Occurrence and associated minerals

Kalyuzhnyite-(Ce) occurs in the moraine of the Darai-Pioz glacier in the upper reaches of the Darai-Pioz River, Tien-Shan mountains, near the junction of the Turkestansky, Zeravshansky and Alaisky ridges (39°30′N 70°40′E). This area is in the Rasht (formerly Garm) district, Central Tajikistan. The Darai-Pioz alkaline massif belongs to the Upper Paleozoic Alaysky (Matchaisky) intrusive complex. Information on the geology of the Darai-Pioz massif can be found in Pautov et al. (Reference Pautov, Agakhanov, Karpenko, Uvarova, Sokolova and Hawthorne2019) with reference to relevant earlier publications. Kalyuzhnyite-(Ce) was found in boulders (up to 2 m across) of quartz rock, i.e. silexite boulders composed of 90–95% medium- to coarse-grained quartz of ice-like appearance (quartz grains vary from 2 mm to 2 cm) characteristic for moraine deposits of the Darai-Pioz glacier. The following minor and accessory minerals are present: large (up to 10 cm across) golden-brown tabular and lamellar crystals of polylithionite, pink plates of sogdianite–sugilite, pale-yellow to orange aggregates and tabular crystals of reedmergnerite, black crystals of aegirine, orange–brown semi-transparent lenticular crystals of stillwellite-(Се), grass-green or yellowish-green semi-transparent and transparent crystals of leucosphenite, dark-green crystals of turkestanite and aggregates of large white grains of microcline. Also present are fine-grained brown or greyish-brown aggregates of Mn-bearing pectolite, quartz, Sr-bearing fluorite and a variety of rare minerals. Kalyuzhnyite-(Ce) occurs in these pectolite-rich aggregates (Fig. 1a,b), associated with quartz, fluorite, pectolite, baratovite, aegirine, leucosphenite, neptunite, reedmergnerite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite. The origin of the silexite boulders with pectolite aggregates, in which kalyuzhnyite-(Ce) was found, is debatable as these rocks have not been investigated in situ. The problem of their genesis has been discussed by Pautov et al. (Reference Pautov, Agakhanov, Pekov and Karpenko2022).

Figure 1. Back-scattered electron images of polished sections of kalyuzhnyite-(Ce) (Kaly-Ce): (a) grain 1 (holotype catalogue number 98144) in association with mendeleevite-(Ce) (Mdl-Ce) and quartz (Qz); (b) grain 2 in association with pectolite (Pct) and quartz.

Physical properties

Kalyuzhnyite-(Ce) occurs as equant grains with poorly developed faces in a quartz–pectolite aggregate (Fig. 1a,b). The two grains of kalyuzhnyite-(Ce) up to 70 μm were found by scanning electron microscopy in a small hand-sample. One part of grain 1 (Fig. 1a) was used for the crystal-structure work (crystal 1) and the second part of grain 1 was used for the electron-microprobe analysis (crystal 2). The microprobe mount of crystal 2 was deposited as a holotype Kalyuzhnyite-(Ce), catalogue number 98144. The second smaller grain 2 up to 50 μm (Fig. 1b) was used for Raman spectroscopy (in a thin section).

The grains are colourless and have a vitreous lustre. Kalyuzhnyite-(Ce) has an uneven fracture and does not fluoresce under cathode waves or ultraviolet light. The cleavage is {010} perfect and no parting was observed. The hardness of kalyuzhnyite-(Ce) was not measured due to the very small size of grain 2. The mineral is brittle and D _calc. = 3.120 g/cm³.

Individual grains show no visible twinning. Kalyuzhnyite-(Ce) is nonpleochroic. In reflected light, kalyuzhnyite-(Ce) is grey. Reflectance values were measured with a UMSP-50 Opton microspectrophotometer using the Opton SiC standard 474251 (with a spectral slot width of 10 nm) and are given in Table 1. Kalyuzhnyite-(Ce) has very weak bireflectance.

Table 1. Reflectance values (%) for kalyuzhnyite-(Ce).

The reference wavelengths required by the Commission on Ore Mineralogy (COM) are given in bold.

Raman spectroscopy

The Raman spectrum of kalyuzhnyite-(Ce) (Fig. 2) was obtained on a randomly oriented crystal (polished section, Fig. 1b) using Thermo DXR2xi Raman imaging confocal microscope with a green laser (532 nm) at room temperature. The output power of the laser beam was 10 mW (at 100% power), holographic diffraction grating was used with 1800 lines cm^–1, spectral resolution was 2 cm^–1, the range was from 50 to 1800 cm^–1. The diameter of the focal spot on the sample was 2 μm. The back-scattered Raman signal was collected with a 100× objective; signal acquisition time for a single scan of the spectral range was 2.0 s and the signal was averaged over 20 scans. The spectrum was processed using Omnic software.

Figure 2. Raman spectrum of kalyuzhnyite-(Ce).

Lines in the Raman spectrum of kalyuzhnyite-(Ce) can be grouped into three frequency ranges by analogy with data obtained for some multi-ring silicates of Na, Sr, Ti and REE (Sitarz et al., Reference Sitarz, Mozgawa and Handke1998; Frost and Xi, Reference Frost and Xi2012; Tobbens et al., Reference Tobbens, Kahlenberg, Kaindl, Sartory and Konzett2008). The 800–1100 cm^–1 range bands are due to Si–O bond-stretching modes, the 500 to 800 cm^–1 vibrations are due to O–Si–O bending modes. The spectral region lower than 500 cm^–1 corresponds to Me–O stretching vibrations and lattice modes, while some bonds from 300 to 500 cm^–1 can correspond to SiO₄-unit vibrations.

Chemical composition

The chemical composition of kalyuzhnyite-(Ce) was determined using a JEOL 773 electron microprobe (energy dispersive spectroscopy mode, an accelerating voltage of 20 kV, a specimen current of 2 nA and a beam diameter of 5 μm; Fersman Mineralogical Museum). The following standards were used: microcline USNM 143966 (Si,K), albite 107 (Na), SrTiO₃ (Sr), BaSO₄ (Ba), PbTiO₃ (Pb), CsTbP₄O₁₂ (Cs), ilmenite USNM 96189 (Ti), anorthite USNM 137041 (Ca), LiNbO₃ (Nb), LaPO₄ (La), CePO₄ (Ce), PrPO₄ (Pr), NdPO₄ (Nd), SmPO₄ (Sm), GdPO₄ (Gd), HoPO₄ (Ho), ErPO₄ (Er), and MgF₂ (F). The data (10 analyses) were reduced and corrected by the PAP method of Pouchou and Pichoir (Reference Pouchou, Pichoir and Armstrong1985). The chemical composition of kalyuzhnyite-(Ce) is the mean of 10 determinations and is given in Table 2. The empirical formula calculated on 26.11 (O + F) apfu (atoms per formula unit) is Na_1.07K_0.37Cs_0.30Sr_1.21Ca_0.37Pb_0.24Ba_0.06(Ce_0.43Nd_0.41Pr_0.08La_0.06Sm_0.03Gd_0.01Er_0.01Ho_0.01)_Σ1.04(Ti_0.97Nb_0.04)_Σ1.01Si_8.06O_25.21F_0.90H_6.42 with Z = 4. The structural formula based on assigned site-populations is (Na_0.80□_0.20)_Σ1(K_0.37Cs_0.30)_Σ0.67[(Ca_0.25Pb_0.22Sr_0.14)Na_0.25Ln ³⁺_0.08]_Σ0.94(Sr_0.98Pb_0.02)_Σ1(Ln ³⁺_0.96Ca_0.04)_Σ1(Ti_0.97Nb_0.03)_Σ1.00(Si₈O₂₁)OF_0.50[F_0.40(H₂O)_0.10](H₂O)_3.11, where Ln ³⁺_1.04 = (Ce_0.43Nd_0.41Pr_0.08La_0.06Sm_0.03Gd_0.01Er_0.01Ho_0.01)_Σ1.04, □ = vacancy and Ba_0.06 does not belong to this mineral species (see below). The simplified formula is (Na,□)(K,Сs)(Ca,Pb,Sr,Na)SrLn ³⁺Ti(Si₈O₂₁)OF(H₂O)₃, where Ce is the dominant lanthanoid. The ideal formula of kalyuzhnyite-(Ce), NaKCaSrCeTi(Si₈O₂₁)OF(H₂O)₃ (see below), requires (wt.%) Na₂O 3.06, K₂O 4.58, CaO 5.46, SrO 10.08, Ce₂O₃ 15.97, TiO₂ 7.78, SiO₂ 46.78, F 1.85, H₂O 5.26, O ≡ F –0.78, Total 100.00.

Table 2. Chemical composition and unit formula for kalyuzhnyite-(Ce).

*The empirical formula is calculated on 26.11 (O + F) apfu.

**H₂O is calculated from structure-refinement results: H₂O = 3.21 pfu; E.s.d. = estimated standard deviation.

Powder X-ray diffraction

Powder X-ray diffraction data were obtained by collapsing single-crystal experimental data into two dimensions. Data (in Å for MoKα) are listed in Table 3. Unit-cell parameters are therefore the same as from the single-crystal data (Table 4).

Table 3. Simulated powder X-ray diffraction data* for kalyuzhnyite-(Ce).

* Powder data were obtained by collapsing single-crystal X-ray diffraction data into two dimensions. Intensities are listed for reflections with I _est. ≥ 10.

Table 4. Miscellaneous refinement data for kalyuzhnyite-(Ce).

*Twin components are related by the transformation matrix (1̄ 0 0, 0 1̄ 0, 1 0 1).

X-ray single-crystal data collection and structure solution and refinement

X-ray single-crystal data for kalyuzhnyite-(Ce) were collected from a twinned crystal with a Bruker APEX II ULTRA three-circle diffractometer with a rotating-anode generator (MoKα), multilayer optics and an APEX II 4K CCD detector. The intensities of reflections with –26 ≤ h ≤ 26, –15 ≤ k ≤ 15 and –20 ≤ l ≤ 20 were collected with a frame width of 0.3° and a frame time of 6 s up to 2θ ≤ 60.21°, and an empirical absorption correction (SADABS, Sheldrick, Reference Sheldrick2015) applied. There were few observed reflections at high 2θ, and refinement of the structure was done for 2θ ≤ 55°, −24 ≤ h ≤ 24, –14 ≤ k ≤ 15 and –19 ≤ l ≤ 19. The crystal-structure solution by direct methods and refinement were done with the Bruker SHELXTL Version 2014/3 software (Sheldrick, Reference Sheldrick2015) in space group P2/c. We refined the structure as two components related by the TWIN matrix ($\bar{1}$ 0 0, 0 $\bar{1}$ 0, 1 0 1). The crystal structure of kalyuzhnyite-(Ce) was refined to R ₁ = 2.74%, the twin ratio being 0.5037(9) : 0.4963(9) (Table 4). Details of data collection and structure refinement are given in Table 4. The occupancies of ten cation sites were refined with the following scattering curves: M1 site: Ti; M2 site: Nd; M3 site: Sr; M4 site: Na; M(5,6) sites: Pb; M(7,8) sites: Ca; A(1,2) and B(1,2) sites: K and Cs, respectively. The occupancies of the F and X sites were refined with the scattering curve of F, and W(1–9) sites, with the scattering curve of O. Refinement of the F, X and W1 site-occupancies converged to integer values (within 3 e.s.d.) and were subsequently fixed at full occupancy. Scattering curves for neutral atoms were taken from the International Tables for Crystallography (Wilson, Reference Wilson1992). Final atom coordinates and anisotropic displacement parameters are given in Table 5, selected interatomic distances and angles in Table 6, refined site-scattering values and assigned site-populations in Table 7, and bond-valence values in Tables 8 and 9. A list of observed and calculated structure factors and a Crystallographic Information File (CIF) have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary Material (see below). The structure diagrams were drawn using ATOMS 64 software (Dowty, Reference Dowty2016).

Table 5. Atom coordinates, site occupancies (%) and anisotropic displacement parameters (Ų) for kalyuzhnyite-(Ce).

*U _iso

Table 6. Selected interatomic distances (Å) and angles (°) in kalyuzhnyite-(Ce).

φ = O, F and H₂O; *M5–W3 and M5–W7 distances are weighted by 60 and 40%, respectively (54%-occupancy of the M5 site: 28%- and 26%-occupancies of the W3 and W7 sites); **M6–W8 and M6–W4 distances are weighted by 43 and 57%, respectively (56%-occupancy of the M6 site: 24%- and 32%-occupancies of the W8 and W4 sites).

Symmetry codes: a: –x + 1, –y + 1, –z + 1; b: x, –y + 1, z+½; c: x, –y + 1, z–½; d: –x, –y + 1, –z; e: x + 1, y, z; f: –x, –y + 1, –z + 1; g: x, y, z; h: –x, y, –z+½; i: x, y + 1, z; j: –x, –y, –z + 1; k: –x, –y, –z; l: x, y–1, z; m: –x + 1, y, –z+½; n: –x + 1, y–1, –z+½; o: –x + 1, –y, –z + 1; p: –x + 1, y, –z + 3/2; q: –x, y + 1, –z+½.

Table 7. Refined site-scattering and assigned site-populations for kalyuzhnyite-(Ce).

φ = O, F and H₂O.

*Ln ³⁺_1.04 = (Ce_0.43Nd_0.41Pr_0.08La_0.06Sm_0.03Gd_0.01Er_0.01Ho_0.01)_Σ1.04, with f_av = 59.16 el. (f_av = average scattering curve for Ln ³⁺_1.04).

**Site scattering was refined, adjusted in accord with the chemical analysis (Table 2), and then fixed at the last stages of the refinement.

Table 8. Bond-valence values* (vu) for selected anions** in kalyuzhnyite-(Ce).

* Bond-valence parameters (vu) are from Brown (Reference Brown, O'Keeffe and Navrotsky1981); bonds to oxygen were used for cations bonded to O22 and W1; bonds to fluorine were used for cations bonded to F; occupancies of cation sites were taken into account for all calculations.

** Anions which do not coordinate Si.

Table 9. Bond-valence values* (vu) for the X anion involved in short-range order in kalyuzhnyite-(Ce).

* Bond-valence parameters (vu) are from Brown (Reference Brown, O'Keeffe and Navrotsky1981).

Site-population assignment

Si is assigned to the eight tetrahedrally coordinated Si(1–8) sites, with <Si–O> = 1.614 Å (Tables 5, 6).

The four M sites in the heteropolyhedral sheet (Table 5) are considered next. At the [6]-coordinated M1 site, the refined scattering is 21.97 electrons per formula unit (epfu) and the mean bond-length <M1–φ> = 1.983 Å (φ = O, F). In accord with the grand mean bond-length <^[6]Ti⁴⁺–O> = 1.971 Å> observed in inorganic structures (Gagné and Hawthorne, Reference Gagné and Hawthorne2020), we assign all available Ti (Table 2) and minor Nb to the M1 site: Ti_0.97Nb_0.03 apfu, with calculated site-scattering of 22.57 epfu (Table 7). At the [8]-coordinated M2 site, the refined scattering is 57.1 epfu and the mean bond-length <M2–φ> = 2.494 Å (φ = O, F). In accord with the grand mean bond-length <^[8]Ce³⁺–O> = 2.495 Å> observed in inorganic structures (Gagné, Reference Gagné2018), we assign REE = Ln ³⁺_0.96, where Ce is the dominant lanthanoid (Ln = lanthanoids), and minor Ca: 0.04 apfu, to the M2 site: Ln _0.96Ca_0.04 pfu [^[8]Ce³⁺: r = 1.143 Å, ^[8]Ca²⁺: r = 1.12 Å, Shannon (Reference Shannon1976)] (Tables 5–7). There is a good agreement between refined and calculated site-scattering for the M2 site: 57.1 and 57.59 epfu, respectively (Table 7). On the basis of the refined site-scattering values and observed bond-distances, we assign Sr_0.98Pb_0.02 and Ca_0.80□_0.20 pfu to the M3 and M4 sites, respectively (Table 7).

Consider the interstitial M(5–8), A(1,2) and B(1,2) sites. The refined site-scattering value at the M5 site is slightly higher than at the M6 site: 15.3 versus 14.7 epfu; the mean bond-length values: 2.515 and 2.531 Å, are very similar. Therefore, we allocate more Pb (the heaviest scatter) to the M5 site: Pb_0.16Ca_0.11□_0.23 pfu, and less Pb to the M6 site: Sr_0.14Ln_0.08Pb_0.06Ca_0.11□_0.22 pfu (Table 7). Based on the refined site-scattering values and observed bond-distances, we assign Na_0.25□_0.75 and Ca_0.14□_0.86 pfu to the M7 and M8 sites, respectively (Table 7).

The refined site-scattering values at the [11]- and [${9}$]- coordinated A(1,2) and B(1,2) sites sum to 21.01 epfu (Table 7). The cations to be assigned to these sites are Cs_0.30, K_0.37 and Ba_0.06, with individual calculated scattering of 16.5, 7.03 and 3.36 epfu, respectively, and a total calculated scattering 26.89 epfu (Table 2). The A(1,2) sites, with total refined site-scattering of 15.7 epfu and derived cations’ radii of 1.85 and 1.88 Å, respectively [e.g. calculated as <A1–O> – r(^[4]O^2–): 3.320–1.38 = 1.85 Å] must be occupied by Cs [^[11]Cs: r = 1.85 Å, Shannon (Reference Shannon1976)]. Hence, we assign Cs_0.20□_0.30 and Cs_0.10□_0.40 pfu to the A1 and A2 sites, with calculated site-scattering values of 11.0 and 5.50 epfu, respectively, with a sum of 16.5 epfu (Table 7). The B(1,2) sites, with total refined site-scattering of 5.4 epfu and derived cation radii of 1.74 and 1.60 Å, respectively, must be occupied by K [^[11]K: r = 1.62 Å, ^[9]K: r = 1.55 Å, Shannon (Reference Shannon1976)]. Hence, we assign K_0.20□_0.30 and K_0.17□_0.33 pfu to the B1 and B2 sites, with calculated site-scattering values of 3.80 and 3.23 epfu, respectively, and a sum of 7.03 epfu (Table 7). There is a good agreement between refined and calculated site-scattering values for (1) the Cs-bearing A(1,2) sites: 15.7 versus 16.50 epfu; (2) the K-bearing B(1,2) sites: 5.4 versus 7.03 epfu and (3) A + B sites: 21.1 versus 23.53 epfu (Table 7). We were not able to assign Ba_0.06 apfu (calculated scattering is 3.36 epfu; ^[11]Ba: r = 1.57 Å, ^[9]Ba: r = 1.47 Å) to the A and B sites and suggest that Ba_0.06 does not occur in the crystal structure of kalyuzhnyite-(Ce).

Description of the structure

Cation sites

The crystal structure of kalyuzhnyite-(Ce) contains three groups of cation sites: M sites of the heteropolyhedral sheet, Si sites of the Si–O sheet and interstitial M(5–8), A(1,2) and B(1,2) sites.

Heteropolyhedral sheet

There are four cation sites in the heteropolyhedral sheet: the Ti-dominant M1 site, the Ce-dominant M2 site, the Sr-dominant M3 site, and the Na-dominant M4 site (Fig. 3a). The M1 site is occupied by Ti_0.97Nb_0.03 apfu, ideally Ti apfu. The M1 atom is octahedrally coordinated by six anions: five O atoms and one F atom, <M1–φ> = 2.983 Å (φ = O and F) (Tables 5–7). The M2 site is occupied by Ln _0.96Ca_0.04 apfu, ideally Ce apfu (Ce is the dominant lanthanoid). The M2 atom is coordinated by eight anions: seven O atoms and one X anion, <M2–φ> = 2.494 Å (φ = O, F and H₂O) (Tables 5–7). The M3 site is occupied by Sr_0.98Pb_0.02 pfu, ideally Sr apfu. The M3 atom is coordinated by eight anions: seven O atoms and one F atom, <M3–φ> = 2.631 Å (φ = O and F) (Tables 6, 7). The M4 site is occupied by Na at 80%, its ideal composition is Na apfu. The M4 atom is coordinated by seven anions: six O atoms and one X anion, <M4–φ> = 2.591 Å (φ = O, F and H₂O) (Tables 6, 7). Ideally, cations of the heteropolyhedral sheet, ^{M 4}Na + ^{M 3}Sr + ^{M 2}Ce + ^{M 1}Ti, sum to (NaSrCeTi)¹⁰⁺ apfu (Table 7).

Figure 3. Main structural units in the crystal structure of kalyuzhnyite-(Ce): (a) the heteropolyhedral sheet (NaSrCeTiOF)⁷⁺; (b) the double (Si₈O₂₁)^10– sheet. Ti-dominant M1 octahedra are pale yellow, [8]-coordinated Ce-dominant M2 and Sr-dominant M3 polyhedra are green and pink, respectively; [7]-coordinated Na-dominant M4 polyhedra are blue, F atoms are shown as small yellow spheres and SiO₄ tetrahedra are orange.

Si–O sheet

In the Si–O sheet (Fig. 3b), there are eight Si(1–8) tetrahedrally coordinated sites occupied by Si with a <Si–O> distance of 1.614 Å (Tables 5, 6). We consider the Si–O sheet a complex anion (see below).

Interstitial sites

Interstitial cations occur at the eight partly occupied M(5–8), A(1,2) and B(1,2) sites within the double Si–O sheet (Fig. 4a,b; Table 5). There are short distances between cations which occur at the four interstitial sites (Table 6): M6–B1 = 0.65(3) Å and B2–A2 = 1.03(2) Å, so these sites can be only alternately occupied. There is a short distance between two points of the M7 site: M7–M7′ = 0.88(4) Å (Table 6), hence the M7 site must be occupied at ≤ 50%. The M5 and M6 sites are 54% and 56% occupied; their compositions are Pb_0.16Ca_0.11□_0.23 and Sr_0.14Ln _0.08Pb_0.06□_0.22 pfu, respectively. The M5 and M6 sites are each coordinated by four basal O atoms of Si tetrahedra, an X ligand and H₂O groups at the partly occupied W(3,7) sites [M5] and W(4,8) sites [M6] (Fig. 5a; Table 6). Because of short O–O distances [W3–W7 = 1.67 Å and W4–W8 = 1.79 and 1.68 Å (Table 6)] the W3–W7 and W4–W8 sites can be only alternately occupied. The partial occupancies of the W3 site (28%) + W7 site (26%) (Table 5) sum to 54%, equivalent to the 54%-occupancy of the M5 site. The partial occupancies of the W4 site (32%) + W8 site (24%) (Table 5) sum to 56%, equivalent to the 56%-occupancy of the M6 site. Therefore, we consider [W3 + W7] and [W4 + W8] as two aggregate ligands (H₂O groups) for the M5 and M6 atoms, respectively. Hence M5 and M6 atoms are each coordinated by four O atoms of Si tetrahedra, an X ligand (see Anion sites below) and an H₂O group, with <M5–φ> = 2.515 Å and <M6–φ> = 2.531 Å (Tables 6, 7). The M7 and M8 sites are occupied by Na at 25% and Ca at 14%, respectively (Fig. 5b; Table 7). The M7 and M8 atoms are coordinated by four O atoms and two H₂O groups at the W(2,4) sites [M7] and W(1,6) sites [M8], with <M7–φ> = 2.53 Å and <M8–φ> = 2.380 Å (Tables 6, 7). The composition of the four M(5–8) sites is [(Ca_0.25Pb_0.22Sr_0.14)Na_0.25Ln _0.08)]_Σ0.94, ideally Ca apfu (Table 7). The two [11]-coordinated A1 and A2 sites are occupied by Cs at 40% and 20%, respectively (Fig. 5c; Tables 5–7). The Cs atoms are coordinated by eight O atoms, two H₂O groups (W1 site) and a F atom, with <A1–φ> = 3.230 Å and <A2–φ> = 3.255 Å (Tables 6, 7). The [9]-coordinated B1 and [11]-coordinated B2 sites are occupied by K at 40% and 34%, respectively (Fig. 5d,e; Tables 5–7). The B1 atom is coordinated by six O atoms, two H₂O groups (W2 site) and one X ligand, with <B1–φ> = 3.12 Å and the B2 atom is coordinated by eight O atoms, two H₂O groups (W1 site) and a F atom, with <B2–φ> = 2.983 Å (Tables 6, 7). The A(1,2) + B(1,2) sites are occupied by (K_0.37Cs_0.30)_Σ0.67, ideally K apfu (Table 7). Ideally, interstitial cations, ^{M (5–8)}Ca + ^{A (1,2)B(1,2)}K, sum to (KCa)³⁺ pfu.

Figure 4. General view of the crystal structure of kalyuzhnyite-(Ce): (a) linkage of two double Si–O sheets and a heteropolyhedral sheet with interstitial cations at partly occupied M(5–8), A(1,2) and B(1,2) sites and H₂O groups; (b) positions of interstitial cations and H₂O groups within the ten-membered rings of SiO₄ tetrahedra of the double Si–O sheet. Legend as in Fig. 3, dominant cations at partly occupied sites are shown as spheres: Pb (bright yellow) at the M5 site; Sr (pink) at the M6 site; Na (blue) at the M7 site; Ca (bright pink) at the M8 site; Cs (pale orange) at the A(1,2) sites and K (green) at the B(1,2) sites; an H₂O group at the W1 site is shown as a central O atom (red sphere) with two H atoms (small grey spheres), and O–H bonds are shown as black lines.

Figure 5. Details of the coordination of the interstitial cations in the crystal structure of kalyuzhnyite-(Ce): (a) [7]-coordinated Pb at the M5 site and Sr at the M6 site, respectively; (b) [6]-coordinated Na at the M7 site and Ca at the M8 site, respectively; (c) [11]-coordinated Cs at the A(1,2) sites; (d) [9]-coordinated K at the B1 site; (e) [11]-coordinated K at the B2 site. Legend as in Fig. 4a, W(2,4,6,7,8) are H₂O groups at partly occupied sites, the X anion has composition [F_0.40(H₂O)_0.10] pfu.

Cations of the heteropolyhedral sheet and interstitial complex ideally sum to (NaSrCeTi)¹⁰⁺ + (KCa)³⁺ = (NaKCaSrCeTi)¹³⁺.

General topology of the crystal structure

There are two main structural units in the crystal structure of kalyuzhnyite-(Ce): a heteropolyhedral sheet and a double Si–O sheet (Fig. 3a,b). The heteropolyhedral sheet parallel to (010) is composed of Ti-dominant M1 octahedra, Ce-dominant M2 polyhedra, and Sr-dominant M3 polyhedra and Na-dominant M4 polyhedra which share edges and vertices (Fig. 3a, Tables 5–7). We sum the cation and anion parts of the heteropolyhedral sheet to derive its ideal composition: (NaSrCeTi)¹⁰⁺ + (OF)^3– = [NaSrCeTi(OF]⁷⁺. The double Si–O sheet parallel to (010) is composed of ten-membered rings of SiO₄ tetrahedra (Figs 3b, 4b). The ten-membered rings occur at y ≈ + 0.265 and y ≈ –0.265 and connect via two SiO₄ tetrahedra which form an Si4–O15–Si5 bridge oriented along b (Fig. 4a; Table 6). The composition of this double Si–O sheet is (Si₈O₂₁)^10–. Such a double Si–O sheet is new – a sheet with this topology and composition has never been described in minerals before (Hawthorne et al., Reference Hawthorne, Uvarova and Sokolova2019); we call this the kalyuzhnyite sheet. The heteropolyhedral and Si–O sheets alternate along b; there is one heteropolyhedral sheet and one Si–O sheet per b unit-cell parameter (Fig. 4a). The sheets connect via common vertices of M(1–4) polyhedra and SiO₄ tetrahedra to form a framework. Summation of the ideal compositions of the heteropolyhedral sheet and double Si–O sheet, gives ideal composition of the framework: [NaSrCeTiOF]⁷⁺ + (Si₈O₂₁)^10– = NaSrCeTi(Si₈O₂₁)OF]^3–. The interstitial complex is composed of M(5–8), A(1,2) and B(1,2) cations and H₂O groups at the W(1–9) sites. The interstitial complex is located within the double Si–O sheet (Figs 4, 5). The larger cations, Pb and Sr at the M(5,6) sites, Cs at the A(1,2) sites and K at the B(1,2) sites occupy centres of ten membered rings of SiO₄ tetrahedra (Fig. 4b). Smaller cations, Na and Ca at the M7 and M8 sites and H₂O groups at the W(1–9) sites occur in the large channels along c (Figs 4a, 5b). The ideal composition is derived by a sum of cation and anion/H₂O parts of the interstitial complex to give: (KCa)³⁺ + (H₂O)₃ = [(KCa) (H₂O)₃]³⁺.

The general structural formula of kalyuzhnyite-(Ce)

The structural formula on the basis of assigned site-populations (Table 7) is (Na_0.80□_0.20)_Σ1(K_0.37Cs_0.30)_Σ0.67[(Ca_0.25Pb_0.22Sr_0.14)Na_0.25Ln ³⁺_0.08]_Σ0.94(Sr_0.98Pb_0.02)_Σ1(Ln ³⁺_0.96Ca_0.04)_Σ1(Ti_0.97Nb_0.03)_Σ1.00(Si₈O₂₁)OF_0.50[F_0.40(H₂O)_0.10](H₂O)_3.11, where Ln ³⁺_1.04 = (Ce_0.43Nd_0.41Pr_0.08La_0.06Sm_0.03Gd_0.01Er_0.01Ho_0.01)_Σ1.04 and Ce is the dominant lanthanoid. The simplified formula is Na(K,Cs,□)(Ca_,Na,Pb,Sr)SrLn ³⁺Ti(Si₈O₂₁)OF(H₂O)₃. On the basis of the structure refinement results and bond-valence calculations, the ideal formula of kalyuzhnyite-(Ce) can be written as the sum of the cation and anion components: (NaKCaSrCeTi)¹³⁺ + [(Si₈O₂₁)OF(H₂O)₃]^13– = NaKCaSrCeTi(Si₈O₂₁)OF(H₂O)₃, Z = 4. The validity of the ideal formula is supported by the good agreement between the total charges for cations in the ideal and empirical formulae: 1⁺ (Na) + 1⁺ (K) + 2⁺ (Ca) + 2⁺ (Sr) + 3⁺ (Ce) + 4⁺ (Ti) = 13⁺ versus 1.07⁺ (Na_1.07) + 0.67⁺ (K_0.37Cs_0.30) + 1.64⁺ (Ca_0.37Pb_0.24Sr_0.21) + 2⁺ (Sr_1.00) + 3.12⁺ (Ln _1.04) + 4.08⁺ (Ti_0.97Nb_0.04) = 12.58⁺.

The ideal formulae of kalyuzhnyite-(Ce), above, was based on the sum of the cation and anion components. The ideal formula of kalyuzhnyite-(Ce) (Z = 4) based on ideal compositions of cation and anion sites ^{M 4}Na, ^{A (1,2)B(1,2)}K, ^{M (5–8)}Ca, ^{M 3}Sr, ^{M 2}Ce, ^{Si (1–8)}Si₈, ^{O (1–21)}O₂₁, ^{O 22}O, ^F+XF and ^{W (1–9)}(H₂O)₃, can be written as NaKCaSrCeTi(Si₈O₂₁)OF(H₂O)₃.

Relation to other sheet silicates

Kalyuzhnyite-(Ce) has no analogues. The double (Si₈O₂₁)^10– sheet consists of ten-membered rings of SiO₄ tetrahedra. It has unique topology and composition, and its Si–O double sheet has not previously been described in minerals (Hawthorne et al., Reference Hawthorne, Uvarova and Sokolova2019). We can relate kalyuzhnyite-(Ce) to penkvilksite, Na₂TiSi₄O₁₁⋅2H₂O (Bussen et al., Reference Bussen, Men'shikov, Merkov, Nedorezova, Uspenskaya and Khomyakov1974; Merlino et al., Reference Merlino, Pasero, Artioli and Khomyakov1994; Cadoni and Ferraris, Reference Cadoni and Ferraris2008) and tumchaite, Na₂(Zr,Sn)Si₄O₁₁⋅2H₂O (Subbotin et al., Reference Subbotin, Merlino, Pushcharovsky, Pakhomovsky, Ferro, Bogdanova, Voloshin, Sorokhtina and Zubkova2000), two silicate minerals with a single corrugated sheet of ten-membered rings of SiO₄ tetrahedra; the composition of this single sheet is (Si₄O₁₁)^6–.