Occurrence and associated minerals

Rundqvistite-(Ce) was found 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 ranges (39°27′N 70°43′E). This area is in the Rasht (formerly Garm) district, Central Tajikistan. The alkaline Darai-Pioz 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, Reference Pautov, Agakhanov, Pekov, Karpenko, Siidra, Hawthorne and Faiziev2023) with reference to relevant earlier publications, and the genesis of these rocks was discussed by Pautov et al. (Reference Pautov, Agakhanov, Pekov and Karpenko2022).

Rundqvistite-(Ce) was found in quartz–pectolite aggregate in a silexite-like peralkaline pegmatite. The following minor and accessory minerals are present: black crystals of aegirine, orange–brown semi-transparent lentil-like crystals of stillwellite-(Се), aggregates of large white grains of microcline, 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, grass-green or yellowish-green semi-transparent and transparent crystals of leucosphenite, and dark-green crystals of turkestanite. Rundqvistite-(Ce) occurs in fine-grained brown or greyish-brown aggregates of Mn-bearing pectolite, quartz, Sr-bearing fluorite, aegirine and a variety of rare minerals: baratovite, neptunite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite.

Physical properties

Rundqvistite-(Ce) occurs as elongated grains up to 0.1 mm long and up to 0.03 mm thick embedded in quartz–pectolite aggregate (Fig. 1). The mineral is transparent, colourless and has a white streak and a vitreous lustre. Rundqvistite-(Ce) does not fluoresce under cathode rays or ultraviolet light. The mineral is brittle, the fracture is uneven; no cleavage or parting were observed. The average micro-indentation hardness is VHN₅₀ = 612 (with range of 572–653). Measurements were done on the PMT-3 instrument, calibrated on NaCl at a loading of 50 g. Although micro-indentation and Mohs hardness are different hardness measurements (Broz et al., Reference Broz, Cook and Whitney2006), the first representing the resistance to indentation and the second the resistance to scratching, the present value of micro-indentation hardness roughly corresponds to a Mohs hardness of ~5. The measured density by flotation in Clerici solution is 3.70(2) g/cm³. The calculated density is 3.709 g/cm³.

Figure 1. Back-scattered electron image of a polished section of rundqvistite-(Ce) [Run-(Ce)] in association with pectolite [Pct] and quartz [Qz].

Rundqvistite-(Ce) is optically biaxial (–) with refractive indices (λ = 590 nm) α = 1.644(2), β = 1.659(2) and γ = 1.662(2); 2V_calc. = 48°. It is non-pleochroic. Dispersion is medium: r < v. The compatibility indices (1 – Kp/Kc) = 0.000 (for D _meas.) and 0.002 (for D _calc.) are rated as superior (Mandarino, Reference Mandarino1981).

Chemical composition

The chemical composition of rundqvistite-(Ce) was determined using a JEOL Superprobe JCXA-733 electron microprobe with energy dispersive spectroscopy (EDS) (using an ultrathin ATW2 window and an INCA control system, Fersman Mineralogical Museum). Measurements were done using EDS with an accelerating voltage of 20 kV, a probe current of 2 nA and a beam diameter of 3 μm. To avoid damage of the very thin (0.014 mm) rundqvistite-(Ce) flakes under the electron beam, we chose EDS as a less destructive method of chemical analysis due to the lower current (when compared to wavelength dispersive spectroscopy). The following standards were used: microcline USNM 143966 (Si), albite 107 (Na), SrTiO₃ (Sr), PbTiO₃ (Pb), ilmenite USNM 96189 (Fe), anorthite USNM 137041 (Ca), ZnS (Zn), LaPO₄ (La), CePO₄ (Ce), PrPO₄ (Pr), NdPO₄ (Nd), SmPO₄ (Sm), EuPO₄ (Eu), GdPO₄ (Gd), TbPO₄ (Tb) and DyPO₄ (Dy). Contents of other elements with atomic numbers > 8 are below the detection limits.

The data (10 point analyses) were reduced and corrected by the PAP method of Pouchou and Pichoir (Reference Pouchou, Pichoir and Armstrong1985). The chemical composition of rundqvistite-(Ce) is the mean of 10 determinations and is given in Table 1. The empirical formula calculated on 24 O apfu is Na_3.10Sr_2.29Ca_0.54Pb_0.06(Ce_0.55La_0.19Nd_0.17Pr_0.06Sm_0.02Gd_0.02Eu_0.01Tb_0.01Dy_0.01)_Σ1.04Zn_1.92Fe_0.06Si_8.00O₂₄ Z = 2. The structural formula based on refined site-occupancies (see below) is (Na_2.94Sr_0.06)_Σ3.00[(Sr_2.23Ca_0.54Pb_0.06Na_0.13)_Σ2.96Ln ³⁺_1.04]_Σ4.00[(Zn_1.92Fe²⁺_0.06)_Σ1.98Si₈O₂₄], where Ln ³⁺_1.04 = (Ce_0.55La_0.19Nd_0.17Pr_0.06Sm_0.02Gd_0.02Eu_0.01Tb_0.01Dy_0.01)_Σ1.04. The simplified formula is (Na,Sr)₃(Sr,Ln ³⁺,Ca)₄[(Zn,Fe)₂Si₈O₂₄] where Ce is the dominant lanthanoid (Ln ³⁺). The ideal formula of rundqvistite-(Ce), Na₃(Sr₃Ce)(Zn₂Si₈O₂₄), requires (wt.%): Na₂O 7.68, SrO 25.66, Ce₂O₃ 13.55, ZnO 13.44, SiO₂ 39.67, total 100.

Table 1. Chemical composition and unit formula* for rundqvistite-(Ce).

* The empirical formula was calculated on 24 O apfu.

Powder X-ray diffraction

Powder X-ray diffraction data were obtained using an RKU-86 camera, CrKα radiation and a V-filter (Table 2). Unit-cell parameters calculated from the powder-diffraction data are a = 5.196(1), b = 7.889(2), c = 26.012(9) Å, β = 90.04(4)° and V = 1066.3(4) ų.

Table 2. Powder X-ray diffraction data for rundqvistite-(Ce).

X-ray single-crystal data collection and structure refinement

X-ray single-crystal data for rundqvistite-(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 crystal of rundqvistite-(Ce) used for single-crystal X-ray diffraction measures 0.014 × 0.022 × 0.034 mm. The intensities of reflections with –7 ≤ h ≤ 7, –11 ≤ k ≤ 11 and –37 ≤ l ≤ 37 were collected with a frame width of 0.3° and frame time of 26 s up to 2θ ≤ 61.99°, and an empirical absorption correction (SADABS, Sheldrick, Reference Sheldrick2015) was applied. The crystal structure was refined to R ₁ = 2.76% in space group P2₁/c using atom coordinates of vladykinite, Na₃Sr₄(Fe²⁺Fe³⁺)Si₈O₂₄ (Chakhmouradian et al., Reference Chakhmouradian, Cooper, Ball, Reguir, Medici, Abdu and Antonov2014), on the basis of 3184 unique reflections [F > 4σ(F)] with the Bruker SHELXTL Version 2014/3 software (Sheldrick, Reference Sheldrick2015). The structure was refined as two components related by the TWIN matrix (–1 0 0, 0 1 0, 0 0 1); the twin ratio being 0.5153(12) : 0.4847(12). Details of data collection and structure refinement are given in Table 3. The occupancies of five cation sites were refined with the following scattering curves: Na and M1 sites: Na; M(2,3) sites: Sr; and the T site: Zn. Refinement of the Na site-occupancy converged to an integer value (within 3 e.s.d.) and was 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 4, selected interatomic distances and angles in Table 5, refined site-scattering values and assigned site-populations in Table 6 and bond-valence values in Table 7. A list of observed and calculated structure factors and a crystallography information file (cif) have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary Material (see below).

Table 3. Miscellaneous refinement data for rundqvistite-(Ce).

* Twin components are related by the transformation matrix (–100, 010, 001)

Table 4. Atom coordinates and anisotropic displacement parameters for rundqvistite-(Ce).

Table 5. Selected interatomic distances (Å) and angles (°) for rundqvistite-(Ce).

Symmetry operators: a: x – 1, y, z; b: –x + 1, y + ¹/₂, –z + ³/₂; c: x – 1, y + 1, z; d: x, y + 1, z; e: –x + 1, –y + 1, –z + 1; f: –x, –y + 1, –z + 1; g: x + 1, y, z; h: –x + 2, y – ¹/₂, –z + ³/₂.

Table 6. Refined site-scattering and assigned site-populations for rundqvistite-(Ce).

* Site-scattering was refined, converted to an integer and then fixed at the last stages of the refinement;

** Ln ³⁺_1.04 = Ce_0.55La_0.19Nd_0.17Pr_0.06Sm_0.02Gd_0.02Eu_0.01Tb_0.01Dy_0.01 (60.93 epfu); f _av = 58.59 el.

Table 7. Bond-valence* values for rundqvistite-(Ce).

* Bond-valence parameters (vu) are from Brown (Reference Brown, O'Keeffe and Navrotsky1981); bond-valence contributions calculated from Zn (T), Na (Na, M1) and (Sr + Ce³⁺) (M2,M3); coordination numbers are shown for non-[4]-coordinated anions.

Site-population assignment

Silicon was assigned to four tetrahedrally coordinated Si sites, with <<Si–O>> = 1.632 Å (Tables 4, 5). In accord with the refined site-scattering of 58.4 epfu (electrons per formula unit), all available Zn and Fe²⁺ (Table 1) were assigned to the T site, with <T–O> = 1.953 Å (Tables 4–6). In accord with the structure-refinement results (see above), the Na site is occupied solely by Na. The M1 site has a slightly higher refined site-occupancy and slightly longer mean bond-length: 23.62 epfu and 2.522 Å, (i.e. 11.81 epfu per one atom) than the Na site: 11.00 epfu and 2.447 Å. Hence we assign Na_1.94Sr_0.06 apfu to the M1 site (Tables 4–6). Based on the refined site-scattering values and observed bond-distances, we assign (Sr_1.07Ca_0.26Na_0.04Pb_0.02)_Σ1.39Ln ³⁺_0.61 and (Sr_1.16Ca_0.28Na_0.09Pb_0.04)_Σ1.57Ln ³⁺_0.43 apfu to the M2 and M3 sites, respectively (Tables 4–6). Sum of Ln ³⁺_0.61 and Ln ³⁺_0.43 apfu equals Ln ³⁺_1.04 = (Ce_0.55La_0.19Nd_0.17Pr_0.06Sm_0.02Gd_0.02Eu_0.01Tb_0.01Dy_0.01) apfu, with an aggregate scattering of 60.93 epfu (Table 6). The bond-valence sums for the anions at the twelve O sites vary from 1.71 to 2.20 vu (valence unit) (Table 7), and hence they are O atoms.

Description of the structure

The crystal structure contains 2 groups of cation sites: Si and T sites of the tetrahedral Si–Zn–O sheet and interstitial Na and M(1,2,3) sites. In the Si–Zn–O sheet, the Si(1–4) sites are occupied by Si, with <<Si–O>> = 1.632 Å and the T site is occupied mainly by Zn, ideally Zn₂ apfu (Tables 3, 4). The Zn atom is tetrahedrally coordinated by four O atoms. Zinc and Si tetrahedra form four-membered, five-membered and eight-membered rings in the ratio 1:4:1 (Fig. 2a). The ideal formula of the Si–Zn–O sheet is (Zn₂Si₈O₂₄)^12–. In the crystal structure of rundqvistite-(Ce), a sheet of Si and Zn tetrahedra parallel to (100) is the main structural unit.

Figure 2. Crystal structure of rundqvistite-(Ce): (a) the (Zn₂Si₈O₂₄)^12– sheet of SiO₄ and ZnO₄ tetrahedra at x ≈ 0, Na and Sr atoms above the sheet are shown as blue and green spheres; (b) the layer of Na-dominant Na and M1 octahedra and [8]-coordinated Sr-dominant M(2,3) polyhedra at x ≈ 0.5; Na-dominant octahedra are blue, Sr-dominant polyhedra are green. Drawn with Atoms 6.4 software (Dowty, Reference Dowty2016).

The interstitial cations occur at the four cation sites situated at x ≈ 0.5, i.e. between two Si–Zn–O sheets at x ≈ 0 and x ≈ 1.0. The Na site is occupied by Na, giving Na_1.0 apfu, with <^[6]Na–O> = 2.447 Å, and the Na-dominant M1 site ideally gives Na₂ apfu, with <^[6]M1–O> = 2.522 Å (Tables 4–6). Hence the Na and M1 sites ideally give Na₃ apfu. The [8]-coordinated M2 and M3 sites, with <^[8]M2–O> = 2.607 Å and <^[8]M3–O> = 2.599 Å, respectively, give [(Sr_2.23Ca_0.54Pb_0.06Na_0.13)_Σ2.96Ln _1.04]_Σ4.00 apfu, ideally (Sr₃Ce) apfu. Therefore, Na, M1, M2 and M3 interstitial cations sum to [Na₃(Sr₃Ce)]¹²⁺ apfu. The [6]-coordinated Na and M1 polyhedra and [8]-coordinated M2 and M3 polyhedra share common edges to form a layer (Fig. 2b).

The layer of the [6]- and [8]-coordinated Na and M(1–3) polyhedra and the sheet of Si(1–4) and T tetrahedra connect via common vertices along a, forming a framework.

We write the ideal structural formula of rundqvistite-(Ce) as the sum of the interstitial cations and the Si–Zn–O sheet: Na₃(Sr₃Ce) + (Zn₂Si₈O₂₄) = Na₃(Sr₃Ce)(Zn₂Si₈O₂₄) for Z = 2.

Relation to other species

Rundqvistite-(Ce), ideally Na₃(Sr₃Ce)(Zn₂Si₈O₂₄), is a structural analogue of vladykinite, ideally Na₃Sr₄(Fe²⁺Fe³⁺)Si₈O₂₄ (Chakhmouradian et al., Reference Chakhmouradian, Cooper, Ball, Reguir, Medici, Abdu and Antonov2014). Rundqvistite-(Ce) and vladykinite are related by the following substitution: ^[8]Ce³⁺ + ^[4](Zn²⁺)₂ ↔ ^[8]Sr²⁺ + ^[4](Fe²⁺Fe³⁺). Table 8 lists comparative data for rundqvistite-(Ce) and vladykinite.

Table 8. Comparison of rundqvistite-(Ce) and vladykinite.

*This work; **Chakhmouradian et al. (Reference Chakhmouradian, Cooper, Ball, Reguir, Medici, Abdu and Antonov2014).

There is an interesting relation between rundqvistite-(Ce) and samfowlerite, ideally Ca₁₄Mn₃Zn₂(Zn,Be)₂Be₆(SiO₄)₆(Si₂O₇)₄(OH,F)₆ (Rouse et al., Reference Rouse, Peacor, Dunn, Su, Chi and Yeates1994). Samfowlerite is monoclinic, P2₁/c, a = 9.068(2), b = 17.992(2), c = 14.586(2) Å, β = 104.86(1)°, V = 2312(4) ų and Z = 2. Rouse et al. (Reference Rouse, Peacor, Dunn, Su, Chi and Yeates1994) described a sheet of “vertex-sharing TO₄ tetrahedra (T = Si, Be, Zn)” which form 4-, 5- and 8-membered rings. Hawthorne et al. (Reference Hawthorne, Uvarova and Sokolova2019) compared topologies of tetrahedral sheets in vladykinite and samfowlerite and wrote the ideal formula of samfowlerite as follows: Ca₁₄Mn₃[(Be₇Zn)Zn₂Si₁₄O₅₂(OH)₆]; composition of the Si–Be–Zn–O–OH sheet is given in square brackets. In samfowlerite, there are two sites that contain Zn: the Zn1 site is occupied solely by Zn giving Zn₂ apfu and the Zn2 site is occupied by Be > Zn giving (Be_1.25Zn_0.75), ideally Be₂ apfu. Rouse et al. (Reference Rouse, Peacor, Dunn, Su, Chi and Yeates1994) noted that “substitution of Be for Zn at the Zn2 site suggests that the space group P2₁/c is only that of an average structure, and that the true structure is of lower symmetry, with ordering of Zn and Be on separate sites.” Here we can write the ideal composition of the Si-Be–Zn–O–OH sheet as [Be₈Zn₂Si₁₄O₅₂(OH)₆]^34–. The (Zn₂Si₈O₂₄)^12– sheet in rundqvistite-(Ce) and [Be₈Zn₂Si₁₄O₅₂(OH)₆]^34– sheet in samfowlerite are shown in Fig. 3a,b. The two sheets have three common elements: Si, Zn and O.

Figure 3. The sheets of 4-, 5- and 8-membered rings of tetrahedra in (a) rundqvistite-(Ce), projection on the (100) plane and (b) samfowlerite, projection on the (102) plane. Si tetrahedra are orange, Zn and Zn-dominant tetrahedra are purple, Be and Be-dominant tetrahedra are yellow. The unit cell of rundqvistite-(Ce) and the repeat of the samfowlerite sheet are shown in thin solid lines and dashed lines, respectively.