Synthesis of Fe-deficient eudialyte analogues: Relationships between the composition of the reaction system and crystal-chemical features of the products

Abstract

Eudialyte-related compounds have been synthesised hydrothermally at T = 600°C and P = 2 kbar from a stoichiometric mixture of Na₂CO₃, CaO, Fe₂O₃, ZrOCl₂ and SiO₂ using the Na:Ca:Fe:Zr:Si ratios corresponding to the eudialyte end-member Na₁₅Ca₆Fe₃Zr₃Si(Si₂₅O₇₃)(OH)Cl₂⋅H₂O, in the presence of 1 M aqueous solutions of NaCl and NaF. The synthesis was carried out in platinum ampoules over 10 days. Natural raslakite (a Ca-deficient member of the eudialyte group) was used as a seed and added in amounts corresponding to 2 wt.% of the whole charge. The products were characterised by powder X-ray diffraction, IR and Raman spectroscopy, morphological features, and electron probe microanalyses. In experiments with NaCl, almost pure eudialyte-type compounds were obtained. Synthesis in the presence of a NaF solution resulted in the formation of a F-dominant eudialyte-type compound related to raslakite as the main product and aegirine, vlasovite and lalondeite as by-products. All synthesised eudialyte-type compounds are Zr-rich and Fe-deficient, similar to eudialyte-group minerals from hyperagpaitic rocks related to foyaites. The increased NaCl contents in the reaction system results in increased Ca content in the synthesised eudialyte-related compounds. The crystal-chemical formulae of the synthesised eudialyte-type compounds are derived based on general regularities established earlier for eudialyte-group minerals.

Introduction

In recent years, eudialyte-group minerals (EGM) have attracted increased attention. On the one hand as typical accessory (and sometimes rock-forming) minerals of agpaite complexes they are relatively widespread: numerous discoveries have been made in dozens of alkaline complexes. On the other hand, unlike most other rock-forming minerals, EGM have the ability to concentrate a number of rare elements (Zr, Hf, Nb and rare earth elements), and are therefore of practical interest as a potential source of these elements. However, the most interesting feature of eudialyte-related minerals is the unique complexity and variability of their crystal structures within one overall structure motif. The EGM are a very effective model on which many crystal-chemical, geochemical and genetic regularities have been established (Rastsvetaeva et al., 2012; Rastsvetaeva et al., 2020; Rastsvetaeva and Chukanov, 2020; Chukanov et al., 2020). The EGM are trigonal, with space groups R3m, R $\bar{3}$m or R3, and the unit-cell parameters a = 14.1–14.2 and c = 30.0–30.8 or ~60 Å (the c parameter is doubled in samples with modular structures). The crystal structures of EGM are based on a heteropolyhedral framework composed on Si₃O₉ and Si₉O₂₇ rings of tetrahedra, M1₆O₂₄ rings of edge-sharing octahedra connected via M2O_4–7 polyhedra and isolated ZO₆ octahedra. Various cations with charges from +1 to +6 and coordination numbers 4 or 6 can occur at the M3 and M4 sites situated near the centres of the Si₉O₂₇ rings. Large extra-framework cations occupy the N1–5 sites. Different extra-framework anions and water molecules occur at the O′, X1 and X2 sites. In most cases, in EGM so-called ‘blocky’ isomorphism is realised when one group of atoms and ions is replaced by another one, which is accompanied by a change in the valence state and/or coordination numbers of cations. In reality, the N1–5 and M2–4 sites do not have fixed positions but are micro-regions in which blocky isomorphism is realised. For these sites, splitting is very typical.

The general formula of EGM is (Z = 3; see Johnsen et al., 2003): N1₃N2₃N3₃N4₃N5₃M1₆M2_3–6M3M4Z ₃(Si₂₄O₇₂)O′_4–6X1X2 where: N1–5 = Na, K, Н₃О⁺, Ca, Mn²⁺, Sr, Ba and rare earth elements (REE); M1 = Ca, Mn²⁺, Fe²⁺, REE, Na and Sr; M2 = Mn²⁺, Fe²⁺, Fe³⁺, Na, Zr, Ta, Ti, K and H₃O⁺; M3 and M4 = Si, S, Nb, Ti, W and Na; Z = Zr, Ti and Nb; O′ = O, OH or H₂O; X1 and X2 = F, Cl, H₂O, ОН, CO₃ and SO₄.

Currently, the eudialyte group contains 31 mineral species approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association. Of these, 18 minerals were discovered in agpaitic rocks of three large alkaline massifs (Khibiny, Lovozero and Kovdor) located on the Kola Peninsula, Russia, and most of the remaining EGM were also identified there as accessory components of pegmatites.

From the petrological point of view, three main kinds of EGM can be distinguished. Type 1 is most common. This includes eudialyte s.s. and dominates in the Khibiny massif, and can be termed the Khibiny type. In this type, univalent cations (Na and in some cases subordinate K) prevail at the N sites, the M1 site is occupied mainly with Ca (typically, 5 to 6 Ca atoms per formula unit, apfu), the M2 site contains bivalent cations (Fe²⁺, rare Mn²⁺) and the M3 and M4 sites are occupied predominantly by Si. Thus, homovalent isomorphism is typical for the Khibiny-type EGM.

Type 2 (Lovozero type) is a Ca deficient EGM variety (with <5 Ca apfu) typical of peralkaline rocks of the Lovozero massif. In these minerals, Ca is partly substituted by Mn²⁺ and/or Fe²⁺. In minerals with Ca < 4 apfu (e.g. raslakite or sergevanite, Chukanov et al., 2003, 2020), Ca alternates with smaller cations in the M1₆O₂₄ ring which results in the division of the M1 site into two independent M1a and M1b sites and a change of symmetry from R3m or R $\bar{3}$m to R3. In Type-2 minerals, the M2 micro-region is commonly Fe,Mn-deficient and accumulates Na and Zr in accordance with the substitution scheme 3(Fe²⁺,Mn²⁺) → 2Na + Zr. The M3 and M4 sites of the Lovozero-type eudialytes can contain significant amounts of niobium and can be Nb-dominant. The Type-2 EGM are especially typical in foyaites.

Type-3 EGM (Kovdor type) are enriched in Ca and contain 6.5 to 10 Ca apfu. Excessive calcium occurs at the N3 and/or N4 sites and, usually, significant amounts of Nb occur at the M3 and/or M4 site. High contents of carbonate anions at the X sites are a specific feature of the Type-3 EGM. These minerals are typical for peralkaline rocks of ultrabasic–alkaline massifs with carbonatites. Carbonatites are considered as the source of calcium and CO₃^2– groups (Rozenberg et al., 2005; Chukanov et al., 2005; Moiseev, Chukanov, 2006). In the structures of some Type-3 EGM, strontium can have the same role as calcium (Rastsvetaeva et al., 2018; Gritsenko et al., 2020). As a rule, Sr-rich Type-3 EGM contain significant amounts of CO₃^2– groups occurring at the X sites.

In previous work, eudialyte has been synthesised as a component of polymineral assemblages, including aegirine, vlasovite and other zirconosilicates, from a mixture with the composition 6SiO₂ + ZrO₂ + 6Na₂CO₃ + CaCO₃ + FeCl₂ + 4H₂O in the presence of Na₂SiF₆ or K₂SiF₆, with temperature range of 450–550°C and H₂O pressures of 85 to 700 bars (Christophe-Michel-Lévy, 1961). In that work, eudialyte was identified by powder X-ray diffraction (PXRD) data. It formed subhedral pseudohexagonal platy crystals ~50 μm across which were almost optically isotropic, with a refractive index of 1.592. Unfortunately, no compositional data were obtained for this sample.

Recently, decomposition of eudialyte in different solvents was studied at 100°C for 48 h (Smirnova et al., 2015). In particular, it was shown that in 2 to 10% hydrochloric acid complete decomposition takes place. The interaction of eudialyte with 1–7% solutions of oxalic acid results in leaching of Ca, Na and REE, though the zirconosilicate framework remains.

In this work, we report the synthesis of eudialyte-type compounds which are close to the Types 1–3 but are depleted in iron.

Experimental

Starting materials

In the experiments, a mixture of Na₂CO₃, CaO, Fe₂O₃, ZrOCl₂, and amorphous SiO₂ obtained by deposition from tetraethyl orthosilicate solution (Trofimova et al., 2011) was used (Table 1). ZrOCl₂⋅8H₂O was preliminarily dehydrated in an oven at a temperature of 150°C for 8 hours, and then added in the required amount to the starting mixture. This was done in order to prevent excess water in the system. Na₂CO₃ and CaO were also subjected to calcination at this temperature. The Na:Ca:Fe:Zr:Si ratio of the mixture corresponded to the eudialyte end-member Na₁₅Ca₆Fe₃Zr₃Si(Si₂₅O₇₃)(OH)Cl₂⋅H₂O.

Table 1. The composition of the starting mixture.

A starting mixture was prepared by gradually adding a premeasured amount of each reagent into a nitric acid solution at 200°C; Fe₂O₃ and amorphous SiO₂ were added last. The mixture was then evaporated until a slurry was formed. The mixture was then placed in alundum crucibles and calcined at 450°C at atmospheric pressure in a muffle furnace for 14 days until complete decomposition of nitrates had taken place. The resulting sol-gel was ground in jasper mortars with ethyl alcohol to improve homogeneity and was then used as a starting material. As a seed, powdered natural raslakite (a Ca-deficient member of the eudialyte group) from the Alluaiv mountain, Lovozero alkaline massif, with the empirical formula (H₃O)_xNa_14.03K_0.24Ca_3.34Fe²⁺_2.30Mn_0.60Zr_3.23Ti_0.24(Si_0.81Nb_0.19)(Si₂₅O₇₃)Cl_1.08(SO₄)_0.12(OH,H₂O)_n (x ≈ 3, n ≈ 3) was added in amounts of 2 wt.% of the whole charge.

Synthesis

All syntheses were carried out in high-pressure reactors with internal heating (UVGD-10000 designed in the D.S. Korzhinskii Institute of Experimental Mineralogy of the Russian Academy of Sciences). Temperature and pressure measurement accuracy was ±2°C and ±50 bar, respectively.

The starting materials (150 mg of the sol-gel and 3 mg of the seed) were placed in platinum ampoules 50 mm in length, with a diameter of 4–5 mm and wall thickness of 0.1–0.2 mm. The required amount of fluid was added, then the ampoules were sealed, weighed and tested for hermeticity.

In order to determine the role of chlorine in the formation of eudialyte, three synthesis routes, with different kinds and amounts of the fluids, were carried out: 100 ml of 1 M NaCl (Route 1), 150 ml of 1 M NaCl (Route 2) and 100 ml of 1 M NaF (Route 3). The filled ampoules were placed in the reactors and held at the required PT parameters (600°C and 2 kbar) for 10 days. The PT parameters were chosen on the basis of data from Kogarko et al. (2002) on the formation of eudialyte-bearing mineral assemblages of the Lovozero massif. Quenching was carried out under isobaric conditions. After the experiments, the ampoules were weighed to control if they had or had not leaked. At the conclusion of the experiment, after unloading the ampoules, the solid products were washed with distilled water and dried in an oven at a temperature of 80°C.

Research methods

Raman spectra were measured at ambient conditions in the Institute of Experimental Mineralogy RAS using apparatus consisting of a Acton SpectraPro-2500i spectrograph with detector cooling up to –70°C, CCD Pixis2K and an Olympus microscope with a continuous solid-state monomeric laser with radiation wavelength of 532 nm and diode pumping. The laser beam diameter was ~4 μm. The spectral range resolution was 3.8 cm^–1. Raman data were collected by repeated exposures of 480 s (4 × 120 s).

Powder X-ray diffraction data were obtained at a temperature of 25°С using a Bruker-D8 diffractometer in scanning mode with a step of 0.02° and 7–40°θ range using CoKα radiation (wavelength = 1.78892 Å). These data were used to determine unit-cell parameters of the synthesised eudialyte-type compounds. Spectroscopic-purity silicon with a = 5.4307 Å was used as an internal standard. The source-to-sample distance was ~14 cm.

To obtain infrared (IR) absorption spectra, powdered samples (~0.2 mg of each) were mixed with anhydrous KBr, pelletised, and analysed using an ALPHA FTIR spectrometer (Bruker Optics) at a resolution of 4 cm^–1. A total of 16 scans were collected for each spectrum. The IR spectrum of an analogous pellet of pure KBr was used as a reference.

Electron probe microanalysis, including imaging of the experimental products in secondary (SEM) and back-scattered electron (BSE) modes were performed locally (D.S. Korzhinskii Institute of Experimental Mineralogy the Russian Academy of Sciences) using a Tescan VEGA-II XMU analytical suite including a digital scanning electron microscope with an energy-dispersive spectrometer (EDS) INCA Energy 450 and semiconducting Si (Li) detector Link INCA Energy and wave-dispersive spectrometer (WDS) Oxford INCA Wave 700; (produced by Tescan Orsay Hld., Brno, Czech Republic, https://www.tescan.com). The SEM is equipped with YAG (Y-Al garnet) secondary and back-scattered electron detectors. Calculations of the results of microanalysis were made using INCA Microanalysis Suite 4.15 (version 18d+SP4) with an option for accounting for possible matrix effects from WDS analysis. The analyses were performed using an accelerating voltage of 20 kV. The current of the absorbed electrons on a reference sample of cobalt was 195–210 pA and the current of the polished samples varied from 110 to 200 pA depending on the microrelief, structure and composition of the samples. The spectra acquisition time was 100 s. The sample-to-detector distance was 25 mm. The size of an electronic ‘spot’ on the surface of a sample varied in the range of 157–180 nm, and the zone of excitation was ~4–5 μm. To check for Na migration during the analysis, additional measurements with a beam defocused to the area of 20 × 20 μm were made. The following standards and analytical lines were used: CaF₂ (FKα); albite (NaKα); wollastonite (CaKα); synthetic Al₂O₃ (AlKα); SiO₂ (SiKα); pure Fe (FeKα); pure Zr (ZrLα); pure Hf (HfLα); and synthetic NaCl (ClKα).

Results

Composition and morphological features of the products

The products obtained during the experiments are fine-crystalline aggregates of greenish-beige colour. In BSE images (Figs 1 and 2), well-formed hexagonal crystals of newly-formed eudialyte-type compounds can be seen. The main crystal forms are pinacoid {0001}, prism {11$\bar{2}$0} and rhombohedron {02$\bar{2}$1}.

Fig. 1. BSE (BSE+SE combined in (с)) images of crystals of synthesised eudialyte-related compounds: (a) Route 1 (polished section, from experiment #23), (b) Route 2a and (c) Route 2b (samples from experiment #24).

Fig. 2. BSE (BSE+SE combined in (b)) images of polished sections of the products obtained in Route 3: (a) eudialyte-type compound (light grey) and lalondeite (dark grey) and (b) crystals of a eudialyte-type compound (light grey) with aegirine inclusions (dark grey) (samples from experiment #25).

Compositional data are given in Table 2. The product obtained in Route 1 is close to Type-1 EGM (Khibiny type) based on the Ca content although unlike typical Type-1 EGM, it is Fe-deficient and enriched in Zr. Taking into account the general crystal-chemical regularities for EGM, one can conclude that in the crystal structure of this compound the M2 site has a mixed occupancy (Fe + Zr + Na).

Table 2. Composition (wt.%) of the synthesised eudialyte-type compounds (S.D. = standard deviation; bdl = ‘below detection limit’).

Note: The compositional data are averaged over four (in Routes 1, 2b, and 3) or two (for Route 2a) spot analyses.

For the EGM obtained by Route 2, two groups of local compositions with contrasting Ca and Fe contents can be distinguished. They correspond to a Ca-rich and Fe-deficient eudialyte variety (Route 2a) and Fe-deficient Nb-free analogue of golyshevite (Route 2b). Thus, these products are intermediate between the Fe-deficient analogues of Type 1 and Type 3.

The eudialyte-related compound obtained in Route 3 corresponds to Type 2 (Lovozero type). It is Ca-and Fe-deficient and contains an excess of Zr (1.02 apfu) compared to the eudialyte end-member. In addition, this product is Na rich, which is also a specific feature of Type-2 EGM.

Infrared spectroscopy

Absorption bands in the IR spectra of eudialyte-related compounds obtained in the Routes 1 to 3 are shown in Fig. 3. The assignment of bands, based on the analysis of IR spectra of several tens of structurally investigated eudialyte-group minerals in accordance with Rastsvetaeva et al. (2012) is as follows: 3400 to 3700 cm^–1 is assigned to O–H stretching vibrations; 1630 to 1650 cm^–1 to H–O–H bending vibrations; 1430 to 1510 cm^–1 to the asymmetric stretching mode of carbonate groups situated at the X sites; 900 to 1100 cm^–1 to Si–O stretching vibrations; 737 to 740 cm^–1 to mixed vibrations of rings of SiO₄ tetrahedra (‘ring band’: Sitarz et al., 2000); 640 to 700 cm^–1 to other mixed modes of the rings of SiO₄ tetrahedra; and 527 to 530 cm^–1 to (Zr,Fe)–O stretching vibrations. Bands below 500 cm^–1 correspond to lattice modes involving predominantly bending vibrations of rings of SiO₄ tetrahedra and Ca–O stretching vibrations.

Fig. 3. Infrared absorption spectra of the seed used in the syntheses (a) and eudialyte-related compounds obtained in Route 1 (b), Route 2 (c), and Route 3 (d).

Low intensities of the bands at 527–530 cm^–1 (Routes 1 to 3) and high intensities of the peaks at 938 and 944 cm^–1 (Routes 1 and 2) reflect low contents of iron at the M2 site and high occupancies of the M3 and M4 sites with Si. A relatively low intensity of the peak at 940 cm^–1 in the IR spectrum of the product obtained in Route 3 might indicate the presence of vacancies at the M3 and M4 sites (Rastsvetaeva et al., 2012).

According to the IR spectroscopy data, all eudialyte-related compounds obtained in this work contain carbonate groups. Clearly Na₂CO₃ was the source of these carbonate-group bands. However, the product obtained in Route 3 is characterised by a low CO₃^2– content, which is typical for the Type-2 EGM.

Raman spectroscopy

The Raman spectra of the seed (natural raslakite) and the eudialyte-related compound obtained in Route 2 are shown in Fig. 4. Both spectra contain bands of stretching vibrations of Si–O–Si and Si–O–Zr fragments (in the ranges of 990–1090 and 520–570 cm^–1, respectively) and apical Si–O bonds (940–970 cm^–1) as well as mixed and bending vibrations of the heteropolyhedral framework (other bands below 870 cm^–1). Bands in the range of 1200 to 3200 cm^–1 correspond to strong hydrogen bonds in hydrated H₃O⁺ complexes (Zundel and Eigen cations) with the O⋅⋅⋅O distances from 2.38 to 2.72 Å, which are very typical components of Na-depleted eudialyte-group minerals (Chukanov et al., 2022).

Fig. 4. Raman spectra of the seed used in the syntheses (a) and the eudialyte-related compound obtained in Route 2 (b).

X-ray diffraction

Data from PXRD for the synthesised materials are given in Table 3. These data, show that the products of Route 1 and Route 2 are almost pure eudialyte-type compounds whereas the product of Route 3 contains significant amounts of impurities, including lalondeite (Na,Ca)₆(Ca,Na)₃Si₁₆O₃₈(F,OH)₂⋅3H₂O, vlasovite Na₂ZrSi₄O₁₁ and Ca-bearing aegirine (Na,Ca)(Fe³⁺,Fe²⁺)Si₂O₆. Other (minor) impurities detected by semiquantitative EDS analysis are pectolite and an unidentified, presumably X-ray amorphous zirconosilicate forming fine intergrowths with vlasovite. Impurity peaks at 2.999 and 2.995 Å observed in the PXRD patterns of the products of Route 1 and Route 2, respectively, might correspond to the -OH analogue of lalondeite (Na,Ca)₆(Ca,Na)₃Si₁₆O₃₈(OH)₂⋅3H₂O.

Table 3. The strongest reflections in the powder X-diffraction patterns of the products of Routes 1 to 3.

Discussion

Preliminary syntheses in the Na–Ca–Zr–Nb–Fe–Mn–Si–O–(Cl,F) system, which were previously conducted by us (2019–2021), were carried out at different P–T conditions (temperature 450–600°C and pressure 1.5–2.5 kbar) with wide ranges of compositions and different fluids (1 M solutions of NaCl, NaF, or NaOH and including high-alkaline fluids, 20 and 40% NaOH), both with and without a seed. These experiments showed that for the crystallisation of eudialyte-type compounds in these systems the presence of both a seed and Cl^– or F^– ions is necessary. In the presence of high-alkaline fluids, other zirconosilicates (parakeldyshite and/or zirsinalite) are formed. This observation is in agreement with the replacement of eudialyte-group minerals with parakeldyshite or zirsinalite at the peralkaline hyperagpaitic stages of the evolution of the igneous rocks and pegmatites of the Lovozero and Khibiny massifs (Khomyakov, 1995). Under hydrothermal conditions, EGM can be either pseudomorphed or dissolved. Alteration of eudialyte was observed at 230°C and pH below 10.5 (Mikhailova et al., 2022). The dissolution process was most intensive in acidic hydrothermal solutions. Thus, EGM are stable only under high-alkaline conditions and data obtained in this work confirm this conclusion.

All eudialyte-related compounds synthesised in this work are Fe deficient compared to eudialyte s.s. This observation could be related partly to the crystallisation of acicular aegirine as a minor impurity in all syntheses products. Another possible cause of low Fe contents in the synthesised eudialyte-related compounds could be the low solubility of Fe₂O₃.

To date, the crystal structures of more than 100 samples of minerals belonging to the eudialyte group have been characterised, which makes it possible to elucidate the predominant regularities in the distribution of diverse elements between different sites in their crystal structures (Johnsen et al., 2003; Rastsvetaeva et al., 2012, 2020, Rastsvetaeva and Chukanov, 2020). On the basis of these regularities, the crystal-chemical formulae of the synthesised eudialyte-type compounds can be written as follows:

Na_13.89–x(H₃O)_y(Ca_5.72Na_0.28)(Fe_1.20Zr_0.95Na_x)(Zr_2.99Hf_0.01)Si₂(Si₂₄O₇₂) (O,OH,CO₃)_zCl_0.83⋅nH₂O (Route 1);

(Na_12.70–xCa_1.16)Ca₆(Fe_1.58Zr_0.40Na_x)(Zr_2.97Hf_0.03)Si₂(Si₂₄O₇₂)(O,OH)_y (CO₃)_zCl_0.88⋅n(H₂O,H₃O) (Route 2a);

(Na_13.16–xCa_2.78)Ca₆(Fe_1.05Zr_0.23Na_x)(Zr_2.99Hf_0.01)Si₂(Si₂₄O₇₂)(O,OH)_y Cl_0.99(CO₃)_z⋅n(H₂O,H₃O) (Route 2b);

Na_13.52–x(Ca_4.76Na_1.24)(Fe_0.92Zr_1.04Na_x)(Zr_2.98Hf_0.02)Si₂(Si₂₄O₇₂)(O,OH)_y F_1.02⋅n(H₂O,H₃O) (Route 3).

As a rule, the enhanced c parameter of ~31 Å corresponds to H₃O-rich eudialyte-group minerals. The product of Route 1 has a c parameter of 30.934 Å which might be an indirect indication of a high H₃O:H₂O ratio in this compound compared to other eudialyte-related compounds synthesised in this work.

According to IR spectroscopy data, all synthesised eudialyte-related compounds belong to high-silicon (i.e. with a low vacancy content at the M3 and M4 sites) and Fe-deficient types. The first two formulae correspond to an Fe-deficient variety of eudialyte s.s., in which Fe that should dominate at the M2 site is mostly replaced by Zr and Na. The other two formulae (Route 2b and Route 3) do not correspond to any known mineral species belonging to the eudialyte group.

The product with the highest Ca content (Route 2b) is associated with golyshevite (Na,Ca)₁₀Ca₉Fe₂Zr₃NbSi₂₅O₇₂(OH)₃(CO₃)⋅H₂O (Chukanov et al., 2005), from which it differs in the absence of Nb and a low Fe content. By analogy with golyshevite, it can be assumed that in this compound, Ca in excess of 6 apfu is concentrated at the N4 site. As can be seen from the compositional data, the product of Route 3 is characterised by the highest content of impurity phases, among which lalondeite, vlasovite and aegirine are identified (Table 3). The formation of lalondeite is associated with the presence of fluorine in the fluid phase used in Route 3. During the crystallisation of lalondeite and aegirine, the stoichiometry of the mineral-forming medium changes. In particular, the Zr:Si ratio increases, which promotes the crystallisation of vlasovite.

As a rule, natural minerals of the eudialyte group are characterised by low fluorine contents: even in samples from the paragenesis with villiaumite NaF, chlorine, as a rule, dominates over fluorine. The only F-dominant EGM is kentbrooksite (Na,REE)₁₅(Ca,REE)₆Mn₃Zr₃[SiNb](Si₂₄O₇₄)F₂⋅2H₂O (Johnsen et al., 1998). It is very probable that chlorine is a component that stabilises the structure of eudialyte. This assumption is consistent with the formation of a large amount of by-products in Route 3.