Introduction
Babunaite-(Nd) NdAsO4 was discovered in a dark pink muscovite-quartz schist near the Nežilovo village in the northern Macedonia, where metasomatic rocks of the Mixed Series of the Pelagonian Massif are distributed. These Mixed Series rocks are a source of new Pb-Zn-bearing oxide minerals, including nežilovite, PbZn2Mn4+2Fe3+8O19 (Bermanec et al., Reference Bermanec, Holstam, Sturman, Criddle, Back and Šćavničar1996), zincohögbomite-2N6S, (Zn,Al,Fe)3(Al,Fe,Ti)8O15(OH) (Armbruster et al., Reference Armbruster, Bermanec, Zebec and Oberhansli1998, Armbruster, Reference Armbruster2002), ferricoronadite, Pb(Mn4+6Fe3+2O16 (Chukanov et al., Reference Chukanov, Aksenov, Jančev, Pekov, Göttlicher, Polekhovsky, Rusakov, Nelyubina and Van2016), zincovelesite-6N6S, Zn3(Fe3+,Mn3+,Al,Ti)8O15(OH) (Chukanov et al., Reference Chukanov, Krzhizhanovskaya, Jančev, Pekov, Varlamov, Göttlicher, Rusakov, Polekhovsky, Chervonnyi and Ermolaeva2018), zincorinmanite-(Zn), ZnSb5+(Fe3+2Zn)O7(OH) (Chukanov et al., Reference Chukanov, Gridchina, Rastsvetaeva, Varlamov, Kasatkin, Pekov, Vigasina, Virus, Jančev and Britvin2025), and minerals of the epidote group, such as piemontite-(Pb), CaPb(Al2Mn3+)(Si2O7)(SiO4)O(OH) (Chukanov et al., Reference Chukanov, Varlamov, Nestola, Belakovskiy, Goettlicher, Britvin, Lanza and Jančev2012).
It is widely accepted that the uniqueness of Nežilovo’s rocks is related to the presence of chalcophile elements, which are usually chemically bound in sulfides, but here form oxides and oxysalts. This is explained by the oxidising conditions during formation, the high chemical activity of barium and the binding of sulfur in baryte, which is distributed widely in these rocks (Chukanov et al., Reference Chukanov, Varlamov, Ermolaeva and Jančev2020, Bermanec et al., Reference Bermanec, Chukanov, Varlamov, Rajačić, Jančev and Ermolaeva2023). Endogenic ores free of sulfur with chalcophile elements are relatively rare. These have a metasomatic origin and are found in the Fe-Zn Franklin and Sterling mines in New Jersey, USA (Tarr, Reference Tarr1929; Palache, Reference Palache1929a, Reference Palache1937; Wilkerson, Reference Wilkerson1962), the Fe-Mn Långban mine, Nordmark (including Jacobsberg) and Pajsberg (including Harstigen) in the Bergslagen mining district, Värmland County, Sweden (Palache, Reference Palache1929b; Holtstam and Langhof, Reference Holtstam and Langhof1999), and the Kombat mine in Namibia (Innes and Chaplin, Reference Innes, Chaplin, Anheusser and Maske1986; Dunn, Reference Dunn1991). All of the above localities are well known thanks to the wide variety of minerals found here, many of which are rare and only occur in this type of ore.
Babunaite-(Nd), NdAsO4, is the seventh new mineral discovered in the Nežilovo area. Initially, we thought we had found a potentially new mineral, ‘gasparite-(Nd)’, which belongs to the monazite supergroup and has previously been reported in Lugau, Salzburg, Austria, but was not approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (Kolitsch et al., Reference Kolitsch, Schachinger and Auer2025). It turned out, however, that this mineral has a scheelite-type structure. The known synthetic NdAsO4 with a scheelite-type structure [tetragonal, I41/a; a = 5.1046(4) Å and c = 11.6032(11) Å]. It can be obtained from the monoclinic phase of NdAsO4 [‘gasparite-(Nd)’] under high-pressure and high-temperature conditions of 11 GPa and 1100–1300°C respectively (Metzger et al., Reference Metzger, Ledderboge, Heymann, Huppertz and Schleid2016; Kolitsch et al., Reference Kolitsch and Holtstam2004). Furthermore, the scheelite-type NdAsO4 has been synthesised from component oxides at 550°C in an evacuated silica ampoule (Mazhenov et al., Reference Mazhenov, Nurgaliev and Muldakhmetov1988).
In this paper, we provide a description of a new mineral, babunaite-(Nd) (IMA2025-032, symbol Bbu-Nd), that was approved by the CNMNC-IMA in 2025. It is named after the Babuna River, near an outcrop of muscovite-quartz schist containing the mineral was found. This paper presents data on the composition and structure of babunaite-(Nd) and discusses questions concerning its genesis.
Methods of investigations
The composition and morphology of babunaite-(Nd) and associated minerals were studied using a Phenom XL scanning electron microscope with an EDS detector, as well as a Quanta scanning electron microscope with a Thermo Fisher Scientific EDS UltraDry X-ray microanalyser. The composition of babunaite-(Nd) was measured using a Cameca SX100 with an accelerate voltage of 15kV, a beam current of 40 nA and a spot size of 2 μm, using the following analytical lines and standards: CaKα = wollastonite, MnKα = rhodonite, WMα = scheelite, AsLβ = skutterudite; VKα = vanadinite; ThMα = Th-Glass; YLα = xenotime; LaLα = La-Glass; CeLα = Ce-Glass; PrLβ = Pr-glass; NdLβ = Nd-Glass; SmLβ = Sm-glass; EuLβ = Eu-Glass; GdLβ = Gd-Glass; TbLα = Tb-Glass; and DyLα = Dy-glass.
Raman spectra of babunaite-(Nd) were recorded using a WITec alpha 300R confocal Raman microscope equipped with an air-cooled solid-state laser (488 and 532 nm) and a CCD camera operating at –61°C. An air Zeiss LD EC Epiplan-Neofluar DIC-100/0.55NA objective was used. The Raman scattered light was focused onto a multimode fibre and a monochromator with a 600 or 1800 mm–1 grating. The laser power at the sample position was ∼25 mW. 15 scans with an integration time of 3 s were collected and averaged, with a resolution of 3.5 cm–1 (600 nm) and 2 cm–1 (1800) being set. The monochromator of the spectrometer was calibrated using the Raman scattering line of a silicon plate (520.7 cm–1).
Single-crystal X-ray studies were carried out using a SuperNova diffractometer (Agilent Technologies) with an Atlas detector (Institute of Physics, Faculty of Science and Technology, University of Silesia, Poland). Measurements were performed under ambient conditions (290 K) using MoKα radiation (λ = 0.71073 Å). The structure solution and refinement were performed using the SHELXL-2018/3 programme (Sheldrick, Reference Sheldrick2015).
Occurrence
Babunaite-(Nd) was discovered in muscovite-quartz schist ∼5 km from the village of Nežilovo in Northern Macedonia. The locality is situated alongside the macadam road to the Čeples campus, near the footpath leading to the source of the Babuna River (N41°41’06.2”, E021°25’31.2”). This area belongs to the high-grade metamorphic region of the Upper Precambrian Pelagonian Massif. According to Arsovski (Reference Arsovski1959, Reference Arsovski1961) and Stojanov (Reference Stojanov1960), the region is divided into two parts. The lower complex comprises gneiss and mica schists that have been intruded by younger granitic and granodioritic bodies. The upper part consists of the so-called ‘Mixed Series’ (an irregular intercalation of gneiss, schist, calciphyre and marble with metariolite relics) and a series of massive marble (Fig. 1). During geological mapping occurrences of Pb-Zn mineralisation were found in the vicinity of the field of massive marble within the Mixed Series rocks through exploratory trenching (Stoyanov, Reference Stojanov1960; Ivanov, Reference Ivanov1961; Jančev, Reference Jančev1975a, Reference Jančev1975b; Reference Jančev1979).
Geological scheme of the Nežilovo area (modified after Jančev, Reference Jančev1979). Blue-grey colour – massive marble series, pink colour – Mixed Series. 1 – marble, 2 – calciphyre, 3 – metariolite, 4 – albite schist and augen gneiss, 5 – banded feldspar schist, 6 – gneiss, 7 – type locality of babunaite-(Nd), 8: a – fault, b – fault zone.
In one such trench, which contains metasomatically altered multicoloured schists and marble-like rocks with piemontite, dolomite, baryte and Pb-Zn mineralisation, a layer of pink mica schist up to 0.5 m thick was found containing babunaite-(Nd) (Fig. 2). A few dozen metres from the babunite-(Nd) type locality, there is a small body of metariolites (Fig. 1), which was first reported by Stojanov (Reference Stojanov1961).
(a) Old inspection pit, where samples for investigation were collected. Pink muscovite-quartz schists are clearly visible (white arrows). Muscovite schists are intercalated with gneiss. (b) Sample of muscovite-quartz schist from which thin-sections were prepared.
The dark pink mica schists are composed of Mn-bearing muscovite and quartz, with minor braunite (Fig. 3). The diverse accessory minerals include: hematite, gahnite, almeidaite, långbanite, zircon, piemontite, piemontite-(Pb), nežilovite, Sb-bearing rutile, fluorapatite and As-bearing fluorapatite, gasparite-(La), chernovite-(Y) and arsenoflorencite-(La). Single crystals of babunaite-(Nd) measure up to 70 μm and often have rounded edges. The transparent, pale-yellow crystals have a strong lustre (adamantine). The crystals exhibit good visible yellow luminescence at 638 nm. The microhardness of babunaite-(Nd) is VHN25 = 578(21) kg/mm2 (mean 16) and a range of 503–579 kg/mm2, which correspond to a hardness of 5 on the Mohs scale. The mineral is brittle and does not show cleavage. The calculated density based on the empirical formula and single-crystal X-ray diffraction (SC-XRD) data is 5.918 g·cm–3. Unfortunately, we were unable to determine the refractive indices of babunaite-(Nd), estimated to be ∼1.9, as our capabilities are limited to measuring the optical properties of minerals with refractive indices of less than 1.8. The optical sign of babunaite-(Nd) is uniaxial (+) and its birefringence, measured by maximum interference colouration, is Δ = 0.035. The calculated mean refractive index is 1.914. In reflected light babunaite-(Nd) is light grey with light yellow internal reflections. The reflectivity is in the range of 10–11.5%.
Back-scattered electron image showing the mineral associations and babunaite-(Nd) occurrence in muscovite-quartz schist. Rock fragments with babunaite-(Nd) marked with a square in (a) and (b) are enlarged in (c) and (d), respectively. Amd – almeidaite, Bbu-(Nd) – babunaite-(Nd), Bnt – braunite, Hem – hematite, Ms – muscovite, Qz – quartz.
The composition of babunaite-(Nd) varies from grain to grain. We plan to conduct a systematic study of REE arsenates and phosphates in the metasomatic rocks of Nežilovo in the future. The data on the holotype babunaite-(Nd), that was used for the structural investigation, are presented in this article (Table 1). Data on associated grains with the maximum Nd, W and Th and minimum W contents found are also provided (Table 1). Significant fluctuations in composition are observed even within a single grain of babunaite-(Nd). The mean empirical crystal chemical formula of the holotype babunaite-(Nd) crystal is: (Nd3+0.39Ca0.14Th0.09Pr3+0.08La0.07Sm3+0.06Y0.06Gd0.05Ce3+0.02Eu3+0.01)Σ0.97(As5+0.95W6+0.05V5+0.01)Σ1.02O4, which contains the following end-members: REEAsO4 = 74% (including 39% NdAsO4), (Ca0.5Th0.5)AsO4 = 18%, CaWO4 = 6% (Table 1, analysis 1). The maximum content of Nd2O3 in babunaite-(Nd) is 29.7 wt.%, or 49% NdAsO4 end-member (Table 1, analysis 2). The maximum content of WO4 is 10.7 wt.%, and minimum content is 2.4 wt.% (Table 1, analyses 3, 4). In gasparite-(La) and chernovite-(Y), which are associated with babunaite-(Nd), the W content is lower than the limit of detection in microprobe analysis. Rare babunaite-(Nd) grains containing zones with high Th and Ca content could indicate a potentially new mineral with the ideal formula (Th0.5Ca0.5)AsO4 and scheelite structure (Table 1, analysis 5).
Chemical composition of (1) the holotype crystal of babunaite-(Nd) and (2–5) crystals of babunaite-(Nd) in the sample showing various compositions. As these crystals have a zonal structure we have selected analyses with the highest Nd content, the highest and lowest W content, and the highest Th content (marked in bold)
Notes: S.D. – standard deviation, n.d. – not detected.
* Dy, Tb are below the detection limit.
Raman spectroscopy
The Raman spectra of babunaite-(Nd) show a quantity of artefacts in the form of broad bands with luminescence from Sm3+, Er3+ and Dy3+ (Fig. 4; MacRae and Wilson, Reference MacRae and Wilson2008). A spectrum obtained using a blue laser (488 nm) indicates the absence of OH groups, which is also confirmed by the structural study of babunaite-(Nd). Raman spectra showing minimal artefact activity were obtained using a green laser (532 nm) in the 100–1150 cm–1 range in the zones with relatively low Th and Ca content (Fig. 5a) and with high Th and Ca of the holotype babunaite-(Nd) (Fig. 5b). Three ranges can be distinguished in the babunaite-(Nd) spectra: (1) 100–300 cm–1 are attributed to the torsional and translational motion of (TO4) group and the lattice vibrations of the (Nd, Ca, Th)-polyhedron. These vibrations are masked by broad bands related to Er3+-luminescence. (2) 300–500 cm–1 corresponds to the bending vibrations ν2 and ν4 in the (ТО4) group with T = As5+, W6+; and (3) 700–950 cm–1 corresponds to the stretching vibrations of ν1 and ν3 in the (ТО4) group (Pradhan and Choudhary, Reference Pradhan and Choudhary1987; Hardcastle and Wachs, Reference Hardcastle and Wachs1995; Rezgui et al., Reference Rezgui, Ouerfelli, Gavinho, Carvalho, Graça and Teixeira2023). The main bands from the stretching vibrations ν1(As–O) and ν1(W–O) occur at 833 cm–1 and 926 cm–1, respectively. In the holotype babunaite-(Nd) spectrum, the band from the bending vibrations ν4(As–O) in (AsO4) is near 440 cm–1, configuration Nb–O–As (Fig. 5a). In the spectrum of babunaite-(Nd) with high Ca and Th contents, an additional band appears at ∼469 cm–1. This band is related to the vibrations ν4(As–O) and the Th–O–As configuration.
Raman spectra of babunaite-(Nd) obtained using green (532 nm) and blue (488 nm) lasers and a monochromator with 600 mm–1 grating.
Raman spectra of (a) holotype babunaite-(Nd) and (b) babunaite-(Nd) with high Th content obtained using a green laser (532 nm) and a monochromator with 1800 mm–1 grating.
Structure of babunaite-(Nd)
Single-crystal X-ray diffraction data were collected from a 0.10 × 0.03 × 0.02 mm babunaite-(Nd) crystal using a SuperNova diffractometer. The experimental details and refinement data are summarised in Tables 2, 3 and 4.
Crystal data and structure refinement details for babunaite-(Nd)
Atomic coordinates, equivalent-isotropic displacement parameters (Å2) and site occupancy for babunaite-(Nd)
Anisotropic displacement parameters (Å2)
Babunaite-(Nd) has the general crystal chemical formula ABO4 and a scheelite-type structure. It crystallises in the tetragonal I41/a space group (a = 5.1363(2) Å, c = 11.5764(8) Å, V = 305.41(3) Å3, with Z = 4). The babunaite-(Nd) structure can be generally described as a framework comprising a system of intersecting, perpendicular, zigzag columns of edge-linking Nd-polyhedra, orientated along [100] and [010], which are interconnected by As-tetrahedra (Fig. 6). In this case, each top of a Nd polyhedron is shared with an As-tetrahedron. Nd3+ is bonded in an 8-coordinate geometry with eight equivalent O2⁻ atoms. A splitting of the oxygen position is observed: О1А (94%) and О1В (6%), with the distance O1A–O1B of the length 0.425 Å. There are four shorter bonds: Nd1–O1A = 2.455(5) Å or Nd–O1B = 2.15(6) Å, and four longer bonds: Nd1–O1A = 2.489(5) Å and Nd–O1B = 2.34(4) Å (Table 5). As5+ is bonded to four equivalent O atoms in a tetrahedral geometry. The As–O1 bond lengths are 1.695(6) Å to O1A and 2.07(7) Å to O1B. O1 is bonded in a three-coordinate geometry to two equivalent Nd3+ and one As5+ atoms. The disordering of the O1 position is probably due to the entry of the relatively larger W6+ ion (0.42 Å) into the tetrahedral position instead of the As5+ ion (0.335 Å) (Shannon, Reference Shannon1976). The splitting of the oxygen position may also be connected with the differences of ionic radius between Nd3+ (1.109 Å), Th (1.05 Å) and Ca (1.12 Å) (Shannon, Reference Shannon1976), as well as processes caused by the radioactive decay of Th.
The structure of babunaite-(Nd): (a) projection on (101); and (b) projection on (110). O1B is not shown. (c) Yellow polyhedra are occupied by Nd3+, Th, Ca and coordinated by 8 oxygens, which are disordered on O1A (94%, red) and O1B (6%, pink). (d) Green tetrahedra are occupied by As5+, W6+, taking into account O1A (red) and O1B (pink). Drawn using CrystalMaker for Windows, version 2.7.7.
Selected bond lengths (Å) and weighted bond valences* (BVS, in valence units) from the empirical formula for babunaite-(Nd)
* BVS calculated on the basis our crystallographic information file (CIF) using the ECoN21 program (Ilinca, Reference Ilinca2022)
The powder X-ray diffraction data were calculated from the result of the single-crystal refinement. These results are presented in Supplementary Table S1.
Discussion
In the pink schists, alongside babunaite-(Nd), other REEAsO4 minerals such as gasparite-(La) and chernovite-(Y) were found. These arsenates belong to the scheelite, monazite and xenotime types, respectively. The structure of pure compounds with stoichiometry VIII–IXM(IVTO4) is determined by the cation radii at the M and T sites. This is clearly reflected in the rM(Å)–rT(Å) diagram of the Bastide type, in which isostructural compounds occupy orthogonal fields, giving the diagram the appearance of a settlement plan with orthogonal zoning (Clavier et al., Reference Clavier, Podor and Dacheux2011; Errandonea, Reference Errandonea2017). However, the diagram of the Bastide type for M(TO4) compounds, which are used in physics and chemistry of the solid state, is not informative for minerals. Minerals in databases are usually represented by the ideal formula, but the natural phase as a rule occurs as a solid solution. For example, there are two polymorphic modifications of xenotime-(Gd) (tetragonal) and monazite-(Gd) (monoclinic), both with the ideal formula Gd(PO4). However, holotype xenotime-(Gd) contains only 38% and holotype monazite-(Gd) contains only 30% of Gd(PO4) end-member, respectively (Ondrejka et al., Reference Ondrejka, Uher, Ferenc, Majzlan, Pollok, Mikuš, Milovská, Molnárová, Škoda, Kopáčik, Kurylo and Bačík2023, Reference Ondrejka, Bačík, Majzlan, Uher, Ferenc, Mikuš, Števko, Čaplovičová, Milovská, Molnárová, Rößler and Matthes2024). In the rM(Å)–rT(Å) diagram, ideal compositions with stoichiometry M(TO4) are shown (Fig. 7). There are only three compounds for which stable dimorphs exist under geological conditions: ThSiO4, GdPO4 and NdAsO4. These are huttonite and thorite (Finch et al., Reference Finch, Harris and Clark1964); monazite-Gd and xenotime-Gd (Ondrejka et al., Reference Ondrejka, Uher, Ferenc, Majzlan, Pollok, Mikuš, Milovská, Molnárová, Škoda, Kopáčik, Kurylo and Bačík2023, Reference Ondrejka, Bačík, Majzlan, Uher, Ferenc, Mikuš, Števko, Čaplovičová, Milovská, Molnárová, Rößler and Matthes2024); babunaite-(Nd) and ‘gasparite-(Nd)’ (Kolitsch et al., Reference Kolitsch, Schachinger and Auer2025), respectively. It is well known that there is a discontinuity in the composition of the phosphates of the xenotime group, which contain heavy rare earth elements (HREE, i.e. Gd–Lu) and Y with relatively small ionic radii, and the phosphates of the monazite group, which contain light rare earth elements (LREE, i.e. La–Eu) with relatively big ionic radii (Fig. 7). The limit of LREE and HREE phosphate miscibility decreases with increasing temperature (Gratz and Heinrich, Reference Gratz and Heinrich1997; Heinrich et al., Reference Heinrich, Andrehs and Franz1997; Pyle et al., Reference Pyle, Spear, Rudnick and McDonough2001; Mogilevsky, Reference Mogilevsky2007). A similar situation is observed for minerals in the REE(AsO4) series, where an increase in temperature is accompanied by an increase in Y content in gasparite (Pagliaro et al., Reference Pagliaro, Comboni, Battiston, Krüger, Hejny, Kahlenberg, Gigli, Glazyrin, Liermann, Garbarino, Gatta and Lotti2022). Interestingly, the ionic radius of the M cation in the pairs huttonite–thorite and monazite-(Gd)–xenotime-(Gd) is very close: Th4+ = 1.05 Å and Gd3+ = 1.053 Å, corresponding to the boundary between LREE and HREE. Therefore, these dimorphic minerals are formed under pressure/temperature (P/T) conditions typical for geological processes. A mineral with the formula NdAsO4 (with an ionic radius of Nd = 1.109 Å, LREE), using the crystal chemical criteria, should have a monazite-type structure, as confirmed by the finding of ‘gasparite-(Nd)’ (Kolitsch et al., Reference Kolitsch, Schachinger and Auer2025). Known minerals, such as the REE and Y arsenates, have a monoclinic structure of the monazite type in the gasparite group [gasparite-(Ce), Ce(AsO4) 21/n, a = 6. 937(3) Å, b = 7.137(4) Å, c = 6.738(6) Å, and β = 104.69(5)° (Graeser and Schwander, Reference Graeser and Schwander1987); gasparite-(La), La(AsO4), 21/n, a = 6. 7646(4) Å, b = 7.2184(9) Å, c = 6.0070(4) Å and β = 104.51(1)° (Vereschagin et al., Reference Vereshchagin, Britvin, Perova, Brusnitsyn, Polekhovsky, Shilovskikh, Bocharov, van der Burgt, Cuchet and Meisser2019)] and a tetragonal structure of the xenotime-type [chernovite-(Y), Y(AsO4), I41/amd, a = 7.039(11) Å and c = 6.272(22) Å (Goldin et al., Reference Goldin, Yushkin and Fishman Fishman1967)]. Arsenates with a scheelite-type structure are not found in Nature. The known synthetic phase with a scheelite-type structure is NdAsO4 [tetragonal, I41/a; a = 5.1046(4) Å and c = 11.6032(11)], which is considered a high-pressure phase. It can be obtained from the monoclinic phase of NdAsO4 under high-pressure and high-temperature conditions of 11 GPa and 1100–1300°C (Metzger et al., Reference Metzger, Ledderboge, Heymann, Huppertz and Schleid2016). Such conditions could not be realised in the formation processes of the Mixed Series rocks. The genesis of the complex polymineral paragenesis of the Mixed Series remains to be fully resolved and merits further study. All petrological elements and ore occurrences of the Mixed Series as well as massive marbles, underwent metamorphism by complex polyphase processes (Stojanov, Reference Stojanov1960; Bermanec et al., Reference Bermanec, Chukanov, Varlamov, Rajačić, Jančev and Ermolaeva2023), reaching the conditions of the kyanite–graphite subfacies (Jančev, Reference Jančev1985). We believe that the formation of babunaite-(Nd) is associated with the stabilisation of its structure by the impurity of tungsten (W). Phases of stoichiometry VIIIM IVTO4 with tetrahedrally coordinated anions and an ionic radius greater than 0.4 Å (W, Mo) have a scheelite-type structure (Fig. 7; Errandonea, Reference Errandonea2017). Babunaite-(Nd) is a unique mineral, it is both the first arsenate and the first REE phase in the scheelite group.
All known minerals M(TO4), M= R2+, R3+, R4+; T = R4+, R5+, R6+ in the diagram rM(Å) – rT(Å).
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2026.10222
Acknowledgements
The authors would like to thank the reviewers and editors for their valuable feedback, which helped to improve the manuscript.
Competing interests
The authors declare none.
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