey shaded doublets represent Fe2+ cations in different positions. The hyperfine parameters for samples ALF and ALF_AK are: CS = 0.323(2) -1.03(5) mm/s ; QS = 0.89(6) - 2.6(1) mm/s and CS = 0.33(1) - 1.01(3) mm/s ; QS = 0.86(4) - 2.45(9) mm/s, respectively. The data are very similar for both samples (fig. 1) which indicates that no significant change, i.e. oxidation or diffusion of iron, occurred during the hydrothermal treatment of the sample. Fe3+/Fetotal ratios are virtually the same in both samples, 0.75, which confirms that no oxidation occurred during the treatment. As for the sample annealed at 650°C in air (ALF_650), all iron was oxidized to Fe3+ (fig. 1, fig. 2) which is consistent with the literature (Dollase, 1973). The hyperfine parameters for ALF and ALF_AK are roughly consistent with data from literature (Kartashov et al., 2002 ; Malczewski & Grabias, 2008 ; Nagashima & Akasaka, 2010 ; Škoda et al., 2012 ; fig. 2), hence it could be possible to attribute different doublets to specific crystallographic sites. Thus, in this investigation, both Fe3+ and Fe2+ could be assigned to both M1 and M3 positions (fig. 2). Nevertheless, it is not possible to unambiguously assign different doublets to specific crystallographic sites without additional data, e.g. microprobe or crystal structure refinement data. It can be seen that, while data from several sources analyzing epidote (Nagashima & Akasaka, 2010), allanite-(Ce) (Malczewski & Grabias, 2008) and allanite-(Nd) (Škoda et al., 2012) are extremely close and the doublet assignments are the same, the situation is a little bit different for ferriallanite-(Ce) (Kartashov et al., 2002) (fig. 2). Thus, additional investigations are necessary in order to unambiguously assign the doublets to different crystallographic positions. Dark grey doublets (fig. 1) represent Fe2O3, which is an impurity in allanite and could be easily distinguished from paramagnetic Fe3+ and Fe2+ in Mössbauer spectra due to the marked differences in hyperfine parameters (CS = 0.41(2) mm/s and QS = -0.21(5) mm/s ; fig. 2). These are minor impurities since this phase was not observed in the XRPD pattern (Čobić et al., 2010, sample N1) possibly due to nanometer grain size as a result of metamictization induced alterations. Also, this phase remains stable under both annealing in air and hydrothermal treatment.

allanite-(Ce); metamictization; 57Fe Mössbauer spectroscopy

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