Occurrence

The Malmkärra iron mine is situated close to the tarn Stora Malmtjärnen, Norberg, Västmanland County, Sweden (60°3’34’’N, 15°50’45’’E, 200 m a.s.l.). The deposit is hosted in a synclinal marble layer (dolomite), intercalated with felsic metavolcanic beds, and is divided into separate ore bodies due to local faulting (Geijer, Reference Geijer1936). The underlying bedrock consists of plastically deformed mica-schist, with major quartz, muscovite and cordierite, along with accessory tourmaline, magnetite and allanite-(Ce). It grades into a Na-rich, less altered metavolcanic rock in certain sections. The magnetite-skarn ore has locally replaced the carbonate along its contact with the country rock (Geijer, Reference Geijer1936; Holtstam et al., Reference Holtstam, Andersson, Broman and Mansfeld2014).

The oldest record of the Malmkärra mine is from 1664. Magnetite ore was mined (~100,000 t of Fe) until production stopped in 1936 (Geijer and Magnusson, Reference Geijer and Magnusson1944). Geijer (Reference Geijer1927) first noted the REE mineralisation, later referred to as Bastnäs-type (Geijer, Reference Geijer1961). The mineralisation occurs as bands of REE-bearing silicates deposited between magnetite-rich, tremolite-bearing skarns and dolomitic marble, with patches of Mg silicates (humite and serpentine) in recrystallised carbonate rock (‘ophicalcite’). Malmkärra is the type locality for three rare mineral species so far: västmanlandite-(Ce) (Holtstam et al., Reference Holtstam, Kolitsch and Andersson2005), ulfanderssonite-(Ce) (Holtstam et al., Reference Holtstam, Bindi, Hålenius, Kolitsch and Mansfeld2017) and gadolinite-(Nd) (Škoda et al., Reference Škoda, Plášil, Čopjaková, Novák, Jonsson, Galiová and Holtstam2018). According to a recent estimation (Rönnåsen, Reference Rönnåsen2023), there is still 83,000 t of waste rock present in the mine area, with an average of 0.55% REE.

Fluorbritholite-(Nd) was discovered in a boulder from the old dumps (Fig. 1). All observed occurrences of the new mineral relate to a thin skarn zone situated at the contact between the ‘ophicalcite’ and magnetite ore. Associated minerals in this skarn band include calcite, dolomite, magnetite, lizardite, talc, fluorite, baryte, scheelite and gadolinite-(Nd), accompanied by an allanite-group mineral and a gatelite-group mineral.

Figure 1.

Colour image of cut slab of boulder from Malmkärra, GEO-NRM #20220221. Dark spots are mostly serpentine, greyish areas carbonate. Fluorbritholite-(Nd) was obtained from the magnetite-rich part encircled.

The Palaeoproterozoic (Orosirian) Bastnäs-type REE−Fe (± Cu, Bi, Mo, Co and Au) skarn mineralisations in the area originated from reactions involving hot (T > 400°C) magmatic–hydrothermal fluids containing REE−Si−chloro−fluoro ion complexes and primary carbonate (Holtstam and Broman, Reference Holtstam and Broman2002; Holtstam et al., Reference Holtstam, Andersson, Broman and Mansfeld2014; Sahlström et al., Reference Sahlström, Jonsson, Högdahl, Troll, Harris, Jolis and Weis2019).

Appearance and physical properties

Fluorbritholite-(Nd) has an irregular morphology, forming anhedral grains of small size (rarely up 250 μm across, Fig. 2). The colour is brownish pink, with a white streak. The crystals are transparent with a vitreous to greasy lustre. The hardness (Mohs) is estimated as 5, in analogy with fluorbritholite-(Ce). Fluorbritholite-(Nd) is brittle with an uneven or subconchoidal fracture. No parting or cleavage have been observed. Density was not measured because of the minute sample size; the calculated value is 4.92(1) g⋅cm⁻³. In a petrographic thin section, the mineral is nonpleochroic and uniaxial (–). The refractive indices were not measured due to limited amount of type material (only tiny fragments could be extracted from the one large grain in the type sample). Overall n _calc = 1.795 from Gladstone–Dale coefficients (Mandarino, Reference Mandarino1981), very close to the highest available index liquid (1.80).

Figure 2.

Back-scattered electron image of fluorbritholite-(Nd) (Fbri-Nd), an allanite-like mineral (Alnl), magnetite (Mag), fluorite (Flr) and dolomite (Dol). The white arrow points to a gatelite-like mineral. Sample GEO-NRM #20220221.

Chemical data

The chemical composition (Table 1) was determined using a JEOL JXA-8230 electron-microprobe operated in wavelength-dispersive mode (20 kV acceleration voltage, 10 nA sample current and 5 μm beam size). The number of spot analyses was 12. Natural and synthetic reference materials were used (Table 1). Measurements involved the following X-ray spectral lines and analysing crystals: Si and Al (Kα, TAP); Y and As (Lα, TAP); P, Ca and Cl (Kα, PETH); La, Ce and Lu (Lα, LIF); Gd and Tb (Lβ, LIF); Pr, Nd, Sm, Dy, Ho and Er (Lβ, LIFL); Yb (Lα, LIFL); F (Kα, LD1); Br (Kα, LIFL). Peak overlaps between various REE and F−Ce were corrected empirically. Thulium was not measured but calculated from chondrite normalisation. Iron, Ti, Al, Mg, U, Pb and Th were sought but found to be below the detection limit. The H₂O content was not measured directly because of the small sample volume; it was inferred from structural, chemical and spectroscopic data.

Table 1.

Chemical data (in wt.%) for fluorbritholite-(Nd).

*Calculated value from chondrite normalisation ** H₂O calculated on the basis of (OH + F + Cl + Br + O) = 13 apfu.

The empirical formula calculated on the basis of 8 cations can be written as: (Ca_1.62Nd_0.97Ce_0.83Y_0.52Sm_0.30Gd_0.23Pr_0.17La_0.16Dy_0.11Er_0.03Tb_0.03Ho_0.01Yb_0.01)_Σ4.99(Si_2.92P_0.08As_0.01)_Σ3.01O₁₂[O_0.48F_0.26(OH)_0.14Cl_0.10Br_0.02]_Σ1.00 The ideal mineral formula is Ca₂Nd₃(SiO₄)₃F, which requires (in wt.%) CaO 13.88, Nd₂O₃ 62.45, SiO₂ 22.31, F 2.35, F ≡ O − 0.99, sum 100.00.

Raman spectroscopy

Micro-Raman measurements were conducted using a Horiba (Jobin Yvon) LabRam HR Evolution instrument on a randomly oriented crystal. The sample was excited with frequency-doubled 532 nm and 785 nm Nd−YAG lasers utilising an Olympus 100 × objective (numerical aperture = 0.9). The lateral resolution of the unpolarised confocal laser beam was in the order of 1 μm. Raman spectra of the sample were collected through two acquisition cycles with single counting times of 45 s. Spectra were generated in the range of 100 to 4000 cm⁻¹ utilising a 600 grooves/cm grating and a thermoelectric-cooled charge-coupled device (CCD). The wavenumber calibration was performed using the 520.7 cm⁻¹ Raman band on a polished silicon wafer with a wavenumber accuracy usually better than 0.5 cm⁻¹.

The spectra are largely overwhelmed by fluorescence. There is a distinct peak at 852 cm⁻¹ in the 532 nm spectrum (Fig. 3), resulting from symmetric stretching of SiO₄ groups, and a weak one at 950 cm⁻¹ corresponding to a contribution from the minor PO₄ group (ν₁ symmetric stretching; cf. Boughzala et al., Reference Boughzala, Salem, Kooli, Gravereau and Bouzouita2008). A sample of P-rich fluorbritholite-(Ce) from Norra Kärr, Sweden, has bands close to these positions (RRUFF no. R070412). A weak signal at 3300–3400 cm⁻¹ in the 785 nm spectrum (Fig. 4) can be related to O−H stretching vibration modes.

Figure 3.

Raman spectrum of fluorbritholite-(Nd), obtained with a 532 nm laser.

Figure 4.

Raman spectrum of fluorbritholite-(Nd), obtained with a 785 nm laser.

X-ray crystallography

Single-crystal X-ray diffraction

Single-crystal X-ray diffraction data were collected with a Bruker D8 Advance from a 0.060 × 0.055 × 0.045 mm fragment, extracted from the largest available grain previously analysed with the electron microprobe (Fig. 1). The crystal structure of fluorbritholite-(Nd) was refined using SHELXL-2013 (Sheldrick, Reference Sheldrick2008) starting from the atom coordinates of fluorbritholite-(Y) (Pekov et al., Reference Pekov, Zubkova, Chukanov, Husdal, Zadov and Pushcharovsky2011) to R ₁ = 0.043 for 704 unique reflections. Crystal parameters and refinement conditions are given in Table 2. Refined atom coordinates, site occupancies and equivalent isotropic displacement parameters are reported in Table 3. Selected interatomic distances are provided in Table 4. The bond valence sums (site populations given in the notes to the table), computed with the parameters given by Brese and O'Keeffe (Reference Brese and O'Keeffe1991), are listed in Table 5. Considering the correspondence in atomic number (‘mean’ atomic number of [REE + Y] = 56.8 in fluorbritholite-(Nd)), we used La (atomic number = 57) as the average REE in the refinement. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 2.

Single-crystal data and experimental details.

Note: wR ² = [Σ w (|F _o|² – |F _c|²)² / Σ w (|F _o|⁴)]^½.

Table 3.

Refined fractional atom coordinates, equivalent displacement parameters (Ų) and site occupancies in fluorbritholite-(Nd).

Table 4.

Selected interatomic distances (Å) in fluorbritholite-(Nd).*

* The O1/Cl1 positions, giving rise to the 7-fold coordinated M2 polyhedron, are mutually present.

Table 5.

Bond valence sums (BVS) for fluorbritholite-(Nd) calculated with the parameters of Brese and O'Keeffe (Reference Brese and O'Keeffe1991).*

* M1 = REE_0.66Ca_0.34; M2 = REE_0.70Ca_0.30, with REE = Nd_0.31Ce_0.26Y_0.16Sm_0.10Gd_0.07Pr_0.05La_0.05

Powder X-ray diffraction

Powder X-ray diffraction data (Table 6) were collected with an Oxford Diffraction Excalibur PX Ultra diffractometer fitted with a 165 mm diagonal Onyx CCD detector and using copper radiation (CuKα, λ = 1.54138 Å) from the same crystal used for the single-crystal experiment. The hexagonal P6₃/m unit-cell parameters refined from powder data are a = 9.5966(7) Å, c = 6.9816(8) Å and V = 556.83(7) Å³.

Table 6.

Observed and calculated X-ray powder diffraction data (d in Å) for fluorbritholite-(Nd).*

* The calculated diffraction pattern is obtained with the atom coordinates reported and the site occupancies given in Table 3 (only reflections with I _rel ≥ 3 are listed); the strongest observed Bragg reflections are given in bold.