Zr-Rich Eudialyte from the Lovozero Peralkaline Massif, Kola Peninsula, Russia

: The Lovozero peralkaline massif (Kola Peninsula, Russia) has several deposits of Zr, Nb, Ta and rare earth elements (REE) associated with eudialyte-group minerals (EGM). Eudialyte from the Alluaiv Mt. often forms zonal grains with central parts enriched in Zr (more than 3 apfu) and marginal zones enriched in REEs. The detailed study of the chemical composition (294 microprobe analyses) of EGMs from the drill cores of the Mt. Alluaiv-Mt. Kedykvyrpakhk deposits reveal more than 70% Zr-enriched samples. Single-crystal X-ray diffraction (XRD) was performed separately for the Zr-rich (4.17 Zr apfu) core and the REE-rich (0.54 REE apfu) marginal zone. It was found that extra Zr incorporates into the octahedral M1A site, where it replaces Ca, leading to the symmetry lowering from R 3 m to R 32. We demonstrated that the incorporation of extra Zr into EGMs makes the calculation of the eudialyte formula on the basis of Si + Al + Zr + Ti + Hf + Nb + Ta + W = 29 apfu inappropriate.


Introduction
Eudialyte-group minerals (EGMs) are trigonal Na-Zr-Ca cyclosilicates, which usually host Mn, Fe, Sr, REE, Y, Cl, F, CO 3 , H 2 O, etc. [1]. The chemical composition of eudialytegroup minerals varies in a wide range and strongly depends on the composition of the mineral-forming media. Therefore, EGMs may be considered as geochemical indicators of magmatic crystallization conditions. EGMs may experience secondary transformation due to the ion-exchange processes according to various substitution schemes [2][3][4][5][6]. The possibility of post-crystallization changes results in a wide range of chemical compositions [7] and different space groups [8], which gave rise to the 31 mineral species according to the International Mineralogical Association (IMA) list of approved minerals [9].
EGMs are rock-forming or typical accessory minerals for different types of rocks in the worldwide peralkaline massifs [10]. They have been found in all rock types of the Lovozero peralkaline massif (Kola Peninsula, Russia) [3]. During the evolution of the Lovozero massif, EGMs crystallized at all stages; they indicate the changes in melt composition, oxygen fugacity and temperature. Five new EGM species were discovered in the Lovozero massif (alluaivite, dualite, ikranite, sergevanite, voronkovite), and it seems likely that more new species are about to be discovered [11][12][13][14].
EGMs attract global interest as a prospective source of Zr, Nb, Ta and rare-earth elements (REE) [15]. Its significant deposits are located in Pajarito (New Mexico, USA) [16], Lovozero (Kola Peninsula, Russia) [17], Ilimaussaq (South Greenland) [18], Mont Saint-Hilaire (Canada) [19] and Norra Kärr (Sweden) [20]. The relative ease of extraction of EGMs by magnetic separation [21] and the significant share of heavy rare-earth elements (HREE) makes the exploration of EGM deposits economically feasible [22]. Eudialyte is easily decomposable in acids, but the formation of amorphous silica gel reduces the level of the REE and zirconium extraction into solution [23]. The economic viability has yet to be demonstrated under industry scales, though several promising multi-step leaching techniques were developed [24,25].
In this work, we present the results of studying the chemical composition and crystal structure of Zr-rich (up to 4.17 apfu Zr) and REE-enriched zones (up to 0.30 apfu REEs) of EGM samples. The EGM samples were taken from the drill cores of the Mt. Alluaiv-Kedykvyrpakhk eudialyte deposit, the north-west part of Lovozero massif, Kola Peninsula, Russia ( Figure 1). We discussed the main substitution mechanisms associated with the extra Zr incorporation into EGMs based on the data of 294 microprobe analyses in the view of general petrology and geochemistry of the Lovozero massif [24].
Layered complex (77% of massif volume) is composed of alternated sub-horizontal layers or "rhythms". Each rhythm is a sequence of rocks (from top to bottom): lujavritefoyaite-urtite or lujavrite-foyaite. The transitions between the listed rocks within the rhythm are gradual, and the boundaries between the rhythms are sharp and are often marked by pegmatites. Lujavrite is a coarse-to medium-grained mesocratic nepheline syenite with a trachytoid texture (laths of alkali feldspar are oriented parallel to each other). Foyaite is a coarse-to medium-grained massive leucocratic nepheline syenite, and urtite is an almost monomineralic nepheline rock.
The Eudialyte complex (18% of massif volume) overlies the Layered complex and consists of lujavrite rock enriched in EGM (eudialyte lujavrite). Lenses and sheet-like bodies of foyaite, as well as porphyritic and fine-grained nepheline syenites, are irregularly located among eudialyte lujavrite.
Poikilitic complex (5% of the massif volume) consists of poikilitic feldspathoid (nepheline, sodalite and vishnevite) syenites. The rocks of this complex form lens, sheetlike bodies or irregularly shaped bodies that are located inside the rocks of Layered and Eudialyte complexes.
There are a large number of xenoliths of Devonian volcaniclastic rocks [32,34], both unaltered and intensely metasomatized (fenitized) found among the rocks of the Layered and Eudialyte complexes.
The Alluaiv-Kedykvyrpakhk eudialyte deposit is part of the Eudialyte complex. The ore consists of a eudialyte lujavrite, whose EGM content can reach 80% of the rock volume. The interest in the Lovozero EGMs as a source of Zr and REEs is due to the gigantic amounts of these minerals, their comparatively easy ore processing and the possibility of product recovery [35].

Materials and Methods
Two EGM samples (LV-153/178 and LV-117/226) from the most common rocks of the Eudialyte complex with high Zr and REE content were selected for chemical and single-crystal XRD studies.
The thin polished sections were analyzed using the scanning electron microscope LEO-1450 (Carl Zeiss Microscopy, Oberkochen, Germany), with energy-dispersive microanalyzer Quantax 200 (Bruker, Massachusetts, USA) to obtain BSE (Back Scattered Electron, Carl Zeiss Microscopy, Oberkochen, Germany) images and pre-analyze all detected minerals.  Polished sections were analyzed using Leica M205 polarizing stereomicroscope equipped by Leica DFC295 camera.
Chemical contents (in atoms per formula unit, apfu) were calculated with the MINAL program of D. Dolivo-Dobrovolsky [36]. Statistical analyses were carried out with the STATISTICA 8.0 software (Statsoft company, Dell, Round Rock, TX, USA) [37].
The crystal structures of the EGMs were studied using a Rigaku/Oxford Diffraction XtaLAB Supernova diffractometer at room temperature. More than a hemisphere of diffraction data were collected using CuKα-radiation and scanning along ω with a step of 1 • and the exposure time of 10 s. Empirical absorption correction was applied in the CrysAlisPro [38] program complex using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The unit cells were refined by the least-squares methods. The structure was refined in the SHELX program [39]. The crystal structure was drawn using the Diamond program [40]. Occupancies of the cation sites were calculated from the experimental site-scattering factors in accordance with the empirical chemical composition.
The crystal structure of the eudialyte was first reported in 1971 by Golyshev et al. [41] and Giuseppetti et al. [42], who proposed three possible space groups, R3m, R3m and R32, but refined the structure in the R3m space group only. According to the systematic investigation, EGMs may crystallize in the R3m, R3m or R3 space groups [7]. The crystal structures of the LV-117/226 and LV-153/178 samples were refined in the R3m, R3m, R32 and R3 space groups.
For the LV-153/178 sample, the lowering of symmetry did not improve the R-value and the R3m space group was chosen for the final refinement. The crystal structure of LV-153/178 was refined to R 1 = 0.037 for 1300 independent reflections with F 2 > 4σ(F 2 ), respectively, with the site nomenclature following the IMA recommendations [1].
The crystal structure of the LV-117/226 sample was initially refined in the R3m space group to R 1 = 0.080. The refinement in the R3 space group resulted in R 1 = 0.083 and physically unrealistic displacement parameters for the M1A,B sites. The refinement in the R3m space group to R 1 = 0.072 did not sufficiently improve the structure model. The best refinement was performed in the R32 space group with R 1 = 0.034 for 2591 independent reflections with F 2 > 4σ(F 2 ).
The crystal structure data are deposited in the CCDC under the entry numbers 2082760-2082761.

Chemical Composition
The calculation of the EGMs formulas was performed according to recommendations by Johnsen and Grice [7]. The general EGM formula can be written as:  Table 1 provides microprobe results for structurally investigated EGMs. The calculation based on Si + Al + Zr + Ti + Hf + Nb + Ta + W = 29 apfu shows the excess of Zr (more than 3 apfu) in all investigated samples.

Crystal Structure: General Scheme
The crystal structure of EGMs can be described as a stacking of complex ZTMT modules perpendicular to [001] with t q = 1/2a + 1/3c (Figure 3a) [43].  [41,42]. The nine-membered rings may be centered by additional M4B tetrahedra occupied by Si or be vacant. Ordering of the additional Si-centered tetrahedra may result in the doubling of the c parameter~60 Å [11]. The Z-layers are sandwiched between two T-layers and composed of ZrO 6 octahedra and N1A,B polyhedra. The M-layer (Figure 3b) consists of octahedral 6-and 9-membered rings based upon M1O 6 octahedra or alternating M1O 6 and M2O 6 octahedra (that can be replaced by square pyramids and squares). One-half of the ninemembered rings are usually centered by additional M4A octahedra. Two different types of the M-layers were observed in several "megaeudialytes" [44,45]. The Cl − and OH − anions are located in the cavities of the M layer and may reach up to 2 apfu. The eudialytetype MT-framework usually contains significant numbers of split and low-occupied sites (N1A,B, M2A,B, M3 and M4). rings may be centered by additional M4B tetrahedra occupied by Si or be vacant. Orderin of the additional Si-centered tetrahedra may result in the doubling of the c parameter~6 Å [11]. The Z-layers are sandwiched between two T-layers and composed of ZrO6 octahe dra and N1A,B polyhedra. The M-layer (Figure 3b) consists of octahedral 6-and 9-mem bered rings based upon M1O6 octahedra or alternating M1O6 and M2O6 octahedra (tha can be replaced by square pyramids and squares). One-half of the nine-membered ring are usually centered by additional M4A octahedra. Two different types of the M-layer were observed in several "megaeudialytes" [44,45]. The Cl − and OHanions are located i the cavities of the M layer and may reach up to 2 apfu. The eudialyte-type MT-framewor usually contains significant numbers of split and low-occupied sites (N1A,B, M2A,B, M and M4).

Crystal Structure: LV-153/178
T-layers. The Si1, Si3 and Si5 sites are fully populated by Si atoms with the <Si-O> distances in the range 1.594-1.645 Å. The additional tetrahedral M4 site is partially populated owing to the short M3-M4 distance of~1.30 Å. The refined occupancy of the M4 site is Si 0.27 or 0.54 Si apfu. The total refined Si content is 24.54 apfu that is less than 25.56 Si apfu calculated from the chemical data. The additional octahedral M3 site has an occupancy of Nb 0.33 or 0.65 Nb apfu and an average Nb-O distance of 1.808 Å. The nine-coordinated N4 site is populated by Na with a small admixture of Sr (Table 3).
Z-layers. The octahedral Z1 sites are fully populated by Zr. The average <Z1-O> distance is 2.071 Å. The N1 site is split into 8-coordinated N1A and 7-coordinated N1B subsites with the refined occupancies of Na 0.75 and Na 0.25 , respectively, and the <Na-O> distances of 2.644 and 2.659 Å, respectively (Figure 4).   M-layer. The octahedral M1 site is populated by Ca with a minor admixture of REE that is in agreement with the previously reported XANES data [15]. The refined REE content is 0.54 apfu, which exceeds 0.25 REE apfu derived from the chemical data. The additional electron density may be explained by a possible admixture of Zr. The M1-O distances are in the range 2.316-2.392 Å. The M2 site split into the M2A and M2B sites with the M2A-M2B distance of 1.685 Å. The M2A octahedral site has four short M2A-O8 bonds of 2.110 Å and two long distances of 2.670 Å to the low-occupied O11 site. The refined scattering value for the M2A site is 17.60 e − corresponds to the refined occupancy of Fe 0.66 . According to [46], the extra Zr-content is associated with the low occupied five-coordinated M2B site (Figure 4)

Discussion
An important feature of the geochemistry of the Lovozero massif, which distinguishes it from the neighboring Khibiny massif, is the low calcium content. The average calcium content in the massif rocks is 1.22 wt.%, and the Na/Ca ratio is 9.39 [32]. This is reflected in the chemical composition of rock-forming and accessory minerals. Ca-bearing minerals are not characteristic of the massif rocks [47], and the bulk of this element is included as an admixture in sodium-bearing minerals (e.g., rock-forming aegirine and magnesium-arfvedsonite).
EGM's crystallize in the rocks of the Lovozero massif during cooling foiditic magma at the temperature range 750-900 • C according to nepheline geothermometer data [48,49]. The geochemical behavior of Ca during crystallization of the Lovozero Massif rocks can be conventionally compared with the behavior of incompatible elements. In the early stages of rock crystallization, Ca was dispersed in rock-forming minerals (for example, as a diopside component in aegirine) and accumulated only by the later stages of crystallization. Ca sufficiently accumulates and forms its own minerals, for example, fluorapatite and titanite, only in the most evolved rocks. The EGMs require calcium for crystallization, 6 apfu of Ca are necessary for building an octahedral ring. Owing to the flexibility of the eudialyte structure, it can crystallize in conditions of extreme calcium deficiency. Thus, low-calcium eudialyte is formed, such as oneillite, raslakite and voronkovite. The calcium deficiency in these minerals is compensated by manganese and/or iron. Complex substitutions and the presence of extra Zr content with Ca-deficiency make some problems for the EGM normalization formula.
The normalization scheme based on Si + Al + Zr + Ti + Hf + Nb + Ta + W = 29 apfu proposed in [7] produces strong correlation between Zr and Si contents for the EGM samples from Mt. Alluaiv (Figure 5a). This correlation is misleading if the same graph is based on atomic amounts. The absence of correlation between the Si and Zr contents is demonstrated in Figure 5b. The same problems with the normalization of the EGM formula were noted in [3]. These authors propose to use the atomic amounts of cations to trace changes in the composition of EGM during magmatic evolution. As an alternative way for the calculation of the EGM formula, we recommend a normalization scheme based on the Si content determined directly from the single-crystal XRD refinement for selected samples (two last columns in Table 1). The EGMs are usually distinctly zoned (Figure 2b) with cores that are Ca-deficient and Zr-enriched and marginal zones enriched by REE [3] (Table 1). Factor analysis of the data ( Figure 6) on the composition of eudialyte from rocks of the eudialyte complex showed a decrease in Ca content with an increase in Zr content (as well as Al, Fe, Mg and Na). High Zr concentrations are observed in the cores of zonal eudialyte crystals from eudialyte lujavrites. The outer rims of these crystals are relatively enriched in calcium, as well as La, Ce, Ti, Nb and Mn. The same elements are usually enriched in homogeneous eudialyte grains from evolved rocks of the eudialyte complex, namely foyaites, porphyritic and fine-grained nepheline syenites. The Zr-rich EGMs are primary and possess no signs of secondary solid-state alteration [50].
Forming of zonal EGM crystals with normal Ca content (6 apfu) at the late rims of EGM may be connected with (Figure 2b) Ca accumulation during crystallization of foiditic magma or as a result of partial melting/fenitization of Ca-rich xenoliths of Devonian volcaniclastic rocks [51].
The observed chemical variations in core and marginal zones of EGM well agrees with the observed two-phase and three-phase (davinciite-rastsvetaevite-"hydrorastsvetaevite") concentrically zoned crystals, where the central part is represented by davinciite, while the outer lighter rims are formed by rastsvetaevite and "hydrorastsvetaevite" [52,53].  (Table 1). Factor analysis of the data (Figure 6) on the composition of eudialyte from rocks of the eudialyte complex showed a decrease in Ca content with an increase in Zr content (as well as Al, Fe, Mg and Na). High Zr concentrations are observed in the cores of zonal eudialyte crystals from eudialyte lujavrites. The outer rims of these crystals are relatively enriched in calcium, as well as La, Ce, Ti, Nb and Mn. The same elements are usually enriched in homogeneous eudialyte grains from evolved rocks of the eudialyte complex, namely foyaites, porphyritic and fine-grained nepheline syenites. The Zr-rich EGMs are primary and possess no signs of secondary solid-state alteration [50]. According to our chemical data, the Zr content in EGMs may reach up to 4.2 apfu. The Zr amount linearly increases with the increasing Al content (Figure 7a). It seems that the incorporation of extra Zr may be connected with several complex substitution schemes, including vacancies and the possible incorporation of Al into the M4 site. The negative correlations with Zr were observed for REE, Ca and Nb (Figure 7b-d). It should Forming of zonal EGM crystals with normal Ca content (6 apfu) at the late rims of EGM may be connected with (Figure 2b) Ca accumulation during crystallization of foiditic magma or as a result of partial melting/fenitization of Ca-rich xenoliths of Devonian volcaniclastic rocks [51].
The observed chemical variations in core and marginal zones of EGM well agrees with the observed two-phase and three-phase (davinciite-rastsvetaevite-"hydrorastsvetaevite") concentrically zoned crystals, where the central part is represented by davinciite, while the outer lighter rims are formed by rastsvetaevite and "hydrorastsvetaevite" [52,53].
According to our chemical data, the Zr content in EGMs may reach up to 4.2 apfu. The Zr amount linearly increases with the increasing Al content (Figure 7a). It seems that the incorporation of extra Zr may be connected with several complex substitution schemes, including vacancies and the possible incorporation of Al into the M4 site. The negative correlations with Zr were observed for REE, Ca and Nb (Figure 7b-d). It should be noted that the Zr-rich EGMs are depleted in Ca (Figure 7b), and this correlation is the most significant (R 2 = 0.34). In the crystal structures of EGMs, Ca and REE occupy the M1 octahedral site [1,7,8,15], and the incorporation of additional Zr is also connected with the M1 site. Nb occupies the octahedral M3 site [7]. The decrease in the Nb content correlated with the increasing Zr content may be connected with the vacancies at the M3 site. Zr placed into the five-coordinated M2A site according to the previous studies [46]. However, the M1 site contains excessive electron density (refined REE content is 0.54 apfu instead of 0.25 REE apfu from chemical data). It seems that extra Zr content is associated with the M1 site, which has more typical for Zr octahedral coordination. The five-coordinated Zr (Figure 4) in inorganic structures is unlikely, which forces us to place all extra Zr into a more appropriate octahedrally coordinated M1 site. According to the new data for Ca-deficient EGM from the Lovozero massif, the ordering of Mn into the M1 site leads to lowering of the total EGM symmetry to the R3 space The marginal zones of EGM grains with the small excess of Zr (0.2 apfu) or non-zonal grains with normal Zr-content (3 apfu) crystallize in the R3m space group with additional Zr placed into the five-coordinated M2A site according to the previous studies [46]. However, the M1 site contains excessive electron density (refined REE content is 0.54 apfu instead of 0.25 REE apfu from chemical data). It seems that extra Zr content is associated with the M1 site, which has more typical for Zr octahedral coordination. The five-coordinated Zr ( Figure 4) in inorganic structures is unlikely, which forces us to place all extra Zr into a more appropriate octahedrally coordinated M1 site.
According to the new data for Ca-deficient EGM from the Lovozero massif, the ordering of Mn into the M1 site leads to lowering of the total EGM symmetry to the R3 space group with the splitting of the M1 site into two subsites [54]. In the studied structure, the M1A and M1B sites differ in site-scattering factors (by more than 3 e − ) and in polyhedral volumes (16.17 vs. 15.61 Å 3 ). The incorporation of extra Zr into the M1A octahedral site is supported by the total refined Zr content (Z1 + M1A) of 4.15 apfu, which is in excellent agreement with the chemical data (Table 1). Such type of ordering results in local symmetry lowering for the six-membered octahedral ring and consequently lowering of symmetry from R3m in Zr-poor 153/178 sample to R32 in the crystal structure of Zr-rich LV-117/226 EGM sample (Figure 8). The complex substitution schemes in EGMs can be associated with the blocky isomorphism [55], which includes block-by-block substitutions involving groups of atoms with different coordination and topologies, and in the latest works are also called local heteropolyhedral substitutions [56]. This type of substitution usually involves the speciesdefining M2, M3 and M4 sites [1,8,57].
According to our structural data, at least one-half of the additional M4 sites are populated by H2O. Summarizing our chemical ( Figure 6) and structural data, we propose the possible ways of incorporation of extra Zr into the eudialyte structure via local heteropolyhedral substitutions: The positive correlation between the Zr and Al contents in the EGMs from Mt. Kedykvyrpakhk ( Figure 6) and their negative correlation with Ca and REE may be connected with the following complex substitution: M4 Al 3+ + M1 Zr 4+ ↔ M4 Si 4+ + M1 REE 3+ Both M3 and M4 sites are predominantly vacant and, taking into account the positive correlation between Zr and Al and the negative correlation with Ca 2+ and Nb 5+ , the following complex substitution scheme can be proposed: The complex substitution schemes in EGMs can be associated with the blocky isomorphism [55], which includes block-by-block substitutions involving groups of atoms with different coordination and topologies, and in the latest works are also called local heteropolyhedral substitutions [56]. This type of substitution usually involves the speciesdefining M2, M3 and M4 sites [1,8,57].
According to our structural data, at least one-half of the additional M4 sites are populated by H 2 O. Summarizing our chemical ( Figure 6) and structural data, we propose the possible ways of incorporation of extra Zr into the eudialyte structure via local heteropolyhedral substitutions: The positive correlation between the Zr and Al contents in the EGMs from Mt. Kedykvyrpakhk ( Figure 6) and their negative correlation with Ca and REE may be connected with the following complex substitution: M4 Al 3+ + M1 Zr 4+ ↔ M4 Si 4+ + M1 REE 3+ Both M3 and M4 sites are predominantly vacant and, taking into account the positive correlation between Zr and Al and the negative correlation with Ca 2+ and Nb 5+ , the following complex substitution scheme can be proposed: As already mentioned, 70% of the EGM samples from the rocks of the Lovozero massif are hyperzirconium, i.e., their Zr content exceeds 3 apfu [3]. Figure 9 shows a schematic section along the line I-II (see Figure 1b) and the distribution of elements in the composition of the eudialyte group minerals. The Ca (Figure 9a) and Zr (Figure 9b) are antagonists. The richest in zirconium are EGMs from eudialyte lujavrite. The highest Ca concentrations are characteristic of EGMs from foyaite and rocks of the poikilitic complex (leucocratic nepheline ± sodalite syenite) (Figure 9c). The Ca-enriched EGMs here have a normal Zr-content. The cross-section was extracted from 3D block models generated by Micromine 2016.1. Interpolation was carried out by an inverse distance weighted method. The meaning units for X and Z axis are given in meters.

Conclusions
The EGMs, owing to their crystal structure flexibility, may be considered as geochemical indicators of crystallization conditions. The compositional variations in EGMs (strong zonation with cores enriched in Zr, Al and Ti and rims enriched in REE, Si, Ca, Sr and Mn) found in this study gives specific information about magmatic crystallization conditions in the Lovozero complex.
During the evolution of foiditic magma in Lovozero massif, Ca accumulated and crystallized at the last stages of EGM formation. The late EGM rims with normal Ca content connected with the process of Ca-accumulation during magma crystallization and/or addition Ca owing to melting/fenitization of Ca-rich xenoliths of Devonian volcaniclastic Interpolation was carried out by an inverse distance weighted method. The meaning units for X and Z axis are given in meters.

Conclusions
The EGMs, owing to their crystal structure flexibility, may be considered as geochemical indicators of crystallization conditions. The compositional variations in EGMs (strong zonation with cores enriched in Zr, Al and Ti and rims enriched in REE, Si, Ca, Sr and Mn) found in this study gives specific information about magmatic crystallization conditions in the Lovozero complex.
During the evolution of foiditic magma in Lovozero massif, Ca accumulated and crystallized at the last stages of EGM formation. The late EGM rims with normal Ca content connected with the process of Ca-accumulation during magma crystallization and/or addition Ca owing to melting/fenitization of Ca-rich xenoliths of Devonian volcaniclastic rocks.
The normal Zr content in EGM is 3 apfu, where Zr populates Z1 octahedral site. Most of the EGM samples (70%) from the rocks of the Eudialyte complex of the Lovozero massif are hyperzirconium, i.e., their Zr content exceeds 3 apfu. Additional Zr incorporates into eudialyte structure into the octahedral M1A site and replaces Ca with the symmetry lowering from R3m to R32.
There are three main substitution schemes associated with the incorporation of Zr into eudialyte crystal structure: (