Amableite-(Ce), Na₁₅[(Ce_1.5Na_1.5)Mn₃]Mn₂Zr₃□Si[Si₂₄O₆₉(OH)₃](OH)₂⋅H₂O, a new eudialyte-group mineral from Saint-Amable Sill, Québec, Canada

Abstract

The new eudialyte-group mineral amableite-(Ce), ideally Na₁₅[(Ce_1.5Na_1.5)Mn₃]Mn₂Zr₃□Si[Si₂₄O₆₉(OH)₃](OH)₂⋅H₂O, was discovered in a peralkaline pegmatite at Saint-Amable Sill, Montérégie, Québec, Canada. The associated minerals are albite, microcline, aegirine, serandite, natrolite, yofortierite, and an unidentified titanosilicate forming minute grains. Amableite-(Ce) occurs as yellow equant or thick tabular crystals up to 2 mm across. The observed crystal forms are {0001}; the subordinate forms are {11$\bar{2}$0}, {10$\bar{1}$1}, and {10$\bar{1}$0}. Amableite-(Ce) is brittle, with a Mohs hardness of 5. D(meas) = 2.89(1), D(calc) = 2.899 g⋅cm^–3. Amableite-(Ce) is optically anomalously biaxial (+) with α ≈ β = 1.603(2) and γ = 1.608(2). The chemical composition is (wt.%, electron microprobe, H₂O measured by means of a modified Penfield method): Na₂O 14.20, K₂O 0.41, CaO 1.89, MnO 8.25, Fe₂O₃ 2.40, La₂O₃ 3.10, Ce₂O₃ 4.19, Pr₂O₃ 0.16, Nd₂O₃ 0.59, SiO₂ 49.41, ZrO₂ 11.17, HfO₂ 0.24, TiO₂ 0.68, Nb₂О₅ 1.54, Cl 0.26, H₂O 1.70, –O≡Cl –0.06, total 100.13. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R₁ = 0.0423. Amableite-(Ce) is trigonal, space group R3, with a = 14.1340(3) Å, c = 30.3780(11) Å and V = 5255.6(3) ų. The crystal-chemical formula is (Na_12.93K_0.27Ce_0.06)_Σ13.26[(Mn_2.49Ce_0.30Ca_0.21)_Σ3.00(Ce_1.14Na_1.04Ca_0.82)_Σ3.00](Mn_1.05Fe_0.90□_1.05)_Σ3.00(Zr_2.85Ti_0.12Hf_0.03)_Σ3.00(□_0.40Nb_0.36Si_0.24)_Σ1.00(Si_0.88□_0.12)_Σ1.00[Si₂₄(O_70.44(OH)_1.56)_Σ72.00][(OH)_2.20(H₂O)_1.27]_Σ3.47Cl_0.22 (Z = 3). Infrared and Raman spectra are given. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %)(hkl)] are: 11.34 (51)(101), 7.06 (76)(110), 4.312 (63)(205), 3.783 (38)(033), 3.538 (43)(027, 220), 2.963 (84)($\bar{3}$45), 2.837 (100)(404). The mineral is named after the discovery locality.

Introduction

Eudialyte-group minerals (EGMs) are important components of some types of peralkaline rocks in which they are the main concentrators of zirconium. Currently, 32 mineral species belonging to the eudialyte group are known (https://www.mindat.org/). Their crystal structures are based on heteropolyhedral frameworks composed of nine- and three-membered rings of SiO₄ tetrahedra, six-membered rings of edge-sharing M1O₆ octahedra and isolated ZO₆ octahedra. The simplified general formula of EGMs is [N1N2N3N4N5]₃M1₆M2₃M3M4Z ₃(Si₉O₂₇)₂(Si₃O₉)₂Ø_4–6X1X2 where N1–5 are extra-framework cations (mainly, Na⁺ or H₃O⁺, sometimes with species-defining H₂O, K⁺, Ca²⁺, Sr²⁺, Mn²⁺ or REE ³⁺ as well as minor H⁺ and/or Ba²⁺); M1 = Ca²⁺, Na⁺, Fe²⁺, Mn²⁺ and REE ³⁺; M2 = Na⁺, Fe²⁺, Mn²⁺, Fe³⁺ or Zr, occurring between rings of M1O₆ octahedra; M3 and M4 are ^[4]Si, ^[4]Nb, ^[4]W or ^[4]Ti atoms occurring at the centres of the Si₉O₂₇ rings; Z = Zr or Ti; Ø, X1 and X2 are additional extra-framework cations (OH^–, Cl^–, F^–, O^2–, CO₃^2–, SO₄^2– and S^2–) and water molecules (Johnsen et al., 2003; Rastsvetaeva et al., 2012). All sites except those belonging to the rings of tetrahedral and octahedra can be partly vacant. Most EGMs are characterised by the space groups R $\bar{3}$m or R3m, but in some species (members of the oneillite subgroup) different M1 cations are ordered in the M1(1) and M1(2) sites alternating in the six-membered rings of octahedra which results in lowering of symmetry from R $\bar{3}$m or R3m to R3 (Johnsen et al., 1999; Chukanov et al., 2003, 2020, 2022, 2023; Khomyakov et al., 2007, 2009).

This paper describes a new oneillite-subgroup member amableite-(Ce), with Na⁺ + REE ³⁺ and Mn²⁺ ordered in the M1(1) and M1(2) sites and Mn²⁺-dominant M2 site. Amableite-(Ce) is named after its discovery locality, Saint-Amable Sill. The mineral and its name (with symbol Ambl-Ce) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2023–075, Chukanov et al., 2024). The holotype specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia with the registration number 6921/1.

Experimental methods and data processing

In order to obtain infrared (IR) absorption spectra of amableite-(Ce) and the related eudialyte-group mineral voronkovite (Khomyakov et al., 2009), powdered samples 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. The IR spectrum of an analogous pellet of pure KBr was used as a reference. The assignment of IR bands was made based on the analysis of IR spectra of several tens of structurally investigated eudialyte-group minerals, in accordance with Rastsvetaeva et al. (2012).

Raman spectra of randomly oriented grains of amableite-(Ce) and other eudialyte-group minerals used for comparison were obtained using an EnSpectr R532 spectrometer based on an OLYMPUS CX 41 microscope coupled with a diode laser (λ = 532 nm) at room temperature (Moscow State University, Faculty of Geology). The spectra were recorded in the range from 100 to 4000 cm^–1 with a diffraction grating (1800 gr mm^–1) and spectral resolution of ~6 cm^–1. The output power of the laser beam was in the range from 5 to 13 mW. The diameter of the focal spot on the sample was 5–10 μm. The back-scattered Raman signal was collected with a 40× objective; signal acquisition time for a single scan of the spectral range was 1 s, and the signal was averaged over 50 scans. Crystalline silicon was used as a standard.

Compositional data (5 spot analyses) were obtained using a digital scanning electron microscope Tescan VEGA-II XMU equipped with an energy-dispersive spectrometer (EDS) INCA Energy 450 with semiconducting Si (Li) detector Link INCA Energy at an accelerating voltage of 20 kV, electron current of 190 pA and electron beam diameter of 160–180 nm. Attempts to use WDS mode, with a higher beam current, were unsuccessful because of the instability of the mineral under the electron beam due to partial dehydration and migration of Na. This phenomenon is typical for high-hydrous sodium minerals with microporous structures. A good agreement was observed between compositional data obtained under these standard conditions and those obtained under more ‘mild’ conditions (with a current lowered to 90–100 pA and electron beam defocused to an area of 30 × 30 μm). The L-lines of Ta are not observed in the spectrum, which indicates the absence of detectable amounts of tantalum in amableite-(Ce).

The H₂O content was determined by means of a modified Penfield method. The CO₂ content was not determined because characteristic bands of carbonate groups (in the range of 1350–1550 cm^–1) are not observed in the IR spectrum of amableite-(Ce).

Powder X-ray diffraction (XRD) data were collected using a Rigaku R-AXIS Rapid II diffractometer (image plate), CoKα, 40 kV, 15 mA, rotating anode with the microfocus optics, Debye-Scherrer geometry, d = 127.4 mm and exposure time of 15 min. The raw powder XRD data were collected using the program suite designed by Britvin et al. (2017). Calculated intensities were obtained by means of STOE WinXPOW v. 2.08 program suite based on the atomic coordinates and unit-cell parameters (Stoe, 2003).

Single-crystal X-ray diffraction studies were carried out using a Rigaku XtaLAB Synergy-S diffractometer (MoKα radiation) with high-stability sharp-focus X-ray source PhotonJet-S and a high-speed direct-action detector HyPix-6000HE. CrysAlisPro software was used for further processing (CrysAlisPro, 2015). An absorption correction was introduced using the SCALE3 ABSPACK algorithm. The obtained data were loaded into the Olex2 program software (Dolomanov et al., 2009) and the crystal structure was solved and refined by the ShelX program package (Sheldrick, 2015). The crystal data, experimental details of the data collection and refinement results are shown in Table 1.

Table 1. Crystal data, data collection information and structure refinement details for amableite-(Ce).

Results

Occurrence, general appearance and physical properties

The material with amableite-(Ce) was collected in the Demix-Varennes quarry, Saint-Amable Sill (45° 40′1″N, 73°20′35″W), Lajemmerais RCM, Montérégie, Québec, Canada (see: Horváth et al., 1998). Amableite-(Ce) crystallised from a peralkaline post-magmatic fluid. It forms yellow thick-tabular, slightly flattened on (0001) crystals up to 2 mm across in cavities of peralkaline pegmatite (Fig. 1). The dominant crystal form is {0001}; the subordinate forms are {11$\bar{2}$0}, {10$\bar{1}$1} and {10$\bar{1}$0}. The associated minerals are albite, microcline, aegirine, serandite, natrolite, yofortierite, and an unidentified titanosilicate forming minute grains.

Figure 1. Amableite-(Ce) crystals (yellow) in association with albite, aegirine and natrolite. Field of view width is 3.5 mm. Sample 6921/1 from the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia. Photographer: V. Heck.

Amableite-(Ce) is brittle, with a Mohs hardness of 5. No cleavage is observed. The fracture is uneven. Density measured by flotation in heavy liquids (mixtures of methylene iodide and heptane) is equal to 2.89(1) g⋅cm^–3. Density calculated using the empirical formula and unit-cell volume refined from single-crystal XRD data is 2.90 g⋅cm^–3.

The new mineral is optically anomalously biaxial (+) with α ≈ β = 1.603(2) and γ = 1.608(2) (λ = 589 nm). Different causes of optical anomalies (microstrains, compositional inhomogeneity, including different degrees of dehydration in different zones or sectors) are discussed in detail by Shtukenberg and Punin (2007). A possible cause of optical anomalies may be deformation of the crystals during their growth or when they were being crushed. In particular, anomalous biaxiality is very typical for eudialyte-group minerals despite that all studied members of this group are trigonal (Rastsvetaeva et al., 2012). Under the microscope, amableite-(Ce) is colourless and nonpleochroic. Dispersion is distinct with r > v.

Infrared spectroscopy

Absorption bands in the IR spectrum of amableite-(Ce) (curve a in Fig. 2) and their assignments are (cm^–1, s – strong band, w – weak band, sh – shoulder): 3520, 3335, 2880w (O–H stretching vibrations); 1635w (H–O–H bending vibrations); 1170sh, 1010s, 991s (Si–O stretching vibrations of silicate rings); 935s (Si–O stretching vibrations of SiO₄ tetrahedra at the M3 and M4 sites); 739 (mixed vibrations of rings of SiO₄ tetrahedra – ‘ring band’); 700sh, 657 (mixed vibrations of rings of SiO₄ tetrahedra combined with Nb–O stretching vibrations); 519sh (^IVMn²⁺–O and/or ^VFe³⁺–O stretching vibrations); 481s, 446s (lattice mode involving predominantly bending vibrations of rings of SiO₄ tetrahedra); and 390sh, 373 (lattice modes involving ^VI(Ca;Mn²⁺)–O stretching vibrations).

Figure 2. Powder infrared absorption spectra of (a) amableite-(Ce) and (b) holotype sample of voronkovite (Khomyakov et al., 2009), an oneillite-type EGM related to amableite-(Ce). The spectra are offset for comparison.

The band of ^IVFe²⁺–O stretching vibrations (in the range 539–545 cm^–1) was not observed. The IR spectrum of amableite-(Ce) differs from that of the related eudialyte-group mineral voronkovite, ideally Na₁₅[(Na,Са)₃Mn₃]Fe²⁺₃Zr₃Si₂(Si₂₄O₇₂)(OH,O)₄Cl⋅H₂O (curve b in Fig. 2), with a lower intensity of the band at 933–935 cm^–1, which reflects significant amounts of vacancies and Nb at the M3 and M4 sites, in accordance with the structural data (see below).

According to the correlation by Libowitzky (1999), the band of O–H stretching vibrations with the maximum at 3335 cm^–1 corresponds to a hydrogen bond with the O⋅⋅⋅O distance of 2.72 Å. Thus, this band should be assigned to the hydrogen bond O24⋅⋅⋅H–O26 with the O⋅⋅⋅O distance of 2.72(3) Å. The band at 3520 cm^–1 may correspond to silanol groups formed as a result of protonation of pending vertices of the SiO₄ tetrahedra, but this assignment is ambiguous because H atoms were not localised.

Raman spectroscopy

A specific feature of Raman spectra of the EGMs selsurtite, ideally (H₃O)₁₂Na₃(Ca₃Mn₃)(Na₂Fe)Zr₃□Si[Si₂₄O₆₉(OH)₃](OH)Cl⋅H₂O (Chukanov et al., 2023), aqualite, ideally (H₃O)₉(K,Ba,Sr)₂Ca₆Zr₃Na₂Si₂[Si₂₄O₆₆(OH)₆](OH)₃Cl⋅H₂O (Khomyakov et al., 2007) and other hydronium-bearing EGMs is a series of bands in the range of 1200–2900 cm^–1 corresponding to strong hydrogen bonds formed by hydronium cations in different local situations including Zundel- and Eigen-like bonds with short O⋅⋅⋅O distances of ~2.4 and ~2.6 Å (Chukanov et al., 2022, 2023; see Discussion section for details). Similar but much weaker bands are observed in the Raman spectrum of amableite-(Ce), but such bands are absent in the Raman spectrum of eudialyte, which does not contain hydrated proton complexes including (H₃O)⁺ cation groups (Fig. 3). Most probably, hydrated protons could form as a result of partial dissociation of silanol (SiOH) groups.

Figure 3. Raman spectra of (a) selsurtite, (b) amableite-(Ce) and (c) eudialyte.

The assignment of other bands in the Raman spectrum of amableite-(Ce) is as follows: 3430 cm^–1 to O–H stretching vibrations of H₂O molecules and/or OH groups; the range 900–1300 cm^–1 to Si–O stretching modes; 780 cm^–1 to mixed vibrations of rings of SiO₄ tetrahedra; 653 and 689 cm^–1 to mixed vibrations of rings of SiO₄ tetrahedra combined with Nb–O stretching vibrations; 573 cm^–1 to Zr–O stretching vibrations and bands ≤ 400 cm^–1 are assigned to lattice modes.

Chemical data

Analytical data are given in Table 2. Contents of other elements with atomic numbers >8 are below detection limits. Based on IR spectroscopy data, all iron is considered as Fe³⁺. Note that Fe³⁺ and Fe²⁺ cations at the M2 site of eudialyte-group minerals are yellow and red (to brownish-red in the case of five-fold coordination) chromophores, respectively (Pol'shin et al., 1991). Thus, the yellow colour of amableite-(Ce) indicates the absence of Fe²⁺ at the M2 site. The trivalent state of iron is also in agreement with a superior Gladstone–Dale compatibility index CI = 1–(K _p/K _c) = –0.014 (Mandarino, 1981). In comparison, with Fe²⁺, the CI would be –0.026.

Table 2. Chemical composition of amableite-(Ce).

*Bivalent state of Mn and trivalent state of Fe are in agreement with the IR spectrum (see above) and structural data. In addition, Mn³⁺ is a very strong purple chromophore whereas amableite-(Ce) is yellow.

**The content of H₂O was determined by means of a modified Penfield method.

The bulk empirical formula of amableite-(Ce) is H_5.76Na_14.00K_0.27Ca_1.03Ce_0.78La_0.58Nd_0.11Pr_0.03Mn_3.55Fe_0.92Ti_0.26Zr_2.77Hf_0.035Nb_0.35Si_25.12Cl_0.22O_75.36. Based on the structural data (see below), it can be written as: (Na_12.93K_0.27Ce_0.06)_Σ13.26[(Mn_2.49Ce_0.30Ca_0.21)_Σ3.00(Ce_1.14Na_1.04Ca_0.82)_Σ3.00](Mn_1.05Fe_0.90□_1.05)_Σ3.00(Zr_2.85Ti_0.12Hf_0.03)_Σ3.00(□_0.40Nb_0.36Si_0.24)_Σ1.00(Si_0.88□_0.12)_Σ1.00[Si₂₄(O_70.44(OH)_1.56)_Σ72.00][(OH)_2.20(H₂O)_1.27]_Σ3.47Cl_0.22.

The simplified formula is (Na,H,K,Ce)₁₅[(Ce,Na,Ca)₃(Mn,Ce,Ca)₃](Mn,Fe,□)₃(Zr,Ti)₃(□,Nb,Si)(Si,□)Si₂₄(O,OH)₇₂](OH,H₂O,O)₃, and the ideal formula is Na₁₅[(Ce_1.5Na_1.5)Mn₃]Mn₂Zr₃□Si[Si₂₄O₆₉(OH)₃](OH)₂⋅H₂O.

X-ray diffraction and crystal structure

Powder X-ray diffraction data for amableite-(Ce) are given in Table 3. The unit-cell parameters refined from the powder data are: a = 14.140(3) Å, c = 30.37(1) Å and V = 5258(4) ų. The crystal structure of amableite-(Ce) (Figs 4 and 5) was refined on the basis of 6462 independent reflections with I > 2σ(I). The final refinement cycles converged to R ₁ = 4.23%. The atomic coordinates occupancies, site-scattering values and equivalent isotropic parameters are given in Table 4. The anisotropic displacement parameters and selected bond lengths are given in Tables 5 and 6, respectively. See Discussion section for further details. The crystallographic information file (cif) has been deposited via the joint Cambridge Crystallographic Data Centre CCDC/FIZ Karlsruhe deposition service https://www.ccdc.cam.ac.uk/structures/ (the deposition number is CSD 2325823) and are also available as Supplementary Material (see below).

Table 3. Powder X-ray diffraction data (d in Å) of amableite-(Ce).

*For the calculated pattern, only reflections with intensities ≥1 are given. **For the unit-cell parameters calculated from single-crystal data. The strongest lines are given in bold.

Figure 4. The crystal structure of amableite-(Ce): general view. Drawn with the DIAMOND program (Brandenburg and Putz, 2005).

Figure 5. A local arrangement involving 6-membered rings of octahedra in the amableite-(Ce) structure. Drawn with the DIAMOND program (Brandenburg and Putz, 2005).

Table 4. Atomic coordinates, occupancies, site-scattering values and equivalent isotropic displacement parameters (Ų) for amableite-(Ce).

*s.s. – site scattering; calc. s.s. – calculated site scattering (electrons per formula unit). ** Fixed parameters.

Table 5. Anisotropic displacement parameters (Ų) for amableite-(Ce).

Table 6. Selected bond lengths (d in Å) in amableite-(Ce).

Discussion

Crystal structure

Amableite-(Ce) is isostructural with the other 12-layered members of the oneillite subgroup of the eudialyte group with the space group R3. Based on the refined site-scattering factors, the crystal chemical formula of amableite-(Ce) can be written as follows (Z = 3):

^{N 1–5}(Na_12.93H⁺_1.74K_0.27Ce_0.06)_Σ15 ^ZrZ(Zr_2.85Ti_0.12Hf_0.03)_Σ3

^{M 1(1)}(Mn_2.49Ce_0.30Ca_0.21)_Σ3 ^{M 1(2)}(Ce_1.14Na_1.04Ca_0.82)_Σ3

^{M 2}(Mn_1.05Fe_0.90□_1.05)_Σ3 ^{M 3}(□_0.40Nb_0.36Si_0.24)_Σ1 ^{M 4}(Si_0.88□_0.12)_Σ1

[^Si1,2,5–10Si₂₄O_70.44(OH)_1.56] ^Ø,O25–30[(OH)_1.78O_1.08Cl_0.22(H₂O)_0.61]

where [^Si1,2,5–10Si₂₄O_70.44(OH)_1.56] are rings of tetrahedra.

The following combination of structural features of amableite-(Ce) distinguishes it from other eudialyte-group minerals:

1. (1) Cation ordering within the six-membered ring of octahedra resulting in a lowering of symmetry (Fig. 5). The six-membered ring is formed by M1(1)O₆- and M1(2)O₆-octahedra with different occupancies. The M1(1)O₆-octahedron is predominantly occupied by manganese (2.49 atoms per formula unit), whereas the M1(2)O₆-octahedron is predominantly occupied by Na and REE, with subordinate Ca.

2. (2) The predominance of Mn²⁺ at the M2 site.

3. (3) The predominance of vacancies over NbO₆ octahedra and SiO₃(OH) tetrahedra at the M3 site.

4. (4) The predominance of Si at the M4 site.

Comparative data for amableite-(Ce) and related eudialyte-group minerals are given in Table 7.

Table 7. Comparative data for amableite-(Ce) and related eudialyte-group minerals with R3 symmetry.

Raman spectroscopy of hydronium and hydrated proton complexes

Numerous ab initio quantum-chemical calculations of hydronium and other hydrated proton complexes, including Zundel (H₅O₂⁺) and Eigen (H₃O⁺⋅3H₂O) cations have shown that these clusters are characterised by variable configurations and strong hydrogen bonds with the O⋅⋅⋅O distances in the range of 2.38–2.8 Å (Vyas, 1978; Komatsuzaki and Ohmine, 1994; Corongiu, 1995; Kim et al., 2002; Sobolewski and Domcke, 2002a, 2002b; Asmis et al., 2003; Christie, 2004; Headrick et al., 2004; Laria et al., 2004; Asthagiri et al., 2005; Ortega et al, 2005; Paddison and Elliott, 2005; Vener and Librovich, 2009; Biswas et al., 2017; Carpenter, 2020). Calculated wavenumbers of vibrational modes corresponding to the hydrated proton complexes are in the range of 1070–3000 cm^–1.

There is a negative correlation between the frequency of O–H stretching vibrations and O⋅⋅⋅O distance between the O atom of OH group and the O atom – acceptor of the hydrogen bond (McClellan and Pimentel, 1960; see Fig. 6). This correlation is nearly linear in the range of the O⋅⋅⋅O distances from 2.4 to 2.8 Å and significantly deviates from the linearity for weaker hydrogen bonds.

Figure 6. The correlations between wavenumbers of O–H stretching vibrations and O⋅⋅⋅O distances for hydrogen bonds in crystals drawn using data by McClellan and Pimentel (1960) (curve a), Libowitzky (1999) (curve b), and Novak (1974) (squares).

The following empirical correlations between O–H stretching frequencies in IR spectra of minerals and O⋅⋅⋅O and H⋅⋅⋅O distances (obtained from structural data) were established by E. Libowitzky (1999):

(1)$$v( {{\rm c}{\rm m}^{- 1}} ) = 3592- 304\cdot 10^9\cdot {\rm exp}[ {- d( {\rm O\cdot{\cdot} \cdot \rm O} ) /0.1321} ] $$

(2)$$v( {{\rm c}{\rm m}^{- 1}} ) = 3632- 1.79\cdot 10^6\cdot {\rm exp}[ {- d( {\rm H\cdot{\cdot} \cdot O} ) /0.2146} ] $$

A similar correlation was obtained by Novak (1974).

In reality, equations 1 and 2 are a very rough approximation and have restricted applicability. In particular, above 3500 cm^–1 substantial deviations from correlations 1 and 2 are common because O–H stretching frequencies depend not only on O⋅⋅⋅O and H⋅⋅⋅O distances, but also on the nature of cations coordinating O–H groups and H₂O molecules, as well as on the O–H⋅⋅⋅O angle, and the influence of these factors becomes most evident in the case of weak hydrogen bonds. The equations 1 and 2 predict that maximum possible values of O–H stretching frequencies for minerals are 3592 and 3632 cm^–1, respectively, but in many minerals including for magnesium serpentines, brucite, kaolinite, amphiboles etc. observed frequencies are much higher and can exceed 3700 cm^–1. However, these correlations can be used for semiquantitative estimations, at least for relatively strong hydrogen bonds.

According to the equation (1), the Raman bands of amableite-(Ce) observed at 1815, 2433 and 2807 cm^–1 correspond to the O⋅⋅⋅O distances of 2.51, 2.56 and 2.61 Å, respectively which is quite close to the distances of 2.50, 2.58 and 2.59 Å for O7⋅⋅⋅O30, O7⋅⋅⋅O27 and O24⋅⋅⋅O29 in strong hydrogen bonds O–H⋅⋅⋅O in amableite-(Ce).

Implications

Minerals belonging to the eudialyte group are considered as potential sources of REE, Zr, Hf, Nb and Ta for industrial use (Lebedev, 2003; Lebedev et al., 2003; Zakharov et al., 2011; Friedrich et al., 2016; Davis et al., 2017; Ma et al., 2019). Almost all the samples of EGMs studied contain detectable amounts of rare-earth elements (Rastsvetaeva et al., 2012). Samples of EGMs from peralkaline pegmatites are characterised by the highest contents of these elements (typically, from 2 to 8 wt.% of REE ₂O₃). Amableite-(Ce) is the third EGM after zirsilite-(Ce), (Na,□)₁₂(Ce,Na)₃Ca₆Mn₃Zr₃Nb(Si₂₅O₇₃)(OH)₃(CO₃)⋅H₂O (Khomyakov et al., 2003) and johnsenite-(Ce), Na₁₂(Ce,La,Sr,Ca)₃Ca₆Mn₃Zr₃W(Si₂₅O₇₃)(OH,Cl)₂(CO₃) (Grice and Gault, 2006) containing REE as a species-defining component.

As a rule, REE-rich EGMs are enriched in Mn whereas REE-poor EGMs are enriched in Fe. This regularity may have a crystal-chemical origin and correspond to the situations where relatively small (Fe- and Ca-centred) or larger (Mn²⁺ and REE-centred) M2- and M1-octahedra share common edges. However, geochemical factors may also play a significant role.