Introduction
Elpidite, ideally Na2ZrSi6O15⋅3H2O, is known only in alkaline, mainly agpaitic igneous rocks. It is a relatively rare mineral, however, in some peralkaline complexes it occurs in significant concentrations and serves locally as the major Zr host phase. Among natural hydrous zirconosilicates, elpidite is the species richest in Si and, thus, silica-rich peralkaline systems are the most favourable for the formation of this mineral (Kynicky et al., Reference Kynicky, Chakhmouradian, Xu, Krmicek and Galilova2011 and references therein), which is in agreement with Na zirconosilicates in the NaOH–ZrO2–SiO2–H2O system (Ilyushin et al., Reference Ilyushin, Dem'yanets, Ilyukhin and Belov1983). The largest elpidite deposits occur in peralkaline granites and their pegmatites where this zirconosilicate can be an important accessory or even a major mineral and a potentially useful component of complex rare-element (Zr–Nb–Ta–REE) ores. The most notable examples are deposits associated with the Khan Bogdo (= Khan Bogd) (Vladykin et al., Reference Vladykin, Kovalenko and Dorfman1981; Kynicky et al., Reference Kynicky, Chakhmouradian, Xu, Krmicek and Galilova2011) and Khaldzan Buregte (= Khaldzan Buregtey or Khaldzan Buragtag) (Kovalenko et al., Reference Kovalenko, Tsaryeva, Goreglyad, Yarmolyuk, Troitsky, Hervig and Farmer1995; Kempe et al., Reference Kempe, Möckel, Graupner, Kynicky and Dombon2015) alkaline granite intrusions in Mongolia and the Strange Lake complex in Canada (Birkett et al., Reference Birkett, Miller, Roberts and Mariano1992; Salvi and Williams-Jones, Reference Salvi and Williams-Jones2001). In nepheline-syenite complexes no significant elpidite deposits are known; however, it can be abundant locally. In the Lovozero alkaline complex in the Kola Peninsula, Russia such accumulations of elpidite were described in quartz-bearing fenites (Ivanyuk et al., Reference Ivanyuk, Pakhomovsky, Yakovenchuk, Men'shikov, Bogdanova and Mikhailova2006) and in large hydrothermally altered peralkaline, highly agpaitic pegmatite bodies. For example, in the Elpiditovoe pegmatite this zirconosilicate was found as almost monomineralic aggregates up to 100 kg (Pekov, Reference Pekov2000).
From the crystal chemical viewpoint, elpidite is a typical microporous, zeolite-like zirconosilicate. Its crystal structure is based on a heteropolyhedral Zr–Si–O framework which consists of tetrahedral Si6O15 ribbons (double chains) and isolated ZrO6 octahedra. Wide channels inside the heteropolyhedral framework contain Nа+ cations and H2O molecules (Neronova and Belov, Reference Neronova and Belov1964; Cannillo et al., Reference Cannillo, Rossi and Ungaretti1973). The presence of the three-dimensional system of these channels causes distinct zeolitic properties of elpidite: this mineral readily exchanges Na+ in aqueous solutions for other large cations (K+, Rb+, Cs+, Ag+ and Pb2+) and demonstrates reversible dehydration in thermal experiments (Turchkova et al., Reference Turchkova, Pekov and Bryzgalov2006; Grigor'eva et al., Reference Grigor'eva, Zubkova, Pekov, Kolitsch, Pushcharovsky, Vigasina, Giester, Ðorðevic, Tillmanns and Chukanov2011; Zubkova et al., Reference Zubkova, Ksenofontov, Kabalov, Chukanov and Nedel'ko2011, Reference Zubkova, Nikolova, Chukanov, Kostov-Kytin, Pekov, Varlamov, Larikova, Kazheva, Chervonnaya, Shilov and Pushcharovsky2019; Cametti et al., Reference Cametti, Armbruster and Nagashima2016). Elpidite is an orthorhombic mineral. In the literature, its ‘basic’ unit cell has been described in two settings: (1) in the unconventional space group Pbm2 (mainly used in early studies) with the following unit-cell parameters: a = 7.1–7.4, b = 14.4–14.7 and c = 7.05–7.15 Å (so-called 7–14–7 Å elpidite); and (2) in the standard setting, space group Pma2 with inversed unit-cell parameters a and b (used in this work). Distortions of the Zr–Si–O framework and variations in the location of extra-framework cations and H2O molecules cause the structural variations reported for this mineral. For different samples of elpidite and its laboratory-modified (cation-substituted or dehydrated) forms, a variety of space groups (Pbmm, Pbm2, Pbcm, Bba2, Cmce) was reported and doubling of the a and/or c unit-cell parameters was found in some cases. Crystal chemical data on all structurally studied samples of elpidite, its monoclinic (C2/m) dimorph yusupovite and modified forms of elpidite were recently summarised and reviewed by Zubkova et al. (Reference Zubkova, Nikolova, Chukanov, Kostov-Kytin, Pekov, Varlamov, Larikova, Kazheva, Chervonnaya, Shilov and Pushcharovsky2019).
In the present paper we describe a new variety of elpidite found in the Khibiny alkaline complex in the Kola Peninsula. Unlike the neighbouring Lovozero complex, where elpidite is locally abundant (Pekov, Reference Pekov2000; Ivanyuk et al., Reference Ivanyuk, Pakhomovsky, Yakovenchuk, Men'shikov, Bogdanova and Mikhailova2006), in Khibiny it is a rare mineral known only in very minor amounts in hydrothermal assemblages related to several peralkaline pegmatite and pegmatoid bodies (Yakovenchuk et al., Reference Yakovenchuk, Ivanyuk, Pakhomovsky and Men'shikov2005). The new elpidite variety characterised here is significantly depleted in Na and additionally hydrated in comparison to examples of this mineral from all earlier known localities. Such chemical deviations from the ideal composition are reflected in the unusual crystal chemical features and physical properties of this elpidite variety.
Occurrence and sample description
The new variety of elpidite was collected in the upper part of the Loparskaya River valley (north-eastern spur of Mt. Yukspor) in the south-western part of the Khibiny alkaline complex. The mineral was found in a cavernous hydrothermally altered axial zone of a relatively small (about two meters across) peralkaline pegmatite body of lenticular shape. It is situated in foyaite and has a mineral composition common for the Khibiny foyaitic pegmatites: microcline, nepheline and black coarse-prismatic alkali clinopyroxene are the major minerals, whereas eudialyte, titanite, astrophyllite, biotite, pale greenish fluorapatite and sphalerite are accessory constituents. The hydrothermal mineral assemblage formed in cavities includes green acicular aegirine, yellow–green fluorapatite, zeolites (natrolite, chabazite-K, heulandite-Ca and heulandite-K), ancylite-(Ce) and elpidite. The latter occurs as aqua-transparent colourless prismatic, lath-like crystals up to 0.1 mm × 0.4 mm × 3 mm, typically assembled in bunches or spherulites up to 6 mm in diameter. They are encrusted by chabazite and heulandite crystals. Under ultraviolet (UV) irradiation, this variety of elpidite fluoresces greenish-yellow, is very bright in shortwave (λ = 245 nm) and medium–intense in longwave (λ = 330 nm) ultraviolet light. The optical characteristics of this variety of elpidite are given in Table 1. Its density measured by flotation in heavy liquids (bromoform + ethanol) is 2.42(2) g cm–3.
Table 1. Some chemical features and physical properties of ‘ordinary’ elpidite and its highly hydrated variety: comparative data.
Infrared spectroscopy
In order to obtain infrared (IR) absorption spectra, powdered samples were mixed with dried KBr, pelletised, and analysed using an ALPHA FTIR spectrometer (Bruker Optics) in the range 360–4000 cm–1 with 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.
The IR spectrum of the highly hydrated elpidite (Fig. 1a) is close to the spectrum of 7–14–7 Å elpidite (space group Pbm2) from Khan Bogdo studied by Sapozhnikov and Kashaev (Reference Sapozhnikov and Kashaev1978) (Fig. 1b) but differs from the latter by the high-frequency shift of the strongest band of Si–O stretching vibrations (1012 → 1020 cm–1), the presence of additional shoulders (at 3395 and 3620 cm–1) corresponding to O–H stretching vibrations and ca. 1.7 times higher integrated intensity in the range 2800–3800 cm–1 (with the bands at 640 and 780 cm–1 used as internal standards).
Fig. 1. Powder infrared absorption spectra of (a) highly hydrated elpidite from Khibiny (this work), (b) 7–14–7 Å elpidite (space group Pbm2) from Khan Bogdo (see: Sapozhnikov and Kashaev, Reference Sapozhnikov and Kashaev1978) and (c) 7–14–14 Å elpidite (space group Pbcm) from Lovozero (see: Zubkova et al., Reference Zubkova, Ksenofontov, Kabalov, Chukanov and Nedel'ko2011).
It is noteworthy that typically the H2O molecule and H3O+ cation cannot be reliably distinguished by means of IR spectroscopy: the band at ~1740 cm−1 is indicative of the monohydrated H3O+ cation (i.e. Zundel cation, H5O2+) with a strong symmetric hydrogen bond, but not of the H3O+ cation forming relatively weak hydrogen bonds. This matter is discussed in detail by Chukanov and Chervonnyi (Reference Chukanov and Chervonnyi2016). Thus the shoulders at 3395 and 3620 cm–1 and enhanced integrated intensity of the IR spectrum of the Khibiny hydrated elpidite in the range of O–H stretching vibrations may be due to both H2O molecules and hydronium cations. The relatively high frequency of the band of Si–O stretching vibrations (at 1020 cm–1) in the IR spectrum indicates a medium-strength hydrogen bond formed by a species at the Ow2 site (i.e. H2O or H3O+) with O7 in the Si6O15 ribbon with the Ow2–O7 distance of 2.811 Å (see below).
The IR spectrum of the 7–14–14 Å elpidite (Fig. 1c) differs significantly from those of 7–14–7 Å samples in additional splitting of some bands in the ranges 600–700 and 1000–1100 cm–1, which correspond to bending and stretching vibrations of the Si6O15 ribbon. The integrated absorbance of the 7–14–14 Å sample from Lovozero in the range 2800–3800 cm–1 is nearly the same as that of the 7–14–7 Å sample from Khan Bogdo.
Chemical composition
The chemical composition of the highly hydrated elpidite from Khibiny was determined by electron microprobe analysis (EMPA) in the Laboratory of Analytical Techniques of High Spatial Resolution at the Department of Petrology, Moscow State University using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer. The analyses were carried out in WDS mode, with an acceleration voltage of 20 kV and a beam current of 20 nA. The electron beam was rastered to 10 μm × 10 μm to minimise damage of the mineral. The following standards were used: NaCl (Na), microcline (K), clinopyroxene (Ca), zircon (Si, Zr), Ti (Ti) and Nb (Nb). The results obtained were checked for the correctness of Na content measurement, in energy-dispersive mode at a low beam current (0.7 nA). The latter gave practically the same values for all measured constituents. The H2O content was determined using thermogravimetric analysis.
The average chemical composition of the Khibiny hydrated elpidite [average of 12 spot analyses; wt.% (range)] is: Na2O 5.45 (5.12–5.78), K2O 0.67 (0.35–0.91), CaO 0.05 (0.03–0.10), SiO2 60.32 (59.40–61.12), TiO2 1.34 (1.11–1.59), ZrO2 18.43 (17.66–18.92), Nb2O5 0.65 (0.39–0.87), H2O 12.80, total 99.71. Contents of other elements with atomic numbers higher than carbon are below detection limits in the majority of analyses. Some analyses show very low concentrations, close to detection limits, of Fe, Y, Yb, Th and U. The empirical formula was calculated on the basis of 6 Si and 15 O atoms per formula unit (apfu) because of (1) the absence of OH– groups and (2) the presence of both H2O0 and H3O+ groups in the mineral (see the crystal structure data below). The H2O:H3O ratio was calculated based on charge balance considerations after the atoms per formula units (apfu) were calculated for Si (6 apfu) and metal cations. The empirical formula is: [(Na1.05K0.08Ca0.01)Σ1.14(H3O)0.74]Σ1.88(Zr0.89Ti0.10Nb0.03)Σ1.02Si6O15⋅3.47H2O.
X-ray crystallography and crystal-structure determination details
Single-crystal X-ray studies of the highly hydrated elpidite were carried out using an Xcalibur S diffractometer equipped with a CCD detector. A full sphere of three-dimensional data was collected. Data reduction was performed using CrysAlisPro Version 1.171.39.46 (Rigaku Oxford Diffraction, 2018). The data were corrected for Lorentz factor and polarisation effects. The crystal structure was solved by direct methods and refined in the frames of the orthorhombic space group Pma2 with the SHELX software package (Sheldrick, Reference Sheldrick2015) to R 1 = 0.0343 for 1654 unique reflections with I > 2σ(I). Crystal data, data collection information and structure refinement details are given in Table 2, the coordinates and thermal displacement parameters of atoms and site occupancies in Table 3 and selected interatomic distances in Table 4. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Table 2. Crystal data, data collection information and structure refinement details for highly hydrated elpidite.
*Calculated from structural data (i.e. no H atoms were inserted in the structure model).
**w = 1/[σ2(F o2) + (0.0632P)2 + 0.3999P]; P = {[max of (0 or F o2)] + 2F c2}/3
Table 3. Coordinates, equivalent (U eq) and anisotropic (U) displacement parameters of atoms (in Å2), site occupancy factors (s.o.f.) and site multiplicities (Q) in the structure of highly hydrated elpidite.
*U iso; Ow1 = H2O; Ow2 = H3O possibly with traces of H2O
Table 4. Selected interatomic distances (Å) in the structure of highly hydrated elpidite.
The powder X-ray diffraction (PXRD) pattern of the sample studied (PXRD data were collected in an RKD camera with the diameter of 57.3 mm using Debye-Scherrer geometry, FeKα radiation) is typical for elpidite. The unit-cell parameters calculated from the PXRD data are: a = 14.553(5), b = 7.325(2), c = 7.135(2) Å and V = 760.6(7) Å3.
Discussion
The crystal structure of the highly hydrated elpidite was refined in the space group Pma2 and thus with the 14–7–7 Å unit cell (in the standard setting: see Introduction). Although the refinement in centrosymmetric space group Pmma was successful [R 1 = 0.0412 for 1455 unique reflections with I > 2σ(I)], careful inspection of this refinement showed that the position of oxygen in the Ow2 site is split, and so is the Na1 site. The lowering of symmetry to Pma2 means splitting can be avoided: the O atom almost fully occupies one site (89%) and the Na1 site is not split and occupied fully by statistically substituting Na cations and H2O molecules. The same symmetry lowering to Pma2 was found for the elpidite sample studied by Neronova and Belov (Reference Neronova and Belov1964), for Ca-enriched variety of elpidite (Sapozhnikov and Kashaev, Reference Sapozhnikov and Kashaev1978) and for elpidite from Lovozero reported by Zubkova et al. (Reference Zubkova, Nikolova, Chukanov, Kostov-Kytin, Pekov, Varlamov, Larikova, Kazheva, Chervonnaya, Shilov and Pushcharovsky2019) but in the latter only the Ow2 site was finally not split while the Na site was split.
In the structure of the highly hydrated elpidite (Fig. 2a), tetrahedral ribbons Si6O15 are linked by isolated ZrO6 octahedra to form the heteropolyhedral Zr–Si–O framework identical to that reported for all previously studied elpidite samples (Zubkova et al., Reference Zubkova, Nikolova, Chukanov, Kostov-Kytin, Pekov, Varlamov, Larikova, Kazheva, Chervonnaya, Shilov and Pushcharovsky2019). The most interesting feature of the sample studied is the arrangement and the content of extra-framework sites. In all previously studied samples of elpidite with the same unit-cell metrics (14–7–7 Å) there are two crystallographically non-equivalent Na sites and two sites usually filled by O atoms representing H2O molecules. Sodium cations occur in the centre of octahedra and/or seven- and eight-fold polyhedra. Both Na sites in the Khibiny hydrated elpidite are occupied by Na statistically substituted by water molecules in the ratio Na0.41(6)H2O0.59(6) for the Na1 site and Na0.78(4)H2O0.22(4) for Na2 which occur in the centre of a seven-fold polyhedron and octahedron, respectively. The Ow1 site is fully occupied by H2O while the Ow2 site contains an O atom with a site occupancy factor (s.o.f.) of 0.89 and this site is assumed to be filled dominantly by the hydronium cation H3O+. The calculated bond-valence sums range from 1.79 to 2.11 valence units for all O atoms forming the heteropolyhedral Zr–Si–O framework. This excludes the possibility that some of oxygen atoms in the framework are protonated to the charge balance in our Na-depleted sample. Thus, the placement of hydronium in the Ow2 site is the only available option to achieve charge balance. The environment of this site allows it to be filled by H3O+. The shortest distances from Ow2 to the nearest neighbours are all longer than 2.8 Å: 2.811(16) Å between the Ow2 and O7 sites (oxygen atom of the Zr–Si–O framework) and two distances of 2.834(11) Å between the Ow2 and Ow1 sites. The absence of Na1 site splitting in hydrated elpidite, which was reported for some other samples of elpidite, means avoiding a short distance between the Ow2 and Na. This splitting provides two possible sites for Na1 and it is worth noting that in the structure of elpidite reported by Neronova and Belov (Reference Neronova and Belov1964) Na cations occupy another site compared with the structure of hydrated elpidite (Fig. 2a) reported here. In the structure reported by Neronova and Belov (Reference Neronova and Belov1964), the H2O site (marked as Ow2 and mainly filled with the O atom of H3O+ in our sample) is included in the coordination sphere of Na1 site (Fig. 2b).
Fig. 2. The crystal structure of highly hydrated elpidite projected along the b axis (a) and the crystal structure of elpidite drawn after Neronova and Belov (Reference Neronova and Belov1964) in non-standard Pbm2 space group (b). The unit cells are outlined.
The highly hydrated variety of elpidite studied from Khibiny differs distinctly from ‘ordinary’ elpidite not only in crystal chemistry but also in its physical properties. Some distinctive features of this new variety of the mineral are: (1) the unusual character of its IR spectrum, especially in the area of vibrations of H-bearing groups (Fig. 1); (2) its lower density; (3) an uncommon, for elpidite, configuration of optical indicatrix: small difference between refractive indices β and α (~0.001; Table 1); (4) very bright greenish-yellow fluorescence in UV light in contrast to ‘ordinary’ elpidite, which either does not fluoresces or, rarely, shows weak yellowish-white fluorescence.
The deficiency of Na and the presence of H3O+ in the elpidite studied can be a sign that the mineral underwent ion exchange at the stage of hydrothermal alteration of the pegmatite. The first step of the natural ion exchange in sodium-bearing zeolites and hydrous zeolite-like silicates with heteropolyhedral frameworks is typically characterised by leaching of Na+ with partial replacement of H2O by H3O+. Hydronium-rich forms of microporous silicates are in general less stable than those with extra-framework metal cations and can be easily replaced by the latter at further stages of ion exchange (Pekov et al., Reference Pekov, Grigorieva, Turchkova, Lovskaya and Krivovichev2008). In particular, this was reported for hydrous zeolite-like zirconosilicates of the hilairite group. The natural ion-exchange transformation of hilairite, ideally Na2ZrSi3O9·3H2O, to calciohilairite, ideally CaZrSi3O9·3H2O, involves an intermediate stage, where a cation-deficient and hydronium-bearing form of the latter mineral is formed: 2Na+ + H2O → 0.5Ca2+ + 1.5□ + H3O+ (Pushcharovsky et al., Reference Pushcharovsky, Pekov, Pasero, Gobechia, Merlino and Zubkova2002; Pekov et al., Reference Pekov, Chukanov, Kononkova and Pushcharovsky2003). Two minerals closely related to elpidite in terms of crystal chemistry exist in nature, namely yusupovite (Na,K,Cs)2ZrSi6O15⋅3H2O (Agakhanov et al., Reference Agakhanov, Pautov, Karpenko, Sokolova, Abdu, Hawthorne, Pekov and Siidra2015) and armstrongite CaZrSi6O15⋅2–3H2O (Kabalov et al., Reference Kabalov, Zubkova, Pushcharovsky, Schneider and Sapozhnikov2000; Mesto et al., Reference Mesto, Kaneva, Schingaro, Vladykin, Lacalamita and Scordari2014). For the latter Roelofsen and Veblen (Reference Roelofsen and Veblen1999) proposed the origin by natural ion-exchange reaction, which takes place in subsolidus alkaline-granite systems: Na2ZrSi6O15⋅3H2O + Ca2+ → CaZrSi6O15⋅3H2O + 2Na+. Taking into account these data and the experimentally tested strong cation-exchange properties of elpidite (Turchkova et al., Reference Turchkova, Pekov and Bryzgalov2006; Grigor'eva et al., Reference Grigor'eva, Zubkova, Pekov, Kolitsch, Pushcharovsky, Vigasina, Giester, Ðorðevic, Tillmanns and Chukanov2011; Zubkova et al., Reference Zubkova, Nikolova, Chukanov, Kostov-Kytin, Pekov, Varlamov, Larikova, Kazheva, Chervonnaya, Shilov and Pushcharovsky2019), we believe that elpidite can undergo ion exchange in Nature, under hydrothermal conditions. The Na-depleted and highly hydrated variety of the mineral studied in the present work is interpreted here as an intermediate product of such natural ion-exchange reaction between ‘ordinary’ elpidite and a low-temperature hydrothermal fluid (solution). Taking into account the above-discussed experimental data and the presence of aluminosilicate zeolites including the wide-porous species chabazite (Pekov et al., Reference Pekov, Turchkova, Lovskaya and Chukanov2004) as overgrowths on our hydrated elpidite, we infer that the temperature of alteration did not exceed 150–200°C.
Conclusions
A new highly hydrated and Na-depleted variety of elpidite was discovered in a hydrothermally altered peralkaline pegmatite at Mt. Yukspor in the Khibiny alkaline complex, Kola Peninsula, Russia. It differs from ‘ordinary’ elpidite in its crystal chemical features, IR spectrum, optical and fluorescent characteristics.
The highly hydrated elpidite studied is characterised by an undistorted heteropolyhedral Zr–Si–O elpidite-type framework with the 14–7–7 Å unit cell and space group Pma2. The choice of the acentric space group was determined by the possibility of avoiding splitting one of the Na sites and one O site representing the H2O molecule. Both crystallographically non-equivalent Na sites are occupied by Na+ cations statistically substituted by H2O molecules. In comparison with the two sites occupied by H2O molecules in previously reported samples of ‘ordinary’ elpidite with the same unit-cell metrics, our additionally hydrated elpidite has one site fully occupied by H2O while the other one is assumed to be dominantly filled by hydronium cations H3O+.
The deficiency of Na and the presence of H3O+ in the variety of elpidite studied are interpreted to indicate that it underwent natural ion exchange at the stage of hydrothermal alteration of its host peralkaline pegmatite, and represents an intermediate product of natural ion-exchange reaction between ‘ordinary’ elpidite and a low-temperature (<150–200°C) hydrothermal fluid (solution).
Acknowledgements
We thank Anatoly N. Zaitsev, Anton R. Chakhmouradian, Sergey V. Kivovichev and two anonymous referees for valuable comments. This study was supported by the the Russian Foundation for Basic Research, grant no. 18-29-12007-mk. Infrared spectroscopy and TGA were performed in accordance with the state task, state registration No. ААAА-А19-119092390076-7.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1180/mgm.2020.96
