Introduction

Ferriallanite-(Ce) is a mineral belonging to the allanite group of the epidote supergroup. The general crystal chemical formula is A(1)A(2)M(1)M(2)M(3)(SiO4)(Si2O7)O(OH), where A represents a 9- or 11-coordinated polyhedral site mostly occupied by Ca, lanthanoids (Ln), Y, Th, U4+ and Mn2+ and M are octahedral sites mostly occupied by Al, Fe3+ and Fe2+. Silicon occurs in isolated SiO4 tetrahedra and Si2O7 groups (Fig. 1). Minerals of the epidote supergroup are monoclinic, space group P21/m. Various solid solutions exist among endmembers, which are based on elemental substitutions in A and/or M positions (Dollase 1971; Bonazzi and Menchetti 1995; Gieré and Sorensen 2004; Armbruster et al. 2006; see Table 1).

Fig. 1
figure 1

Sketch of the crystal structure of allanite-group minerals. It consists of chains of edge-sharing M(1) octahedra and edge-sharing M(2) octahedra (red). These chains are linked by SiO4 tetrahedra and Si2O7 groups (yellow). Slightly deformed M(3) octahedra are shown turquoise. Large cavities in the framework belong to A(1) and A(2) polyhedra where elements such as Ca2+ or REE3+ are present in ninefold and 11-fold coordination to oxygen, respectively. Hydrogen atoms are marked “H”. Sketch based on Dollase (1971) and Reissner et al. (2019), drawn with the JP-Minerals VESTA v3 software (Momma and Izumi 2011)

Full size image
Table 1 Idealized formulae of selected members of the epidote supergroup, following the nomenclature of Armbruster et al. (2006) and Varlamov et al. (2019)
Full size table

The most common compositional relationships in the allanite-ferriallanite-epidote-clinozoisite solid solution can be expressed via the following substitutional vectors:

$$\mathrm{^ACa^{2+} + \;^{M(3)}Fe^{3+} \leftrightharpoons \;^{A}REE^{3+} +\; ^{M(3)}Fe^{2+}}$$
(1)
$$\mathrm{{^{M(1)}}Fe^{3+}\;\leftrightharpoons\; {^{M(1)}}Al^{3}}$$
(2)

These relationships can be depicted in a ΣREE versus Al diagram for such solid solutions proposed by Petrík et al. (1995), which also allows one to estimate the Fe2+/Fe3+ ratio. However, other substitutions involving Mn2+, Mn3+, Mg, Pb2+, V, Cr, Ti, Sn, Sc and others are possible as well (Dollase 1969; Grew et al. 1991; Bonazzi and Menchetti 2004; Ahijado et al. 2005; Miyawaki et al. 2008; Nagashima et al. 2013; Biagioni et al. 2019). The possibility of F– substituting for O2– in epidote-supergroup minerals has been reported as well (Peacor and Dunn 1988), but its extent tends to be generally low except from minerals of the dollaseite group (Pautov et al. 1993; Ercit 2002).

The crystal structure of both allanite-(Ce) and ferriallanite-(Ce) are almost identical to that of other epidote-group minerals (Ueda 1955; Dollase 1971; Franz and Liebscher 2004; Gieré and Sorensen 2004). A(1) is normally occupied by ninefold coordinated Ca, however, the polyhedron's uneven shape causes the coordination number to range from 7 to 11. The A1 and A2 sites are best represented as being ninefold and tenfold coordinated, respectively. For the allanites A(2) position is 11-fold coordinated and occupied by REE3+; individual mineral species are distinguished based on the dominating REE (Ercit 2002; Armbruster et al. 2006). Although Fe3+ occurs partitioned over the M(1) and M(3) octahedral sites in all iron-rich epidote supergroup minerals, it is the dominant cation at the M(1) position in ferriallanites, ferriandrosites and ferriakasakaites (Dollase 1973; Nozik et al. 1978; Kvick et al. 1988; Franz and Liebscher 2004; Gieré and Sorensen 2004; Nagashima et al. 2013). The distorted, large M(3) octahedron accommodates almost all of the Fe2+; however, some Fe2+ may also occur in the M(1) site (Dollase 1973; Kartashov et al. 2003). Among all epidote-supergroup minerals, Fe2+ is an essential constituent in allanite, ferriallanite, vanadoallanite and ueadite where the presence of Fe2+ contributes to an increased unit-cell volume and affects optical properties. According to Kartashov et al. (2003), ferriallanite-(Ce) has the largest unit cell of all REE-rich minerals belonging to the epidote supergroup.

The radionuclides Th and U readily enter the crystal structure of allanite-group minerals, owing to their similar ionic radii with those of REE. The radioactive decay of Th and U causes structural damage. Even minor amounts of radionuclides may thus lead to a transformation from the crystalline to a glassy state over geological periods of time; such glassy state is referred to as metamict (Ewing 1994). Metamict minerals generally have lowered chemical resistance and are more susceptible to secondary alteration processes, compared to their crystalline counterparts. This is especially true for fluid-driven chemical alteration that may result in substantial compositional changes (Gieré and Sorensen 2004; Bonazzi et al. 2009). The accumulation of radiation damage also causes changes of physical properties, such as volume expansion that is detected from shifts of X-ray diffraction maxima to lower 2Θ values (Ewing et al. 1987; Janeczek and Eby 1993; Reissner et al. 2019). Decrease of the short-range order may be detected using Raman spectroscopy. For instance, Raman spectra of radiation-damaged natural zircon show increased band broadening accompanied by band downshifts, depending on the self-irradiation dose. In some cases, band broadening can be used to estimate the degree of radiation damage, provided significant band broadening due to chemical variations is excluded (Nasdala et al. 1995).

Allanite-group minerals are common accessory phases in granitic pegmatites, granites, granodiorites, syenites, diorites, etc. (e.g., Dollase 1971, 1973; Deer et al. 1986; Broska et al. 2000). They can also occur in various metamorphic rocks, especially metasomatic rocks (Deer et al. 1986; Sorensen 1991; Wing et al. 2003) and skarns. The Nya Bastnäs deposit located in Skinnskatteberg, Bergslagen ore region, Sweden, belongs to the Bastnäs-type Fe-REE deposits. Here, typical Fe skarn hosts pods or layers of massive REE mineralisation. The dominant host rock is a metasomatized volcanic rock interlayered with rocks dominated by quartz, occurring adjacent to ore bodies (Holtstam et al. 2014). The allanite-group minerals are assumed to be of metasomatic origin (Andersson 2004; Holtstam et al. 2014; Sahlström et al. 2019). Initially exploited as a source of Cu and Fe, this deposit, in the nineteenth century, became a significant source of REE ores, with cerite-(Ce), bastnästite-(Ce) and other REE-rich phases (Holtstam et al. 2003; Andersson, 2004). Other minerals described from this locality are tremolitic amphibole, accessory sulphides and oxides such as chalcopyrite, bismuthine, molybdenite and magnetite. In some rare cases uraninite is present locally (Holtstam and Ensterö 2002; Ensterö 2003; Andersson 2004). The specimen investigated in the present study (Fig. 2) originates from a massive REE mineralisation consisting of epidote-supergroup minerals and minor gadolinite-(Nd) in actinolite-tremolite matrix.

Fig. 2
figure 2

A specimen containing ferriallanite-(Ce), as a dark brown to black layer up to 3 cm wide, in contact with greenish-white, fibrous amphibole and newly formed weathering products around the edges, from Nya Bastnäs, Sweden. Photo by Kamil Sobek

Full size image

The main aim of the present study was to expand our knowledge regarding crystal-chemical characteristics of allanite-group minerals. To the best of our knowledge, no detailed spectroscopic studies of ferriallanites samples have been undertaken thus far, and the only spectra of ferriallanite-(La) that have been published are of rather low quality (Varlamov et al. 2019). To provide some rationale for the relationship between Raman spectra and composition, the combination of Raman spectroscopy with EPMA was chosen as main tools. To provide basic information about the relative changes of elements distribution in the sample, LA-ICP-MS served as a supplementary method to visualise such changes in the form of qualitative maps. Also, REE-bearing minerals often show distinct luminescence that may be used as a helpful fingerprinting tool for mineral identification. The photoluminescence-based detection of allanite–(Ce) may be possible based on the Nd3+ emission in the near-infrared range (about 10,600–11,900 cm−1; see for instance Lenz et al. 2015; Zeug et al. 2017). Therefore, laser-induced photoluminescence (PL) was also included.

Materials and methods

Samples and preparation

We have investigated a 6 cm long specimen of ferriallanite-(Ce) layer in amphibole matrix originating from a mine dump near Nya Bastnäs, Skinnskatteberg area, Sweden (Fig. 2). Two other samples were studied as references for comparison, metamict allanite-(Ce) from a pegmatite near Domanínek in Czech Republic (Čermák 2013) and acicular, well-crystallized allanite-(Ce) of hydrothermal origin from Oßling, Germany (Nasdala 1993). The Bastnäs specimen was cut into several polished sections (100 μm thickness) and attached to a glass slide. Some fragments were heated in air to check for possible annealing of radiation damage. For this, the fragments were placed in a Pt crucible and heated to 280 °C at a rate of 30 °C/min. At the end of a 96-h run, the furnace was switched off and the sample was allowed to cool to below 50 °C before taken out of the oven. Reference samples were also embedded in epoxy and polished. For EPMA analysis, samples were coated with carbon.

EPMA

Chemical spot analyses, line scans and back-scattered electron (BSE) images were obtained using a CAMECA SX-100 system operated in wavelength-dispersive X-ray spectrometry mode. The accelerating voltage was 15 kV, the beam current 20 nA, and the focal spot of the electron beam had a diameter of 2 μm. The following natural and synthetic materials (analysed X-ray line in parentheses) were used for calibration: Ti (Kα)—anatase; Fe (Kα)—almandine; Mn (Kα)—rhodonite; Ca, Si (Kα)—wollastonite, Na (Kα)—albite; P (Kα)—Ca5(PO4)3F; Al (Kα)—sanidine; Mg (Kα)—Mg2SiO4; F (Kα)—topaz, Cl (Kα)—vanadinite; Y (Lα)—YAG; Zr (Lα)—ZrSiO4; La, Ce (Lα); Pr, Nd, Sm, Gd, (Lβ)—individual La–Yb orthophosphates. Peak counting times (same as their background counting times) varied from 10 to 100 s depending on the element. Raw X-ray intensities were processes for matrix corrections using X-PHI algorithm of Merlet (1994). An empirically determined correction factor was applied to the coincidence of the 2nd order of Ce–Mβ with the FKα line, Ce–Lγ with the Tm–Lα, and with the Yb–Lα line. In addition, Na2O, Sc2O3, P2O5, SnO2, ThO2, UO2, Dy2O3, Ho2O3, Er2O3, ZrO2, Eu2O3, Tm2O3, Yb2O3, Lu2O3 and PbO were sought but their contents were below the EPMA detection limits.

LA-ICP-MS

LA-ICP-MS analyses were performed by using a pulsed Nd:YAG laser system LSX-213 G2 + working at 213 nm wavelength and using a pulse duration of ∼4 ns. The ablation system is equipped with a 2-volume ablation cell (HelEx) designed to enable rapid eluting of the ablation-generated aerosol in a large format cell. Helium was used as a carrier gas with a flow rate of 0.9 l/min. The aerosol was transported from the ablation cell through a 1 m long FEP (Fluorinated Ethylene Propylene) transport tube (2 mm in diameter) to the ICP discharge of the quadrupole mass spectrometer ICP-MS Agilent 7900. The mass spectrometer operated at the forwarded power of 1550 W and Ar gas flow rate of 15 l/min (outer plasma gas). The analysis was performed by a set of continuous line scans using laser pulses covering a defined area of the sample to create qualitative maps. Qualitative maps were obtained with the spot diameter of 4 µm, fluence of 10 J·cm−2, scan speed of 4 µm/s and repetition rate of 20 Hz. The following isotopes were monitored during the LA-ICP-MS scan measurements with the total integration time of 1 s: 24Mg, 27Al, 29Si, 31P, 43,44Ca, 55Mn, 57Fe, 88Sr, 89Y, 90Zr, 139–159Ln, 232Th, 238U. Data were processed in ILaps by Faltusová et al. (2022).

Raman and photoluminescence spectroscopy

Raman and PL spectra, including line scans, were obtained at room temperature by means of a Horiba Jobin Yvon LabRam-HR Evolution. The system was equipped with an Olympus BX41 optical microscope, a Si-based, Peltier-cooled charge-coupled device detector, and a diffraction grating with 1800 grooves per millimetre. Spectra were excited with the 473 nm emission of a diode laser (10.5 mW at the sample surface). Attempts to obtain PL spectra were also made with 532, 633 and 785 nm laser excitation. The instrument was calibrated using the Rayleigh line, resulting in a wavenumber accuracy of 0.5 cm–1. An Olympus 100 × objective (numerical aperture 0.90) was used. Spectra were obtained in a range 110–4000 cm−1 in confocal mode; the lateral resolution was better than 1 μm. The spectral resolution was better than 1.2 cm–1. Line scans were done using an automated, software-controlled x–y stage. All spectra were obtained in areas about 10 μm away from EPMA points (to avoid unwanted effects of the electron-beam impact). Band fitting was done after appropriate background correction, assuming Lorentzian-Gaussian band shapes.

Results

General description

Ferriallanite-(Ce) from Nya Bastnäs shows irregular zoning, which is even visible by the unaided eye. When observed in plane-polarised transmitted light, a wide range of colour sectors (ranging from light green to dark brown) with intensive pleochroism are seen. In cross-polarised transmitted light, these sectors have high second-order interference colours that are, however, widely masked by body colour (Fig. 3).

Fig. 3
figure 3

Pairs of plane-polarised (left) and cross-polarised (right) transmitted-light photomicrographs. a Aggregate consisting of several randomly oriented ferriallanite-(Ce) crystals, originating from Nya Bastnäs. b Edge of the ferriallanite-(Ce) aggregate. The neighbouring, acicular amphibole is seen on the lower left side. c This metamict allanite-(Ce) from Domanínek is shown for comparison. Note the nearly complete absence of interference colours

Full size image

Crystal chemistry

BSE imaging revealed detailed zonation patterns that reflect variations in chemical composition. Textural relations indicate that interior regions with primary growth zoning were replaced by patchily zoned domains. These domains were chosen for more detailed examination (see lines 1 and 2 in Fig. 4). Differences is BSE intensity reflect mainly the variation of Ca (9.5–16.1 wt% of CaO) and REE (14.6–27.1 wt% of REE2O3), which are accompanied by changes in total Fe oxide (15.4–19.1 wt%) and Al2O3 (11.9–17.4 wt%). Results of chemical analyses are summarised in Table 2 (means and ranges), and all individual analysis results are provided in Tables S1–S5 in the electronic supplementary material (ESM).

Fig. 4
figure 4

BSE image of showing internal heterogeneity of ferriallanite-(Ce) from Nya Bastnäs. There are interior regions with well-developed zoning. Abbreviations of mineral names: Ccp – chalcopyrite, Mag – magnetite

Full size image
Table 2 Ranges of major-element compositions (EPMA results in wt%) of ferriallanite-(Ce) from Nya Bastnäs, in comparison with two allanite-(Ce) reference samples
Full size table

Ferriallanite-(Ce) from Nya Bastnäs contains high solid solution contents of allanite and REE-rich epidote according to the Al (Al = 1.39–1.85 apfu), Fe (1.09–1.49 apfu) and low Mg (Mg = 0.04–0.19 apfu) contents. Values show compositional variation between Fe3+ (Fe3+  = 0.47–0.83 apfu) and Fe2+ (Fe2+  = 0.42–0.89 apfu). The mineral is generally Mn- and Ti-poor (Mn 0.01–0.02 apfu, Ti ≤ 0.01 apfu). Total REEs vary in the range 0.47–0.96 apfu. Note that concentrations of light REEs are significant, up to about 20 wt%, whereas heavy REEs were close to or even below the EPMA detection limits. This corresponds to previous observations that in allanite-group minerals, light REEs are generally enriched (Vlach and Gualda 2007; Papoutsa and Pe-Piper 2015). Based on method proposed by Armbruster et al. (2006) crystal-chemical formulae were derived by assigning cations at the individual structural sites for two extreme compositions (circles pale grey and very dark grey, see Fig. 5). The pale grey colour represents the ferriallanite-(Ce) member of the allanite subgroup according to [M3+ + M4+]A2 > 0.50, [M2+]M3 > 0.50 with formulae A1(Ca)A2(REE,Y3+,Ca,Mn2+)M1(Fe3+,Al)M2(Al)M3(Fe2+,Mn2+,Mg,Ti)T1,2,3[Si3O12](OH); however, the variable chemistry also shows trends towards REE-rich epidote in the dark grey areas, where the darkest circles belong to both the allanite and the clinozoisite subgroup by meeting the additional condition [M2+]A2 > 0.5 with formulae A1(Ca)A2(Ca,REE,Y3+)M1(Fe3+,Al)M2(Al)M3(Fe2+,Mn2+,Mg,Ti,Al)T1,2,3[Si3O12](OH). The presence of U and Th was below the detection limits of EPMA (0.09 wt% and 0.07 wt%, respectively).

Fig. 5
figure 5

Chemical composition of ferriallanite-(Ce) and its variations along the two arrows shown in Fig. 4 with all points (total 51 analyses). a Plot of Ca against Al. b Plot of REE against Al. Radial lines emanating from the clinozoisite composition allow to estimate the portion of Fe3+ in total Fe (central y axis; Petrík et al. 1995). The grey levels of the symbols visualise the observed BSE intensities

Full size image

Data obtained by LA-ICP-MS correlate with EPMA results, especially in REE-rich and REE-poor zones. The edge of the BSE-zoned area (marked with white lines in Fig. 6) has slightly increased contents of Y and U. Yttrium and Th are found in elevated concentrations also in chaotically zoned interior regions surrounding the primary growth zonation. Other elements detected did not show significant spatial variations.

Fig. 6
figure 6

Trace-element distribution patterns obtained by LA-ICP-MS from strongly zoned ferriallanite-(Ce). As a visual guide, the edge of the BSE-zoned area is marked by a white line. Note that BSE intensity and La and Ce content correlate

Full size image

Spectroscopy

Raman line scans across REE-rich and REE-poor primary growth zones revealed minor variations in spectral positions of Raman bands, not exceeding 13 cm–1; there are also accompanying minor variations of band shapes and relative intensities. Changes in band positions are particularly noticed in the spectral ranges 800–1100 cm–1 and 500–600 cm–1. An example for accompanying changes in band shapes is the triplet of bands around 430 cm–1. It should be noted that, in spite of ferriallanite-(Ce) contains OH– groups, no significant O–H stretching signal was detected.

The Raman spectrum of the fragment of ferriallanite-(Ce) that was annealed at 280 °C for 4 days is largely similar to the spectrum of the untreated counterpart (Fig. 7). Both spectra are somewhat similar to the Raman spectrum of fresh allanite-(Ce) from Oßling, Germany. In contrast, metamict allanite-(Ce) from Domanínek, Czech Republic, yielded a notably different Raman spectrum that is typical of glassy silicates, with a weak band around 950 cm–1 (Fig. 7).

Fig. 7
figure 7

Raman spectra obtained from unannealed and annealed chips of the Bastnäs (Sweden) ferriallanite-(Ce), in comparison with spectra of a well-crystallized (Oßling, Germany) and a metamict (Domanínek, Czech Republic) allanite-(Ce) reference. Note that spectra of the unannealed and annealed Bastnäs chips are widely similar, which excludes significant annealing-induced structural reconstitution and hence supports the generally low level of radiation damage in this sample

Full size image

Untreated and annealed fragments of the sample were also checked for possible luminescence using blue, green, red and near infrared laser excitation. No significant emission was detected.

Discussion

Results of chemical analyses of allanite-group minerals were converted to mineral formulae using normalization of the sum of metal cations to 8 and total charge to 25. This approach allows one to estimate the Fe3+/Fe2+ ratio, under the assumption that there are no A- and M-site vacancies and no “oxy-allanite” component present (Ercit 2002). Analytical totals close to 100 wt% and the virtual absence of elements that are characteristic for altered allanite-group minerals and common in radiation damaged specimens (Na, Pb, P, Ti; Ueda 1955; Deer et al. 1986), we may assume that our sample did not experience significant chemical alteration. This, however, appears to be in contrast to the irregular zoning exhibited by a major part of the sample. Our sample is much poorer in Mg and REEs and richer in Al, compared to ferriallanite-(Ce) from the Bastnäs deposit studied by Holtstam et al. (2003), which may indicate considerable chemical variability of this mineral.

Ferriallanite-(Ce) yielded Raman spectra that are somewhat similar to the fairly unsharp spectra of ferriallanite-(La) (Varlamov et al. 2019); however, our spectra show much more pronounced and well-defined bands. Spectra depend on chemical compositions of the sample, especially its REEs contents (Fig. 8). Stretching motions in sorosilicates for Si2O7 and SiO4 complexes occur as intense bands above 700 cm−1. These bands belong to the antisymmetric and symmetric stretching vibration of Si–Ob–Si (Ob—bridging oxygens) and of Si–Onb (Onb—non-bridging oxygens) and were observed in ferriallanite-(Ce) from 830 to 1100 cm–1 (compare Lazarev 1972; Le Cléac’h and Gillet 1990 or Makreski et al. 2007). According to Chukanov et al. (2014), bands above 1039 cm–1 are related to stretching vibrations of the Si–Ob–Si depending on the value of the Si–Ob–Si angle, where with the increasing angle, the frequency of corresponding vibrations increases as well. Varlamov et al. (2019) discussed that the force characteristics rather than the size of a cation in the A(2) site determine this angle; however, after a thorough examination of the crystal-chemistry of the epidote supergroup minerals, the change in Si1–O9–Si2 angle is related to the M3 octahedral volume rather than the A2-O bond strength (see Bonazzi and Menchetti 1995). This is especially noticeable in REE-bearing epidote supergroup minerals, where the Si1–O9–Si2 angle has lower values due to the entry of larger divalent cations at M3 rather than trivalent cations in A2, and is consistent with our observation of shifts in Raman bands in the 300–500 cm–1 region that are assumed to be assigned to external modes of M–O interactions (Makreski et al. 2007; see Fig. 9). Bands in the range 550–700 cm−1 were described by Wang et al. (1994) as symmetric stretching of Si-Ob-Si bonds.

Fig. 8
figure 8

Series of 12 Raman spectra obtained along 40 µm line (line 1 in Fig. 4) across zoned ferriallanite-(Ce). Most significant changes in band positions are highlighted by dashed lines

Full size image
Fig. 9
figure 9

Comparison of Raman shifts and chemical parameters. a Plot of the spectral position of the ~ 1045 cm–1 band (related to the Si–Ob–Si angle; Varlamov et al. 2019) against total REE. b Plot of the spectral position of the ~ 315 cm–1 band (related to a M–O external mode; Makreski et al. 2007) against Al. Dotted lines are visual guides

Full size image

Annealing of ferriallanite-(Ce) at 280 °C was mainly done to check for possible annealing-induced spectral changes that, if present, might indicate structural reconstitution of radiation damage. According to Reissner et al. (2019), structural recovery of radiation-damaged allanite-(Ce) is already significant at 280 °C. Annealing at higher temperatures is expected to be more efficient for structural reconstitution; however, potential dehydrogenation and Fe2+ oxidation (Janeczek and Eby 1993; Bonazzi and Menchetti 1994; Bonazzi et al. 2009; Reissner et al. 2019) might affect Raman spectra as well and hence bias interpretation. Inconspicuous differences between spectra of untreated and annealed samples (Fig. 7) suggest that radiation damage must be insignificant which is also evidenced by a comparison with the Raman spectrum of a metamict allanite-(Ce) sample from Domanínek. The observation that Raman bands of ferriallanite-(Ce) are generally broader than bands of non-radiation-damaged allanite-(Ce) from Oßling (Fig. 7) is assigned to “chemical band broadening” due to more elevated contents of cation substitutions (compare Ruschel et al. 2012). The virtual absence of luminescence, in particular of emissions related to REEs, in spite of significant concentrations of REE3+ (Table 2), is explained by quenching, most likely due to elevated concentrations of Fe2+ (Pascoal et al. 1999).

As demonstrated in a study of the migration of U and Pb in U-rich minerals (Holá et al. 2021), LA-ICP-MS imaging of individual isotopes can be advantageous in elucidating the evolution of geological processes. This is particularly beneficial for the detection of isotopes of trace elements with sub-ppm concentrations (Chew et al. 2021), which EPMA is unable to detect. Unfortunately, due to the very small dimensions of the investigated areas of interest and the measured signal-to-noise ratio, our results are merely semi-quantitative. The U and Th content analysed by LA-ICP-MS in ferriallanite-(Ce) from Bastnäs region was reported by Holtstam et al. (2003) as 2.6 ± 0.2 ppm for U and 0.25 ± 0.03 ppm for Th (in total ~ 3 ppm), which on our estimate according to the low signal intensity (counts per second) should correspond to the same values. Differences in signal intensity in and out of the zonation were observed; however, it is important to highlight the fact that due to a different orientation of the ferriallanite-(Ce) crystal in the studied profile, the material could be ablated differently (Tsibidis et al. 2020).

Conclusions

Results of chemical analyses combined with Raman spectroscopy and LA-ICP-MS imaging were found to be useful for studying the crystal chemistry of ferriallanite-(Ce) from Nya Bastnäs, Sweden. Raman spectra for ferriallanite-(Ce) are presented and discussed for the first time. Results of chemical analyses obtained across REE-rich and REE-poor zones show that BSE intensities of the studied ferriallanite-(Ce) correlate with the chemical composition, in particular the Ca/REE ratio. These compositional variations are also reflected in minor differences in Raman spectra and are especially noticeable for bands around the 1045 cm−1 that are related to vibrations of the Si–Ob–Si angle. The apparent absence of PL emissions, assigned to Fe2+-related quenching in ferriallanite-(Ce), shows that in some allanite-group species, the PL-fingerprinting technique cannot be used.