Monazite-(Gd), a new Gd-dominant mineral of the monazite group from the Zimná Voda REE–U–Au quartz vein, Prakovce, Western Carpathians, Slovakia

Abstract

Monazite-(Gd), ideally GdPO₄, is a new mineral of the monazite group. It was discovered near Prakovce-Zimná Voda, ~23 km WNW of Košice, Western Carpathians, Slovakia. It forms anhedral domains (≤100 μm, mostly 10–50 μm in size), in close association with monazite-(Sm), Gd-bearing xenotime-(Y), Gd-bearing hingganite-(Y), fluorapatite and uraninite. All these minerals are hosted in a REE–U–Au quartz–muscovite vein, hosted in phyllites in an exocontact to granites. The density calculated using the average empirical formula and unit-cell parameters is 5.55 g/cm³. The average chemical composition measured by means of electron microprobe is as follows (wt.%): P₂O₅ 29.68, As₂O₅ 0.15, SiO₂ 0.07, ThO₂ 0.01, UO₂ 0.04, Y₂O₃ 1.30, La₂O₃ 3.19, Ce₂O₃ 6.93, Pr₂O₃ 1.12, Nd₂O₃ 10.56, Sm₂O₃ 17.36, Eu₂O₃ 1.49, Gd₂O₃ 22.84, Tb₂O₃ 1.57, Dy₂O₃ 2.27, CaO 0.21, total 99.67. The corresponding empirical formula calculated on the basis of 4 oxygen atoms is: (Gd_0.30Sm_0.24Nd_0.15Ce_0.10La_0.05Dy_0.03Y_0.03Tb_0.02Eu_0.02Pr_0.02Ca_0.01)_0.98P_1.01O₄. The ideal formula is GdPO₄. The monazite-type structure has been confirmed by micro-Raman spectroscopy and selected-area electron diffraction. Monazite-(Gd) is monoclinic, space group P2₁/n, a = 6.703(1) Å, b = 6.914(1) Å, c = 6.383(1) Å, β = 103.8(1)°, V = 287.3(1) ų and Z = 4. The middle REE enrichment of monazite-(Gd) is shared with the associated Gd-bearing xenotime-(Y) to ‘xenotime-(Gd)’ and Gd-bearing hingganite-(Y). This exotic REE signature and precipitation of Gd-bearing mineral assemblage is a product of selective complexing and enrichment in middle REE in low-temperature hydrothermal fluids by alteration of primary uraninite, brannerite and fluorapatite on a micro-scale. The new mineral is named as an analogue of monazite-(La), monazite-(Ce), monazite-(Nd) and monazite-(Sm) but with Gd dominant among the REE.

Introduction

Rare earth elements represent a unique set of elements having coherent behaviour and very similar properties which are responsible for their similar geochemical properties. According to the International Union for Pure and Applied Chemistry, the term rare earth elements (REE) includes lanthanoids (Ln), yttrium (Y) and scandium (Sc). However, due to the substantially smaller ionic radius of Sc³⁺ with respect to the rest of the group, Sc frequently enters different crystal-structural sites, and therefore Sc is commonly not included in the REE in geological sciences, as is the case in this work. Due to the lanthanoid contraction phenomenon, the REE are further divided, by atomic number into light (LREE: La–Nd), middle (MREE: Sm–Dy) and heavy (HREE: Ho–Lu and Y).

Minerals belonging to the monazite group are anhydrous orthophosphates (with monoclinic symmetry, space group P2₁/n). They contain dominant trivalent LREE cations based on their preference of the monazite-type crystal structure (Ni et al., 1995). Among them, monazite-(Ce) is the most common species, whereas monazite-(La), monazite-(Nd) and monazite-(Sm) are rare, found typically in specific pegmatite, post-magmatic or sedimentary environments (e.g. Maksimović and Pantó, 1980; Graeser and Schwander, 1987; Demartin et al., 1991; Pekov, 1998, 2000; Masau et al., 2002; Dowman et al., 2017). Despite having a generally coherent behaviour, selective REE mobilisation and fractionation in aqueous systems is quite common and has been reported previously from different geochemical environments (e.g. Seredin, 1996; Morgan et al., 2012; Göb et al. 2013; Lee et al., 2013; Ondrejka et al., 2018, 2023; Abedini et al., 2019; Migdisov et al., 2019; Anenburg et al., 2020). In contrast to numerous LREE-dominant minerals, there is currently only one Gd-dominant mineral approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC, Pasero, 2023): lepersonnite-(Gd) CaGd₂(UO₂)₂₄(SiO₄)₄(CO₃)₈(OH)₂₄⋅48H₂O, a rare REE-uranyl carbonate from the Shinkolobwe U deposit in the DR Congo (Deliens and Piret, 1982). This mineral, however, contains a relatively low absolute concentration of essential Gd (2.1 wt.% Gd₂O₃) together with minor Dy, Y and Tb.

Here, we describe a new Gd-dominant mineral monazite-(Gd), discovered in a REE–U–Au quartz vein near Prakovce, Zimná Voda, Slovakia. Monazite-(Gd) is the Gd-dominant member of the monazite group, related to monazite-(La), monazite-(Ce), monazite-(Nd) and monazite-(Sm) by substitution of Gd for other REE and having the REE composition distinctly shifted towards the MREE. The new mineral was approved in accordance with recommendations of the IMA–CNMNC (IMA2022-055; Ondrejka et al., 2022) and the Levinson modifier for rare earth minerals (Levinson, 1966; Bayliss and Levinson, 1988). The symbol Mnz-Gd was given to the new mineral. The holotype specimen of monazite-(Gd) (polished thin section ZV-2A) has been deposited in the collection of the Department of Mineralogy, Petrology and Economic Geology, Faculty of Natural Sciences, Comenius University in Bratislava, Ilkovičova 6, Mlynská dolina, 84215, Bratislava, Slovak Republic under the catalogue number MMUK7660. The cotype specimen (polished thin section ZV-2A1) is deposited in the collections of the Slovak National Museum, Natural History Museum, Vajanského nábrežie 2, P.O. BOX 13, 810 06 Bratislava, Slovak Republic under the catalogue number M-20411.

Occurrence

The Zimná voda occurrence was discovered in 1975, during exploration for uranium ores (Novotný and Čížek 1979). The sample containing monazite-(Gd) was collected by Martin Ondrejka, Štefan Ferenc, Tomáš Mikuš and Alexandra Molnárová in September 2017 during reconnaissance of the Zimná Voda REE–U–Au quartz vein, Prakovce, eastern Slovakia. The site is located near the main ridge of the Slovenské Rudohorie Mts., ~5.6 km to the S of the village of Prakovce, 600 m to the NW of the Tri Studne elevation point (969 m a.s.l.) and 400 m NW of Trohánka bivouac shelter, at an altitude of ~950 m a.s.l., ca. 23 km WNW of Košice town. The geographical coordinates of the occurrence are 48.767°N and 20.913°E.

The Zimná Voda REE–U–Au vein mineralisation is located in the Lower Palaeozoic metamorphic rocks of the Bystrý Potok Formation, a part of the Gelnica Group in the Gemeric tectonic unit of the Western Carpathians, which is part of the Alpine–Carpathian Mountain belt (Bajaník et al. 1983; Ivanička et al., 1989). Two quartz veins (Western and Eastern) containing U and Au mineralisation were found in the area. Monazite-(Gd) was found in the Western vein. The vein is located in fine-grained micaceous phyllites, with interbeds of fine-grained quartzites. It has an E–W strike, total length of ~90 m with an average dip of 65° to the S and conforms to the schistosity of the host rocks. The thickness of the vein ranges from 3 to 30 cm. Along with the contact, the rocks are intensively argillitised and locally silicified. The metamorphic rocks were intruded by Permian rare-metal granites that outcrop at a site called Hummel, ~600 m to the SW of the vein. The mineralisation probably originated by fluid-driven hydrothermal mobilisation of REE, U and Au from the surrounding metamorphic rocks induced by the intrusion of the granitic rocks (Rojkovič et al., 1997; 1999). Further details and regional geology can be found in Ondrejka et al. (2023).

Alongside the investigated monazite-(Gd), the following minerals were identified in the Western vein: quartz, uraninite, brannerite, rutile, gold, bismuth, bismuthinite, pyrite, arsenopyrite, cobaltite, glaucodot, molybdenite, galena, tetrahedrite-(Fe), fluorapatite, monazite-(Ce), monazite-(Nd), monazite-(Sm), xenotime-(Y), hingganite-(Y), muscovite, chlorite and tourmaline; supergene minerals are represented by goethite and other undifferentiated iron oxyhydroxides, pharmacosiderite, scorodite, arseniosiderite, zeunerite, other uranyl arsenates–phosphates (most abundant are nováčekite, kahlerite, threadgoldite, rarely autunite, chistyakovaite–arsenuranospathite related mineral phases and phosphuranylite). The gadolinium-bearing mineral assemblage also includes xenotime-(Y) and hingganite-(Y) (Ondrejka et al., 2023). Moreover, a Gd-dominant analogue of xenotime-(Y) is potentially the second new Gd-dominant mineral from this locality; a detailed structural study, necessary for its approval by the IMA–CNMNC, will be the subject of future investigations.

Experimental methods

The chemical composition of selected minerals was studied by a JEOL JXA-8530F electron microprobe analyser (EMPA) in wavelength-dispersive spectrometry (WDS) mode, and X-ray element mapping at the Earth Science Institute, Slovak Academy of Sciences in Banská Bystrica, Slovakia. An accelerating voltage of 15 kV and a probe current of 20 nA were used. The typical spot beam diameter varied from 2 to 8 μm; a more focused ≤1–3 μm beam was used occasionally to avoid any intermediate composition in strongly heterogeneous micro-scale areas. The determination was calibrated using natural and synthetic standards (Table 1), and raw counts were converted to wt.% of oxides using the ZAF matrix correction. Corrections of line interferences were provided using the method by Åmli and Griffin (1975). Elements with reported detection limits are given in Table 1.

Table 1. Conditions used for the electron microprobe analyses.

Depolarised Raman measurements of monazite-(Gd) were performed on a Labram HR800 spectrometer (Horiba Jobin-Yvon), coupled with an Olympus-BX41 optical microscope. Samples were irradiated using a 532 nm frequency-doubled Nd-YAG and 633 nm He–Ne lasers. The system resolution was 2 cm⁻¹; band definition was improved using a 6-fold sub-pixel shift. Peaks were deconvoluted with a Gauss-Lorentzian function in PeakFit ^© (SeaSolve Software).

To determine the principal features of the crystal structure, we used transmission electron microscopy (TEM) on focused ion beam (FIB) lamella at the Institute of Geosciences, Friedrich Schiller University of Jena, Germany. This procedure enables measurement of electron diffraction patterns of the same phases that were analysed previously by EMP. The FIB preparation was conducted using a Quanta 3D FEG FIB-SEM instrument that enabled a progressive abrasion of the targeted area in the mineral using a focused beam of Ga ions, monitored by secondary electron (SE) and back-scattered electron (BSE) imaging. The monazite-(Gd) lamella was prepared by FIB as a TEM foil on a copper FIB lift-out grid. Selected-area electron diffraction (SAED) patterns of monazite-(Gd) were taken at 13 zone axis orientations on a single crystal.

Element contents in the mineral formula are expressed in atoms per formula unit (apfu), the monazite formula was normalised to 4 oxygen atoms.

Appearance and physical properties

Monazite-(Gd) forms anhedral domains (≤100 μm in size, mostly 10–50 μm), usually as intergrowths with other crystals or aggregates of monazite-(Sm) and Gd-bearing xenotime-(Y) (≤150 μm in size), fluorapatite, Gd-bearing hingganite-(Y) and uraninite scattered in quartz–muscovite gangue (Fig. 1). Among four monazite-group species recognised in the Western vein at Zimná Voda, monazite-(Ce) is the most common and is the only LREE phosphate which occurs in all of the six samples studied. Monazite-(Nd), monazite-(Sm) and monazite-(Gd) were found only in the ZV-2 sample, though in multiple thin sections. The lustre, hardness, cleavage and parting of monazite-(Gd) could not be determined, nor could density be measured due to an insufficient quantity for physical measurement. The density of 5.55 g/cm³ was calculated on the basis of average empirical formula and calculated unit-cell volume (see below). The density of the ideal formula GdPO₄ is 5.99 g cm^–3.

Figure 1. Back-scattered electron images of (a) large aggregate of monazite-(Gd) (Mnz-Gd) with xenotime-(Y) (Xtm-Y), Gd-bearing xenotime-(Y) (Gd-rich Xtm-Y) and uranyl arsenates–phosphates (U–As–P) in quartz–muscovite gangue (Qz + Ms). (b) Monazite-(Gd) (Mnz-Gd) and monazite-(Sm) (Mnz-Sm) with xenotime-(Y) (Xtm-Y) and Gd-bearing xenotime-(Y) (Gd-rich Xtm-Y) in quartz–muscovite gangue (Qz + Ms). (a) Holotype specimen, polished thin section ZV-2A; (b) cotype specimen: ZV-2A1; symbols from Warr (2021).

Chemical composition

Chemical analyses were carried-out on crystals of the holotype specimen (ZV-2A thin section) and further crystals of the cotype specimen (ZV-2A1) and the composition is shown in Table 2. The individual crystals are chemically homogeneous, with no distinct variations in the core–rim profile. Locally, there may be a heterogeneous composition reflecting irregular μm-scale domains of different REE distributions. The crystals analysed have a composition with Gd>Sm>Nd>Ce>La (20.1–23.4 wt.% Gd₂O₃, 0.25–0.31 apfu Gd; 16.2–18.3 wt.% Sm₂O₃, 0.21–0.25 apfu Sm, 9.8–11.6 wt.% Nd₂O₃, 0.13–0.17 apfu Nd, 6.6–8.6 wt.% Ce₂O₃, 0.10–0.13 apfu Ce, 3.0–4.3 wt.% La₂O₃, 0.05–0.06 apfu La). The average chemical composition of monazite-(Gd) calculated from six point electron-microprobe analyses is as follows (wt.%): P₂O₅ 29.68, As₂O₅ 0.15, SiO₂ 0.07, ThO₂ 0.01, UO₂ 0.04, Y₂O₃ 1.30, La₂O₃ 3.19, Ce₂O₃ 6.93, Pr₂O₃ 1.12, Nd₂O₃ 10.56, Sm₂O₃ 17.36, Eu₂O₃ 1.49, Gd₂O₃ 22.84, Tb₂O₃ 1.57, Dy₂O₃ 2.27, Ho₂O₃ 0.24, Er₂O₃ 0.20, Tm₂O₃ 0.02, Yb₂O₃ 0.28, Lu₂O₃ 0.01, FeO 0.08, MnO 0.03, CaO 0.21, PbO 0.01, Cl 0.03, total 99.67. The corresponding empirical formula calculated on the basis of 4 oxygen atoms is: (Gd_0.30Sm_0.24Nd_0.15Ce_0.10La_0.05Dy_0.03Y_0.03Tb_0.02Eu_0.02Pr_0.02Ca_0.01)_0.98P_1.01O₄ which leads to the end-member formula GdPO₄ requiring 71.86 Gd₂O₃, 28.14 P₂O₅ and total 100 wt.%.

Table 2. Representative and average electron microprobe analyses and mineral formula of monazite-(Gd).

– not detected

In general, the element distribution shows enrichment towards the MREE, depletion of LREE and negligible HREE (Ho–Lu)+Y abundances. The chondrite-normalised patterns exhibit conspicuous maxima at Sm and Gd (MREE hump) and a distinct downward concave W-type tetrad effect on the first tetrad (La–Nd) (for further details, see Ondrejka et al., 2023). Concentrations of Th and U were usually below the detection limit of the EPMA; Th rarely attains 1.1 wt.% ThO₂ (0.01 apfu Th) in measurements having also increased Si content (≤0.7 wt.% SiO₂, 0.03 apfu Si). These analyses attest to limited ThSiREE_–1P_–1 huttonite substitution. Satisfactory analytical totals (98−100 wt.%) and stoichiometry calculated on an anhydrous basis indicate that any potential role of a tetrahedral array of (OH)^– groups is negligible. Other trace elements (Ca, Sr, Fe, Pb, S and As) have negligible concentrations or are below detection limits.

Crystallography

Monazite-(Gd) is isostructural with other monazite-group minerals; its monoclinic (space group P2₁/n) structure consists of isolated PO₄ tetrahedra interconnected by (REE)O₉ polyhedra (Ni et al., 1995). A larger REE-containing polyhedron in monazite with the coordination of 9 compared to xenotime with the coordination of 8 allows the incorporation of larger cations; therefore, the monazite structure prefers LREE (Miyawaki and Nakai, 1993). Consequently, monazite crystals with MREE enrichment are rare.

Single-crystal X-ray diffraction studies of monazite-(Gd) were not carried-out due to the small size of the crystals. However, the monazite structure is well known; therefore, selected-area electron diffraction (SAED) patterns were used to confirm the unit-cell dimensions and the symmetry of monazite-(Gd) (Fig. 2). Scanning transmission electron microscope (STEM) images at low magnification and high camera length (LMSTEM) were used to image the entire TEM-lamella. It consists of a large single crystal with sub-grains which are cross-cut by a veinlet. The neighbouring or included mineral is chlorite. The STEM images at low camera length (HAADF images) show contrast which is sensitive to the mean atomic number. These STEM images confirm the homogeneity of the sample. Slight contrast occurs at sub-grain boundaries, probably due to slight density changes there. Other changes in contrast can be correlated to thickness differences of the lamella which is an artefact from the FIB preparation.

Figure 2. Electron diffraction patterns of the monazite-(Gd) crystal along the zone axes specified in each panel.

Selected-area electron diffraction (SAED) on various zone axes resulted in 45 unique reflections that allowed for a reasonable refinement of the lattice parameters (Table 3). Note that the values are less precise than those extracted from X-ray diffraction data because the θ angles in electron diffraction are extremely small. The limited number of reflections from only a part of the reciprocal space leads to a relatively large error – larger for c because the c axis is at a high angle to the FIB sample. Thus, the reflections from (001) could not be observed. The unit-cell parameters were calculated with the UnitCell program (Holland and Redfern, 1997) as a = 6.71(2) Å, b = 6.98(2) Å, c = 6.55(5) Å, β = 104.4(6)°, V = 297.1(4) ų and Z = 4.

Table 3. Unit-cell parameters for natural monazite members and synthetic monazite REEPO₄ compounds.

The monazite structure is relatively simple; therefore, there is a near-ideal linear trend of decreasing unit-cell parameters with decreasing ionic radii of REE cations in synthetic REEPO₄ with a monazite structure (Ni et al., 1995; Ushakov et al., 2001). In contrast, the β angle has an inverse near-linear correlation with the REE ionic radii, it increases with smaller radii (Ni et al., 1995). Consequently, it is possible to assume that the unit-cell parameters of the observed mineral probably correspond to the weighted average of the corresponding end-member unit-cell parameters. This assumption allowed the calculation of unit-cell parameters from the weighted sum of end-members as a = 6.703(1) Å, b = 6.914(1) Å, c = 6.383(1) Å, β = 103.8(1)°, V = 287.3(1) ų and Z = 4. These are, in general, lower than unit-cell parameters calculated from SAED. The difference can be attributed to the variations in REE, which, although not significant, is observable. This suggests that the average composition from which the unit-cell size was calculated could be slightly different to the composition of crystal fragments from which SAED patterns were measured. Moreover, the accuracy of the unit-cell parameters is limited by relatively large errors in SAED measurements, as explained above. For comparison, unit-cell parameters for natural monazite members and synthetic monazite REEPO₄ compounds are presented in Table 3.

Raman spectroscopy

The Raman spectrum of monazite-(Gd) has already been published in supplementary material to Ondrejka et al. (2023) and is in good agreement with other monazite-group mineral species. All the Raman bands of the monazite-(Gd) occur in the range of up to 1200 cm^–1 (Table 4). Two groups of lines were observed: from 100 to 700 cm^–1, with dominant bands at 423 cm^–1 and 472 cm^–1 and 950–1100 cm^–1, with the most intensive bands centred at 981 cm^–1 and having an asymmetric shape.

Table 4. Raman bands for monazite-(Gd)

A Raman spectrum of monazite is predicted to contain 36 (18A_g + 18B_g) modes (Begun et al., 1981; Ruschel et al., 2012; Heuser et al., 2014). The bands below 700 cm^–1 correspond to lattice vibrations, translations and rotations, the A-PO₄ movements and symmetric bending (ν₂), and asymmetric bending (ν₄) vibrations of the PO₄ tetrahedra. The symmetric (ν₁) and antisymmetric stretching (ν₃) vibrations region of the PO₄-tetrahedra is expressed in the 900–1100 cm^–1 region (Fig. 3), where ν₁ are the most intensive.

Figure 3. Depolarised 532 nm-excited Raman spectrum of monazite-(Gd) with its peak wavenumbers. (a) Full range spectrum with assigned Raman bands. (b and c) Deconvolution of spectrum to single Gaussian–Lorentzian-shaped bands in the regions of 300–750 cm^–1 and 900–1100 cm^–1. Intensity scale bar in arbitrary units (a.u.).

Discussion

Monazite-(Gd) is a new lanthanoid orthophosphate mineral of the monazite group with a unique Gd-dominance over the other REE cations. It represents only the second Gd-dominant mineral, described in nature, and approved by IMA–CNMNC, after lepersonnite-(Gd), CaGd₂(UO₂)₂₄(SiO₄)₄(CO₃)₈(OH)₂₄⋅48H₂O, from the Shinkolobwe uranium deposit, DR Congo (Deliens and Piret, 1982). However, monazite-(Gd) contains a significantly higher content of Gd: 20.1–23.4 wt.% Gd₂O₃ than lepersonnite-(Gd) (only 2.1 wt.% Gd₂O₃, Deliens and Piret, 1982). Beside monazite-(Gd), monazite-(Ce), monazite-(Nd) and monazite-(Sm) from the Zimná Voda quartz vein are also enriched in Gd (2.5 to 19.1 wt.% Gd₂O₃, 0.03 to 0.25 apfu Gd; Ondrejka et al., 2023). Moreover, rare occurrences of MREE (Nd, Sm and Gd)-rich compositions in the monazite-group minerals have been described from some magmatic rocks. Monazite-(Sm) from the Annie Claim #3 pegmatite, Canada had 12.1−13.5 wt.% Gd₂O₃ (0.16−0.18 apfu Gd; Masau et al., 2002). Monazite-(Ce) to -(Sm) with 4.7−8.9 wt.% Gd₂O₃ (0.06−0.12 apfu Gd) was detected in the Utsumine and Siohira granites and Shimo-ono pegmatite, Japan (Hoshino et al., 2012). There was 5.7−7.5 wt.% Gd₂O₃ (0.07−0.10 apfu Gd) in monazite-(Nd) from Sierra de Cobres carbonatite dykes, Argentina (Del Blanco et al., 1998) and 2.9−4.8 wt.% Gd₂O₃ in monazite-(Ce) to monazite-(Nd) from the Abu Rusheid lamprophyre dyke, Egypt (Ali, 2012). Lesser Gd enrichment of up to 3.25 wt.% Gd₂O₃ (≤0.04 apfu Gd) has also been detected in skeletal monazite-(Ce) from the Oldřich pegmatite vein, Dolní Bory, Czech Republic (Výravský, 2014) and 2.2 to 2.8 wt.% Gd₂O₃ (0.03−0.04 apfu Gd) in low-temperature, authigenic monazite-(Nd) and monazite-(La) from bauxite deposits at Marmara, Greece and Liverovici, Montenegro (Maksimović and Pantó, 1980, 1996).

Such unusual enrichment of MREE (Gd, Nd and Sm) in monazite-group members as well as in some other REE minerals [e.g. xenotime-(Y) and hingganite-(Y)] could be a result of selective complexing of REE during granite to pegmatite sequence solidification in aqueous F–Li–CO₂-bearing fluids (Masau et al., 2002). Moreover, late-hydrothermal alteration of MREE-enriched precursors (uraninite, brannerite and fluorapatite) is considered as the principal factor for precipitation of monazite-(Gd) and associated Gd-rich minerals (Ondrejka et al., 2023).