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Gadolinite microfeatures constrained by pegmatite formation and alteration evolution

Gadolinite dominantly occurs in pegmatites, particularly in the REL-REE subclass (Černy & Ercit, 2005). The structural formula of gadolinite structural formula is ideally REE2Fe2+Be2Si2O10 with some Ca, Th and U substituting for REE. It crystallizes monoclinically (P21/a) (Miyawaki et al., 1984 ; Malczewski, 2010). As many other pegmatite minerals with complex structural and chemical properties, it is prone to alteration triggered by metamictization and chemical processes of metasomatic origin. The pegmatites of southern Norway are famous for their various mineral assemblages frequently containing REE minerals. In this study, three gadolinite samples from pegmatites of Sveconorwegian age (1.1-0.9 Ga) were investigated for their structural and chemical features in order to devise specific co-crystallization or post-crystallization origin of inhomogeneity observed in the samples. One sample is from a granitic pegmatite of Hidra island (SW Norway), which intruded anorthosite/leuconorite of the Rogaland Igneous Complex (Hetherington & Harlov, 2008). The second sample originates from the amazonite-bearing upper Høydalen pegmatite, which belongs to the Tørdal pegmatite field hosted by coarse-grained metamorphosed gabbroic rocks (Raade et al., 1993). The last one is from the Kåbuland pegmatite, situated in the Evje–Iveland pegmatite field and emplaced within the Iveland- Gautestad metaggabbroic complex (Pedersen & Konnerup-Madsen, 2000). All samples were provided by the Natural History Museum of the University of Oslo (courtesy of G. Raade). The gadolinite samples were analysed using X-ray powder diffraction (XRD), scanning electron microscope in BSE mode equipped with an EDS system (SEM-EDS), and Raman spectroscopy with 532 and 633 cm-1 excitation. XRD measurements indicate gadolinite as only mineral phase present in the samples, however, strong metamictization is observed in the case of the samples from Høydalen and Kåbuland. As thermally annealed in the range from 400 to 1000°C, the metamict samples regain gadolinite structure without co-crystallization of other mineral phases. Structural healing is supported by an intensity increase of diffraction maxima and a decrease of their FWHM values as annealing temperature increases. This observation also applies to the sample from Hidra, which does not display a XRD pattern typical for a metamict mineral before thermal treatment. All the samples, when annealed at 1000°C, yield the following range for unit cell parameters: a = 0.9949 – 0.9964 nm, b = 0.7513 – 0.7518 nm, c = 0.4734 – 0.4740 nm,  = 90.01 – 90.06°, V = 0.3538 – 0.3550 nm3. Although XRD data identified only gadolinite, the SEM-EDS study showed a significant mineral inhomogeneity in all three gadolinite samples. Three types of mineralogical and chemical inhomogeneities are observed: (1) Primary and possibly secondary silicate inclusions (plagioclase, Kfeldspar) ; (2) Growth zonation of gadolinite-(Y) ; and (3) Altered gadolinite with a decreased REE and increased Ca content, sometimes accompanied with occurrence of ThSiO4 (Fig. 1). Raman spectra are not entirely analogous for all three samples, but two bands readily occur in all of them: 360-375 cm-1 (bending) and 870-890 cm-1 (stretching). Interestingly, these bands also occur in two metamict samples, implying local structure preservation that is supported by a straightforward recrystallization of the gadolinite crystal structure in annealing experiments. Band intensity increase of the fundamental vibrations in the altered Ca-rich domains of Hidra gadolinite is observed. This might be related to a substitution of Ca for REE, which could cause improvement of vibration statistics for particular bands when compared to the spectra of the unaltered domains with higher REE content. Namely, a significant amount of REE can produce a spread of vibration frequencies for a certain vibration mode due to a wider range of interatomic distances inherited from various ionic radii of REE at their structural position. NGF Abstracts and Proceedings, No. 2, 2017 153 The inhomogeneity of the Hidra sample is characterized by deposition of K-feldspar in the fissures within gadolinite crystals and anhedral fluorite inclusions together with gadolinite alteration favouring Ca substituting for REE. The Høydalen sample presents a rather regular stripe-wise growth zonation of gadolinite composition, which reflects zonal Y distribution together with an entrapped euhedral plagioclase inclusion. The alteration features of the Kåbuland gadolinite indicate loss of Fe and gain in Ca within altered domains. ThSiO4 inclusions show a limited Y content, with Y probably being mobilized from the altered gadolinite along the intra-crystal micro fractures.

gadolinite-(Y), alteration, metamictization, pegmatites, Norway

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