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10 October 2025

Coordination-Driven Rare Earth Fractionation in Kuliokite-(Y), (Y,HREE)4Al(SiO4)2(OH)2F5: A Crystal–Chemical Study

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and
1
Nanomaterials Research Center, Kola Science Center, Russian Academy of Sciences, Fersmana Str. 14, 184209 Apatity, Russia
2
Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
3
Geological Institute, Kola Science Center, Russian Academy of Sciences, Fersmana Str. 14, 184209 Apatity, Russia
*
Author to whom correspondence should be addressed.
Minerals2025, 15(10), 1064;https://doi.org/10.3390/min15101064
This article belongs to the Section Crystallography and Physical Chemistry of Minerals & Nanominerals
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Abstract

The crystal structure of kuliokite-(Y), Y4Al(SiO4)2(OH)2F5, has been re-investigated using the material from the type locality the Ploskaya Mt, Kola peninsula, Russian Arctic. It has been shown that in contrast to previous studies, the mineral is monoclinic, Im, with a = 4.3213(1), b = 14.8123(6), c = 8.6857(3) Å, β = 102.872(4)°, and V = 541.99(3) Å3. The crystal structure was solved and refined to R1 = 0.030 on the basis of 3202 unique observed reflections. The average chemical composition determined by electron microprobe analysis is (Y2.96Yb0.49Er0.27Dy0.13Tm0.07Lu0.05Ho0.05Gd0.01Ca0.01)Σ4.04Al0.92Si2.04O8-[(OH)2.61F4.42]Σ7.03; the idealized formula is (Y,Yb,Er)4Al[SiO4]2(OH)2.5F4.5. The crystal structure of kuliokite-(Y) contains two symmetrically independent Y sites, Y1 and Y2, coordinated by eight and seven X anions, respectively (X = O, F). The coordination polyhedra can be described as a distorted square antiprism and a distorted pentagonal bipyramid, respectively. The refinement of site occupancies indicated that the mineral represents a rare case of HREE fractionation among two cation sites driven by their coordination numbers and geometry. In agreement with the lanthanide contraction, HREEs are selectively incorporated into the Y2 site with a smaller coordination number and tighter coordination environment. The strongest building unit of the structure is the [AlX2(SiO4)2] chain of corner-sharing AlX6 octahedra and SiO4 tetrahedra running along the a axis. The chains have their planes oriented parallel to (001). The Y atoms are located in between the chains, along with the F− and (OH)− anions, providing the three-dimensional integrity of the crystal structure. Each F− anion is coordinated by three Y3+ cations to form planar (FY3)8+ triangles parallel to the (010) plane. The triangles share common edges to form [F2Y2]4+ chains parallel to the a axis. The analysis of second-neighbor coordination of Y sites allowed us to identify the structural topology of kuliokite-(Y) as the only case of the skd network in inorganic compounds, previously known in molecular structures only. The variety of anionic content in the mineral allows us to identify the potential existence of two other mineral species that can tentatively be named ‘fluorokuliokite-(Y)’ and ‘hydroxykuliokite-(Y)’.

1. Introduction

Natural minerals are the key source of rare earth elements (REEs) due to their industrial and technological use, and REE mineral resources have received enormous attention from both geological and economic points of view [,,,,]. Rare earth minerals are very diverse chemically as well as structurally, and a large number of new mineral species have been discovered over recent years [,,,,,,,,,,,,,,,,,,,,,,,]. Of particular interest are minerals that contain heavy REEs (HREEs) such as Yb, owing to their importance in the fabrication of materials for lasers, superconductors, atomic clocks, ion qubits, etc. There are four mineral species that contain Yb as a mineral-forming component (i.e., when Yb dominates over other elements at least in one structural site): xenotime-(Yb), YbPO4 [], samarskite-(Yb), YbNbO4 [], hingganite-(Yb), YbBe(SiO4)(OH) [], and keiviite-(Yb), Yb2Si2O7 []. The origin of the four Yb minerals is connected with granitic pegmatites, where Yb concentrates in Y minerals sensu stricto. The most important geographic localities where Yb minerals have been found are: Ploskaya Mountain (Mt) pegmatites (Kola peninsula, Russia), Stetind and Høydalen pegmatites (Norway), Shatford Lake pegmatites (Manitoba, Canada), Little Patsy pegmatites (Colorado, U.S.), Zudong granites (Jiangxi, China), etc. Among them, amazonitic pegmatites of the Ploskaya Mt are unique as a type locality for two Yb minerals, hingganite-(Yb) and keiviite-(Yb), where xenotime-(Yb) was found as well []. In addition, the Ploskaya Mt is the type locality for kuliokite-(Y) that contains essential amounts of Yb and is the subject of this study.
Kuliokite-(Y) was discovered by Voloshin et al. [] in amazonitic pegmatites of the Kola peninsula (Russia) in close association with thalenite-(Y) and keiviite-(Y). Later, it was also found in two localities in Norway: Høydalen (southern Norway) [] and Stetind (northern Norway) []. The mineral was named after the Kuliok river located in the central part of the Kola peninsula. It is of interest that the mineral has no synthetic analogues and had never been prepared under laboratory conditions. The crystal structure of kuliokite-(Y) was reported by Sokolova et al. [] to be triclinic and non-centrosymmetric, with the space group P1, a = 8.606, b = 8.672, c = 4.317 Å, α = 102.79, β = 97.94, γ = 116.66o, and V = 270.1 Å3. The empirical chemical formula of the mineral was reported as (Y3.58Yb0.10Er0.085Dy0.055Gd0.015Lu0.005Ho0.005)Al1.04Si2.12O7.59F5.08(OH)2.88; the ideal formula is Y4Al(SiO4)2(OH)2F5. Our re-investigation of the natural material from the type locality demonstrated that kuliokite-(Y) is, in fact, monoclinic and contains two REE sites that are selective with respect to the HREEs. Site selectivity is controlled by the coordination features (coordination number and geometry), in agreement with the lanthanoid contraction.

2. Materials and Methods

2.1. Materials

The crystals of kuliokite-(Y) used in this study have been found in amazonitic pegmatites of the Ploskaya Mt, Keivy massif, Kola peninsula, Russian Arctic. Kuliokite-(Y) forms pinkish transparent crystals (Figure 1) in association with albite, aegirine, thalénite, xenotime-(Y), and kainosite-(Y).
Figure 1. Pinkish crystals of kuliokite-(Y) in association with albite (white), amazonite (green), and aegirine (black) from the Ploskaya Mt, Kola peninsula, Russian Arctic. The field of view is ca. 2 × 3 mm2.

2.2. Chemical Composition

The chemical composition of kuliokite-(Y) has been studied by wavelength dispersion spectrometry using a Cameca MS-46 electron microprobe (Geological Institute, Kola Science Centre of the Russian Academy of Sciences, Apatity, Russia) operating at 20 kV, 20–30 nA, and with a 5 μm beam diameter. Table 1 provides the mean analytical results for three kuliokite crystals (where each analysis is the average of 4–7 point measurements). The standards used for the microprobe work are as follows: synthetic Y3Al5O12 (Y and Al), synthetic LiREE(MoO4)2 (REE = Yb, Er, Lu, Tm, Gd), LiREE’(WO4)2 (REE’ = Dy and Ho), diopside (Ca and Si), and fluorite (F).
Table 1. Chemical composition of kuliokite-(Y) (in wt.% and atoms per formula unit (apfu)), and mean site-scattering factors (<SSF>, e−) for the Y site in its crystal structure.
The average empirical chemical formula calculated on the basis of Al + Si + Ca + Y + Gd + Dy + Ho + Er + Tm + Yb + Lu = 7 can be written as (Y2.96Yb0.49Er0.27Dy0.13-Tm0.07Lu0.05Ho0.05Gd0.01Ca0.01)Σ4.04Al0.92Si2.04O8[(OH)2.61F4.42]Σ7.03; the idealized formula is (Y,Yb,Er)4Al[SiO4]2(OH)2.5F4.5, which is close to that reported earlier, Y4Al(SiO4)2(OH)2F5 []. It is notable that our kuliokite-(Y) sample contains less F, but more Yb and Er, than the crystals of the mineral used in the original study by Voloshin et al. []. The differences in chemical composition between the samples will be discussed below.

2.3. Single-Crystal X-Ray Diffraction Analysis

The crystal structure study of kuliokite-(Y) was carried out by means of the Synergy S single-crystal diffractometer equipped with the Hypix detector using monochromatic MoKα radiation (λ = 0.71069 Å) at room temperature. More than a half of the diffraction sphere was collected using a scanning step of 1° and an exposure time of 60 s. The data were integrated and corrected by means of the CrysAlis 1.171.36.20 program package, which was also used to apply empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm []. The structure was solved and refined using the SHELXL2015 software package [].
The crystal structure of kuliokite-(Y) was solved by using direct methods in the monoclinic non-centrosymmetric space group Im, in contrast to the original study by Sokolova et al. [], who reported the structure in the triclinic space group P1. The unit cell vectors (at, bt, ct) determined in this work (Table 2) are related to those determined in [] (as, bs, cs) through the following equations:
at = −cs; bt = 2as + bs + cs; ct = −bs
Table 2. Crystal data and structure refinement for kuliokite-(Y).
The respective transformation matrix is [00 1 ¯ /211/ 1 ¯ 00]. The reciprocal transformation matrix [(at, bt, ct) ⟶ (as, bs, cs)] is [½½½/00 1 ¯ / 1 ¯ 00]. The unit cell parameters for the primitive cell similar to that reported in [] are as = 8.614, bs = 8.686, cs = 4.321 Å, αs = 102.87, βs = 97.96, γs = 116.63°, which are almost the same as those given in the original work [] (see above). The relations between the monoclinic and triclinic cells are shown in Figure 2. The monoclinic symmetry of kulikoite-(Y) revealed in our study is in agreement with the general observations of crystals of the mineral. For instance, Raade et al. [] reported the pseudomonoclinic habit of kuliokite-(Y) crystals from the Høydalen pegmatites. The structure model in the space group Im was checked for missing symmetry elements using PLATON 2023.1 software []; no additional symmetry was found. The racemic twin model was used in the final refinement; the obtained Flack parameter [] is 0.969(12).
Figure 2. The relations between the unit cell vectors of kuliokite-(Y) determined in this work (in black) with those determined previously (red).
The position of one H site was located through the inspection of the difference Fourier maps of electron density; other H sites could not be located. The occupancies of the Y sites were refined using mixed Y-Yb scattering curves for neutral atoms; the procedure implemented in SHELXL [] was applied with the Y and Yb curves used for the same site (two sites with the same coordinates and displacement parameters) and the total occupancy constrained to 1.0. Crystal data, data collection information, and structure refinement details are given in Table 2; atom coordinates and displacement parameters are given in Table 3, and selected interatomic distances are shown in Table 4. Table 5 provides the results of the bond valence analysis; the bond valence parameters for the cation–oxygen and cation–fluorine bonds have been taken from [,], respectively.
Table 3. Atomic coordinates, site occupancies, and displacement parameters (Å2) for kuliokite-(Y).
Table 4. Selected interatomic distances (Å) for the crystal structure of kuliokite-(Y).
Table 5. Bond valence analysis (in valence units = v.u.) for kuliokite-(Y).

3. Results

3.1. Cation Coordination and Site Assignment

The crystal structure of kuliokite-(Y) contains two symmetrically independent Y sites, Y1 and Y2, coordinated by eight and seven X anions, respectively (X = O, F) (Figure 3). The coordination polyhedron of the Y1 site can be described as a distorted square antiprism formed by five O and three F atoms, whereas the Y2X7 polyhedron is a distorted pentagonal bipyramid with 7X = 4O + 3F. Both types of coordination are typical for inorganic rare earth compounds []. The average <Y–X> bond lengths are 2.330 and 2.294 Å for the Y1 and Y2 sites, respectively, which means that the Y2 coordination is more tight and is more suitable for smaller cations. This is in agreement with the observed site-scattering factor (SSF) values for the Y1 and Y2 sites equal to 43.96 and 48.61 e−, respectively. According to this observation, in kuliokite-(Y), HREEs show selective preference for the smaller Y2 site, which agrees with the lanthanoid contraction, i.e., the gradual decrease in the atomic and ionic sizes of lanthanides from La to Lu [,]. Figure 4 shows the changes in ionic radii for the 7- and 8-coordinated REE3+ ions in comparison with Y3+ (based on the recent tabulation of ionic radii by Gagne and Hawthorne []). The ionic radii for the 7- and 8-coordinated Yb3+ cations are 0.959 and 0.991 Å, respectively, which are smaller than the respective radii for the Y3+ cations (0.966 and 1.024 Å, respectively). Thus, the observed fractionation of HREEs in the crystal structure of kuliokite-(Y) is governed by the coordination features (coordination number and geometry) of the Y sites.
Figure 3. The coordination polyhedra of Y atoms in the crystal structure of kuliokite-(Y). Legend: Y, O, and F atoms are shown as gray, red, and green spheres, respectively.
Figure 4. The ionic radii of trivalent lanthanide ions in []- and []-coordinated cations (blue and green, respectively) in comparison to Y3+ (on the right side of the diagram). The values of the ionic radii are taken from [].
The average <SSF> value for the Y sites derived from crystal structure analysis is 46.29 e−, which is somewhat smaller than the average value of 47.31 e− obtained from chemical analysis. However, it agrees well with the value of 46.14 e− observed in the analysis 3 in Table 1. The variability in the HREE content in kuliokite-(Y) was mentioned in the original study [], though our sample has a much higher HREE content than the holotype crystals.

3.2. Structure Description

The crystal structure of kuliokite-(Y) is shown in Figure 5a,b. The strongest building unit of the structure is the [AlX2(SiO4)2] chain of corner-sharing AlX6 octahedra and SiO4 tetrahedra running along the a axis (Figure 5c). The chains have their planes oriented parallel to (001). The Y atoms are located in between the chains, along with the F− and (OH)− anions, providing the three-dimensional integrity of the crystal structure. Each F− anion is coordinated by three Y3+ cations to form planar (FY3)8+ triangles parallel to the (010) plane. The triangles share common edges to form [F2Y2]4+ chains, as shown in Figure 5d. The similar but isolated (FY3)8+ groups have also been observed in thalénite, Y3[Si3O10]F [,], and cappelenite-(Y), BaY6B6Si3O24F2 [].
Figure 5. The crystal structure of kuliokite-(Y) projected along the a axis shown in polyhedral (a) and combined polyhedral and ball-and-stick (b) representations; the [AlX2(SiO4)2] chain (c) and the [F2Y2] chain of (FY3) triangular groups (d). Legend: Y, O, H, and F atoms are shown as gray, red, black, and green spheres, respectively; Y, Al, and Si coordination polyhedra are shown in greenish-gray, light-blue, and yellow colors, respectively.
Due to the relatively high coordination number of Y atoms and their varied coordination, providing a description of the crystal structure in terms of Y coordination polyhedra is difficult and non-transparent. An alternative description of the structural topology of kuliokite-(Y) may be devised using nodal representation [,], where each coordination polyhedron is designated by a node and two nodes are linked together by an edge, if the respective Y polyhedra share common ligands. In the crystal structure of kuliokite-(Y), each Y polyhedron is linked to six adjacent polyhedra by sharing common edges and corners (Figure 6). The Y…Y distances from central to adjacent Y sites do not exceed 4.5 Å.
Figure 6. The atomic environment of the Y1 (a) and Y2 (b) sites in the crystal structure of kuliokite-(Y) within the 4.5 Å sphere. Legends as in Figure 3 and Figure 5. The adjacent Y sites are linked by an edge if their coordination polyhedra share common O or F atoms.
If the Y sites located from each other within 4.5 Å are linked by edges, a three-dimensional Y network is obtained, which is shown in Figure 7a. The network contains 6-coordinated nodes and possesses channels that are parallel to the a axis and accommodate the one-dimensional blue-and-yellow graphs that reflect the topologies of linkage of AlX6 and SiO4 polyhedra of the aluminosilicate chains shown in Figure 5c.
Figure 7. The crystal structure of kuliokite-(Y) shown as the Y network of the skd topology with channels occupied by Al-Si graphs corresponding to the aluminosilicate octahedral–tetrahedral chains (a); the spatial relations between the Al-Si chains and double Y chains (b).
It is remarkable that the topology of the Y network in kuliokite-(Y) is known in coordination chemistry and corresponds to the skd topology stored in the Reticular Chemistry Structure Resource (RCSR) [,]. The skd topology is relatively simple []. According to the TopCryst servise of the Samara Topological Data Center [], the skd net had previously been observed in molecular structures only, where molecules are linked together by hydrogen, Coulomb, or van der Waals bonds [,,]. Therefore, kuliokite-(Y) is the first example of an inorganic crystal structure with the skd topology of interpolyhedral linkage. The ideal space group of the skd network is Imma, which is a supergroup of the space group Im determined herein for kuliokite-(Y). Note that in the skd net, there is only one symmetrical kind of vertex. The symmetry lowering from Imma to Im with splitting of a single Y node into two symmetrically independent Y sites is, at least in part, driven by HREE fractionation.
The backbone of the skd network is the dense double chain formed by Y3 triangles that actually correspond to the two chains of the type shown in Figure 5d. The spatial relations between the double Y chains and the Al-Si chains are shown in Figure 7b.

3.3. Hydrogen Bonding

As was mentioned above, inspection of electron density difference Fourier maps allowed us to locate one H position only, namely one associated with the O6 site. The local environment of the H site is shown in Figure 8. There are no obvious strong hydrogen bonds donated by the OH6 group. There are two F- anions located at 2.178 Å from the H atom; however, these H…F distances are too long for the hydrogen bonds formed by fluorine atoms []. The bond valence sums associated with the O5 and O6 sites are equal to 1.22 and 1.10 v.u., respectively, which agrees well with their assignment to hydroxyl groups.
Figure 8. The local environment around the H site in the crystal structure of kuliokite-(Y), and the coordination of F and OH sites. Legend as in Figure 3 and Figure 5.

4. Discussion

4.1. Rare Earth Fractionation

The crystal structure of kuliokite-(Y) represents a unique, in its transparency, example of REE fractionation governed by coordination mechanisms that involve lanthanide contraction. There are other minerals that display REE fractionation such as tveitite-(Y) [], davidite-(La) [], etc., and kuliokite-(Y) is another prominent example of such behavior.

4.2. Chemical Formula and Nomenclature Considerations

The original chemical formula of kuliokite-(Y) reported by Voloshin et al. [] was given as Y4Al(SiO4)2(OH)2F5. The ideal formula of our sample can be written as Y4Al[SiO4]2(OH)2.5F4.5, which shows a higher OH content compared to the original samples. There are five X sites in kuliokite-(Y): F1, F2, F3, O5, and O6. The F1 and F2 sites form planar triangular groups (FY3) with F atoms located within the planes defined by three Y atoms (Figure 8). It is rather unlikely that these sites may host hydroxyl groups. On the other hand, the O6 site is most likely the preferential site for hydroxyl and cannot be occupied by F− anions. The F3 and O5 sites have very similar trigonal pyramidal coordination environments consisting of two Y and one Al atoms each (Figure 8). These sites can potentially be occupied by both F− and (OH)− anions. If one of these sites is occupied by F and another by OH, the overall formula corresponds to the formula Y4Al(SiO4)2(OH)2F5, which is the original formula of kuliokite-(Y) accepted by the International Mineralogical Association (IMA). If both F3 and O5 sites are occupied by fluorine, the formula should be written as Y4Al(SiO4)2(OH)F6, which corresponds to the potentially new mineral species, ‘fluorokuliokite-(Y)’. In contrast, the occupancy of both F3 and O5 sites by hydroxyl groups would lead to the formula Y4Al(SiO4)2(OH)3F4 that could correspond to the species with the possible name ‘hydroxykuliokite-(Y)’. The sample studied in this work is therefore on the borderline between kuliokite-(Y) and ‘hydroxykuliokite-(Y)’. More analytical and structural research on material from different locations is needed in order to check whether ‘fluorokuliokite-(Y)’ and ‘hydroxykuliokite-(Y)’ exist in nature.

4.3. Optical Orientation

The revision of the symmetry of kuliokite-(Y) requires reconsideration of the optical orientation of the mineral. In the original work, the relations between crystallographic and optical axes were given as follows: cs = γ, β∧bs = 28°, α∧as = 7°. Taking into account the changes in crystallographic axes described by Equation (1) above and the fact that bs∧bt = 30.7° (rather close to the value of 28°), the correct optical orientation can be written as at = γ, β = bt, α∧ct = 12.9° (in obtuse β angle). Since α = 1.656(1), β = 1.700(1), and γ = 1.703(1) [], kuliokite-(Y) has rather strong birefringence with the direction of maximal refraction corresponding to the direction of the aluminosilicate chains and the direction of minimal refraction oriented perpendicular to the planes of these chains. This kind of optical orientation is in agreement with the general principles of optical anisotropy observed for layered or pseudo-layered materials [].

4.4. Comparison with Trimounsite

It is of interest that the crystal structure of kuliokite-(Y), when viewed in terms of its cationic network (Figure 7), is topologically related to the crystal structure of trimounsite-(Y), Y2Ti2(SiO4)O5 [,], a rare yttrium titanosilicate from the Trimouns talc deposit, Ariège, France. In trimounsite (Figure 9a), the basic structural unit is the [Ti2O5(SiO4)] chain consisting of the backbone chain of edge-sharing TiO6 octahedra incrustated by single SiO4 tetrahedra (Figure 9b,c). The titanosilicate chains are linked by Y3+ cations located in between the chains. The topology of linkage of YOn polyhedra in trimounsite-(Y) is identical to that observed in kuliokite-(Y) and corresponds to the skd network topology (Figure 9d). The one-dimensional channels in the skd network are occupied by complex Ti-Si chain graphs shown in Figure 9c.
Figure 9. The crystal structure of trimounsite-(Y) in polyhedral representation (a) and its titanosilicate chain (b); the graph of the chain (c) and the crystal structure shown as the skd-type network with TiSi chain graphs in one-dimensional channels (d). Legend: Y polyhedral and Y nodes are shown in dark green; Ti polyhedral and Ti nodes are shown in light-blue; Si polyhedra and Si nodes are shown in yellow.

5. Conclusions

This crystal–chemical study of kuliokite-(Y) from the holotype locality allowed for the reconsideration of its symmetry as monoclinic with the non-centrosymmetric space group Im. The refinement of site occupancies indicated that the mineral represents a rare case of HREE fractionation among two cation sites driven by their coordination numbers and geometry. In agreement with the lanthanide contraction, HREEs are selectively incorporated into the site with a smaller coordination number and tighter coordination environment. The analysis of second-neighbor coordination of Y sites allowed us to identify the structural topology of kuliokite-(Y) as the only case of the skd network in inorganic compounds, previously known in molecular structures only. The variety of anionic content in the mineral allows us to identify the potential existence of two other mineral species that can tentatively be named as ‘fluorokuliokite-(Y)’ and ‘hydroxykuliokite-(Y)’.
Finally, it should be noted that, as to our knowledge, there are no known synthetic analogues of kuliokite-(Y). Taking into account the non-centrosymmetric nautre of its crystal structure, it may be of interest to synthesize kuliokite and to investigate its physical properties []; doping with HREEs may play an essential role in its preparation.

Author Contributions

Conceptualization, S.V.K. and V.N.Y.; methodology, S.V.K., O.F.G., and Y.A.P.; validation, S.V.K.; formal analysis, S.V.K., O.F.G., and Y.A.P.; investigation, S.V.K., V.N.Y., O.F.G., and Y.A.P.; data curation, S.V.K.; writing—original draft preparation, S.V.K.; writing—review and editing, S.V.K. and V.N.Y.; visualization, S.V.K. and Y.A.P.; supervision, S.V.K.; project administration, S.V.K.; funding acquisition, S.V.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded in the framework of the state tasks FMEZ-2025-0070 (Nanomaterials Research Centre) and FMEZ-2024-0008 (Geological Institute) of the Kola Science Centre, Russian Academy of Sciences.

Data Availability Statement

The crystal structure data for kuliokite-(Y) are available as a CIF–file from the CCDC/FIZ Karlsruhe database under CSD # 2482967 at https://www.ccdc.cam.ac.uk (accessed on 8 October 2025).

Acknowledgments

The X-ray diffraction and chemical analytical studies were performed in the FRC KSC RAS Centre for Collective Use of Equipment.

Conflicts of Interest

The authors declare no conflicts of interest.

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