Occurrence and origin

The Triassic Saima complex (220–230 Ma) is situated on the Liaodong Peninsula within the northeastern margin of the North China Craton, and within it lujavrite hosts typical alkaline rock-type Zr–REE–Nb mineralisation (where REE = rare earth elements, Wu et al., Reference Wu, Wang, Yang, Wu, Zhang, Gu and Zhang2016; Ma and Liu, Reference Ma and Liu2023). The lujavrite, with ~20% exposed area, intruded the main body of nepheline syenite as sheets, stocks or dykes at the northeast and northwest edges of the complex. It is composed of predominantly K-feldspar, nepheline, aegirine, and variable amounts of Zr–REE–Nb-bearing accessory minerals including zircon, eudialyte, pyrochlore, rinkite-(Ce) and wadeite (Wu et al., Reference Wu, Wang, Yang, Wu, Zhang, Gu and Zhang2016). Late metasomatism such as alkali-metasomatism, skarnification and carbonation prevailed through the whole Saima alkaline complex and led to the dissolution of precursor Zr–REE–Nb-bearing minerals (e.g. wadeite and eudialyte), and the precipitation of a series of secondary alteration minerals (e.g. natrolite, calcite, britholite-(Ce) and zircon, Wu et al., Reference Wu, Wang, Yang, Wu, Zhang, Gu and Zhang2015, Reference Wu, Wang, Liu, Guo and Song2018). The geological, mineralogical and geochronological features of the Saima complex have been reported extensively in recent work (e.g. Wu et al., Reference Wu, Yang, Marks, Liu, Zhou, Ge, Yang, Zhao, Mitchell and Markl2010, Reference Wu, Wang, Yang, Wu, Zhang, Gu and Zhang2016; Zhu et al., Reference Zhu, Yang, Sun, Zhang and Wu2016, Reference Zhu, Yang, Sun and Wang2017).

Plumbogaidonnayite occurs as subhedral to anhedral or platy crystals of ~5–50 μm across, commonly forming aggregates (up to 1 mm) in pseudomorphs of altered eudialyte in Saima lujavrite (Fig. 1). It is associated closely with other secondary products after eudialyte alteration, including natrolite, aegirine, gaidonnayite, georgechaoite, zircon, bobtraillite and britholite-(Ce). Plumbogaidonnayite might be crystallised directly from eudialyte alteration, or, more likely, transformed from gaidonnayite or georgechaoite, which occur as the intermediate products after eudialyte alteration by the natural ion exchange 2Na⁺(K⁺) → Pb²⁺(Ca²⁺) + □ (vacancy) as reported in other microporous framework silicate minerals (e.g. vigrishinite and zvyaginite, Pekov and Chukanov, Reference Pekov, Chukanov, Ferraris and Merlino2005; Pekov et al., Reference Pekov, Britvin, Zubkova, Chukanov, Bryzgalov, Lykova, Belakovskiy and Pushcharovsky2013, Reference Pekov, Lykova, Chukanov, Yapaskurt, Belakovskiy, Zolotarev and Zubkova2014). Other hydrothermal Pb-bearing minerals such as galena and gysinite-(La) have also been observed in interstices of microcline in the plumbogaidonnayite-bearing lujavrite samples. The texture and mineral relationships indicate that lead in plumbogaidonnayite was probably derived from external Pb-rich hydrothermal fluids and zirconium from primary eudialyte dissolution (PbO < 1 wt.%, Wu et al., Reference Wu, Wang, Yang, Wu, Zhang, Gu and Zhang2016).

Figure 1. Photomicrograph (a) and back-scattered electron images (b-d) showing the occurrence of plumbogaidonnayite from the Saima lujavrite sample SM 01. (a,b) An aggregate of plumbogaidonnayite as an alteration product in a pseudomorph after eudialyte. (c,d) Plumbogaidonnayite grains (including the holotype crystal selected for Raman spectroscopy and single-crystal XRD determination) associated with other secondary minerals including gaidonnayite, natrolite and britholite-(Ce), and eudialyte relics. Mineral abbreviations after Warr (Reference Warr2021): Ab – albite; Aeg – aegirine; Bri-Ce – britholite-(Ce); Eud – eudialyte; Gdn – gaidonnayite; Mcc – microcline; Ntr – natrolite; Pgdn – plumbogaidonnayite.

Physical and optical properties

Plumbogaidonnayite is transparent, colourless, or light brown in transmitted light with a vitreous lustre. The streak colour is white. It is brittle with a conchoidal fracture, and no cleavage or twinning was observed. The Mohs hardness value is estimated at ~5 in analogy with other gaidonnayite-group minerals. The calculated density of plumbogaidonnayite is 3.264 g/cm³ based on its unit-cell parameters and empirical formula (see below). Optically, it is biaxial (+) with α = 1.61(3), β = 1.63(3) and γ = 1.66(4) (white light). The calculated 2V is 80°, with optical orientation α || a, β || b, and γ || c. Some physical and optical properties could not be tested owing to the small crystal size. According to its measured refraction indices and calculated density, the compatibility index [1 – (K _P/K _C)] yields 0.053, which belongs to the ‘good’ category (Mandarino, Reference Mandarino1981).

Raman spectroscopy

The Raman spectrum of plumbogaidonnayite was obtained using a Renishaw inVia RM2000 spectrometer at the State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, China. Excitation wavelength and working power were set at 532 nm and 20 mW, respectively. Before collection, a pure silicon standard (520 cm^–1) was selected for equipment calibration. In order to get a strong Raman signature and indicate any presence of H₂O, spectrum signals were collected from 100 to 4000 cm^–1 with a 30 s accumulation time and 2–3 accumulations were adopted.

The Raman characteristics for structural framework in plumbogaidonnayite, which is composed of SiO₄ tetrahedra and ZrO₆ octahedra, are similar to those of gaidonnayite, georgechaoite and isostructural synthetic materials (Celestian et al., Reference Celestian, Lively and Xu2019, Fig. 2). The strongest Raman band at 521 cm^–1 is assigned to the symmetric stretching mode of the three-member ring formed by Si1-, Si2- and Zr1-centred polyhedra (see crystal structure below), and the band at 738 cm^–1 probably represents the mixed vibrations of this ring (Sitarz et al., Reference Sitarz, Handke and Mozgawa2000; Kovalskaya et al., Reference Kovalskaya, Ermolaeva, Chukanov, Varlamov, Kovalskiy, Zakharchenko, Kalinin and Chaichuk2023). The second strongest band at 920 cm^–1 is assigned to the stretching mode of the [Zr1O₆]–[Si2O₄] spiral chain extending along the a axis. The moderate peak at 325 cm^–1 possibly corresponds to SiO₄ ν₄ antisymmetric bending or lattice vibrations, and 687 cm^–1 may represent the Si–O–Si bend involving the bridging oxygen. The weak band at 454 cm^–1 can be assigned to lattice vibrations and other weak bands from 900 to 1100 cm^–1 (i.e. bands at 1011, 1034 and 1059 cm^–1) represent asymmetric Si–O stretching vibrations in SiO₄ tetrahedra, as illustrated in some other species, for example some zeolite-group minerals (Dutta and Del Barco, Reference Dutta and Del Barco1985). The bands of H₂O present at 3486 and 1612 cm^–1 correspond to the symmetric O–H stretching mode and H–O–H bending mode, respectively (Carey and Korenowski, Reference Carey and Korenowski1998). In addition, broad bands at 2460 and 3065 cm^–1 are probably assigned to SiO–H stretching vibrations or hydrogen bonds in potential hydrated H₃O⁺ complexes, which are common in hydrous zirconosilicates (e.g. eudialyte-group minerals, Chukanov et al., Reference Chukanov, Vigasina, Rastsvetaeva, Aksenov, Mikhailova and Pekov2022; Kovalskaya et al., Reference Kovalskaya, Ermolaeva, Chukanov, Varlamov, Kovalskiy, Zakharchenko, Kalinin and Chaichuk2023).

Figure 2. The Raman spectrum for plumbogaidonnayite. a.u. = arbitrary units.

Chemical composition

Chemical composition of plumbogaidonnayite was determined using a JEOL-JXA 8530F Plus electron probe micro-analyser (EPMA) in wavelength dispersive spectroscopy (WDS) mode at 15 kV and 50 nA at the State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, China. A defocused beam (5 μm) was chosen for this hydrous mineral to minimise the element diffusion (e.g. K, Na and Ca). Counting times for stable elements on peaks and background were 20 and 10 s and those for K, Na and Ca were 10 and 5 s, respectively. Standards selected for calibration are listed in Table 1. Calculations on the basis of nine oxygen atoms and assuming the occurrence of two H₂O groups for 23 analyses on different plumbogaidonnayite grains gives the following empirical chemical formula: (Pb_0.70Ca_0.17Ba_0.01K_0.11Na_0.01Y_0.01)_Σ1.01(Zr_1.00Hf_0.01Ti_0.01)_Σ1.02Si_3.01O₉⋅2H₂O. The ideal formula of PbZrSi₃O₉⋅2H₂O requires PbO 39.68, ZrO₂ 21.89, SiO₂ 32.03, H₂O_calc 6.40, total 100 (all in wt.%). Some crystals show compositional heterogeneity under back-scattered electron imaging due to variations in K (0.04–0.24 atoms per formula unit), Ca (0.02–0.32 apfu) and Pb (0.62–0.80 apfu, Fig. 1c). Overall, Pb shows a negative relation with K, Na and Ca, implying potential Pb²⁺ → 2K⁺(Na⁺) and Pb²⁺ → Ca²⁺ substitutions in plumbogaidonnayite (Fig. 3).

Table 1. Chemical electron microprobe data (in wt.%) for plumbogaidonnayite.

S.D. = standard deviation; Bdl = below detection limits; Apfu = atoms per formula unit; Const. = constituent.

* H₂O was assumed as 2 apfu according to the ideal formula of plumbogaidonnayite.

Figure 3. Compositional variations for plumbogaidonnayite plotted on a Pb vs. Ca + (Na+K)/2 diagram. apfu = atoms per formula unit.

Powder X-ray diffraction

The powder X-ray diffraction (XRD) data for plumbogaidonnayite was collected at the School of Earth Sciences and Info-physics, Central South University, China, using a Rigaku XtaLAB Synergy diffractometer (CuKα, λ = 1.54184 Å) in powder Gandolfi mode. The working voltage and current were set at 50 kV and 1 mA, respectively. The structural model of a single crystal (see below) was used to index the powder XRD pattern of plumbogaidonnayite (Table 2). The nine strongest lines [d in Å (I, %) (hkl)] are: 6.489 (36) (020), 5.803 (100) (101), 4.661 (27) (021), 4.336 (29) (121), 3.640 (30) (221), 3.114 (79) (112), 2.947 (27) (400), 2.622 (27) (241) and 2.493 (27) (312). Refined orthorhombic unit-cell parameters are: a = 11.7696(5) Å, b = 13.0048(4) Å, c = 6.6588(4) Å, V = 1019.21(5) Å and Z = 4, which were obtained from the powder data using the software program UnitCell (Holland and Redfern, Reference Holland and Redfern1997).

Table 2. Measured and calculated* powder X-ray diffraction data (d in Å, I in %) for plumbogaidonnayite.

* The calculated values were obtained using VESTA 3 (Momma and Izumi, Reference Momma and Izumi2011).

The strongest values are given in bold.

Crystal structure determination

Single-crystal X-ray diffraction data were collected on the same diffractometer equipped with CuKα radiation (λ = 1.54184 Å) at 50 kV and 1mA. As the sample size is very small (20 μm or less), CuKα was chosen due to its strong intensity for an X-ray tube of 50 W power giving good quality diffraction data. A relatively homogeneous plumbogaidonnayite crystal (20 × 20 × 20 μm) was dug from a polished thin section to perform a structure refinement. It contains (in wt.%) SiO₂ 34.96–36.36, ZrO₂ 23.52–24.90, HfO₂ 0.26–0.48, Y₂O₃ 0.02–0.55, CaO 1.84–2.74, PbO 28.55–31.63, Na₂O 0.01–0.11 and K₂O 0.43–1.07 based on eight analysis spots in and around the grain, yielding the average composition (Pb_0.69Ca_0.20K_0.10Na_0.01Y_0.02)_Σ1.03(Zr_1.00Hf_0.01)_Σ1.01Si_3.00O₉⋅2H₂O. The software CrysAlis^Pro (Rigaku Oxford Diffraction, UK) and SHELX (Sheldrick, Reference Sheldrick2015a, Reference Sheldrick2015b) were used for diffraction data processing and structure refinement. The plumbogaidonnayite structure was solved in space group P2₁nb and all sites were first refined with isotropic vibrations. The occupancies of Si, Zr and O were fixed at 1 and those for Pb, Ca, and K were freely refined. The result of (Pb_0.62□_0.38)_Σ1.00(Ca_0.18K_0.14Pb_0.04□_0.64)_Σ1.00ZrSi₃O₁₁ is consistent with the EPMA data. During the refinement, the splitting of Pb into Pb1, Pb2 and Pb3 subsites with low occupancy from the same site was necessary because an unsplit model, similar to those of gaidonnayite and georgechaoite, would lead to unreasonable results with R ₁ = 9.04%, shift = 1.044 and a residual maximum = 7.2 e^–Å^–3 around the Pb site (0.990 and 0.898 Å for Pb1–Pb2 and Pb1–Pb3 distances, respectively). In addition, the occupancy of the Pb (Pb1, Pb2 and Pb3) site by Ca and K were also tested but it required too many Ca (0.62 apfu) and K (0.31 apfu) atoms due to the electron density, which disagreed with the EPMA data. Similarly, the Ca (Ca1, K1 and Pb4) site may also be split due to its unusual displacement parameter (U _eq = 0.311 Ų) in an unsplit model, thus, combined with residual electron densities and peaks around the Ca site, we also split it into Ca1, K1 and Pb4 subsites with much lower U _eq (0.073, 0.13 and 0.10 Ų, respectively) by isotropic refinement. Anisotropic refinement for these subsites was also tried, but the atoms were nearly overlapped again as in the unsplit model and it led to a non-positive-definite result. The crystal structure refinement finally converged to R ₁ = 5.59% for 1788 unique reflections (I>2σ(I)) and 182 parameters. Unit cell parameters refined are: a = 11.7690(4) Å, b = 12.9867(3) Å, c = 6.66165(16) Å, V = 1018.17(5) ų and Z = 4 in P2₁nb. Details for reflections collection and refinement are available in Table 3, and corresponding atom coordinates, site occupancies, equivalent isotropic and anisotropic atomic displacement parameters are provided in Table 4 and Table 5. Selected bond distances and angles are given in Table 6, and bond-valence sums for each atom are presented in Table 7. The structure of plumbogaidonnayite is shown in Fig. 4. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 3. Data collection and structure refinement details for plumbogaidonnayite.

* wR ₂ = {∑[w(F _o²-F _c²)²]/∑[w(F _o²)²]}^1/2; w = 1/[σ²(F _o)² + (aP)² + bP] where a is 0.0454, b is 17.9195 and P is [2F _c² + Max(F _o², 0)]/3.

^# Flack parameter is calculated from Flack (Reference Flack1983).

Table 4. Wyckoff positions, atom coordinates, inferred site occupancies, and equivalent isotropic displacement parameters in the plumbogaidonnayite structure.

Table 5. Anisotropic displacement parameters (in Ų) for plumbogaidonnayite.

Table 6. Selected bond distances (Å) and angles (°) for plumbogaidonnayite.

Table 7. Calculated bond valence sums (in vu) of atoms for plumbogaidonnayite.*

* Bond-valence sums were calculated with the site occupancy given in Table 4, using the parameters of Brese and O'Keeffe (Reference Brese and O'Keeffe1991).

^a (Pb1, Pb2, Pb3)

^b (Ca1, K1, Pb4)

Figure 4. Crystal structure of plumbogaidonnayite (unit cell outlined in black lines) plotted with VESTA 3 (Momma and Izumi, Reference Momma and Izumi2011). (a) The repeating sinusoidal six [SiO₄] tetrahedra single-silicate chain. (b) ZrO₆ octahedra and SiO₄ tetrahedra forming a 7-member ring and 3-member ring from the view along the c axis. (c) Disordered Pb (Pb1, Pb2 and Pb3), Ca (Ca1, K1 and Pb4) and two H₂O groups (O10 and O11) distributed over the space between the ZrO₆–SiO₄ framework (modified after Wu et al., Reference Wu, Gu, Wang, Liu, Xing and Ren2023c).

Plumbogaidonnayite is a new Pb-member of the gaidonnayite-group minerals containing a similar zirconosilicate framework to gaidonnayite and georgechaoite (Chao, Reference Chao1985; Ghose and Thakur, Reference Ghose and Thakur1985). It is composed of repeating sinusoidal six [SiO₄] tetrahedra single-silicate chains extending along [101] and [10$\bar{1}$] (Fig. 4a), and then chains are corner-linked with [ZrO₆] octahedra into a three-dimensional framework. However, splitting and disordering occur at the extra-framework sites (including cations and H₂O groups) in the plumbogaidonnayite structure, in the space between the silicate chains and [ZrO₆] octahedra, which are commonly fully ordered and occupied by Na and K in gaidonnayite and georgechaoite. Of note, the strong disorder in the extra-framework sites would still lead to some physically unreasonable parameters related to these positions, which are assigned based on electron densities and statistical coordinates. For instance, the large U _eq values (0.126(13) and 0.146(15) Ų) for H₂O groups may suggest partial occupancy at the O10 and O11 sites. In addition, some short distances between extra-framework sites, such as Pb2–O10 (2.09(3) Å), K1–O11 (1.47(14) Å), Pb1–Ca1 (2.24(5) Å), Pb2–Ca1 (2.33(5) Å), Pb3–Pb4 (2.29(9) Å) and Pb3–K1 (3.18(15) Å), could indicate the mutually exclusive occupancy of these two positions or potentially partial H₂O groups involved at these disordered cation sites.

Implications

Plumbogaidonnayite is the first naturally discovered divalent cation-dominant member of the gaidonnayite-group minerals, which occurs associated closely with hydrothermal gaidonnayite and georgechaoite after eudialyte alteration. In actuality, the latter two are common alteration products after eudialyte in peralkaline complexes and eudialyte dissolution experiments (Ivanyuk et al., Reference Ivanyuk, Pakhomovsky and Yakovenchuk2015; Borst et al., Reference Borst, Friis, Andersen, Nielsen, Waight and Smit2016; Mikhailova et al., Reference Mikhailova, Pakhomovsky, Kalashnikova and Aksenov2022), however the absence of plumbogaidonnayite in most cases is probably attributed to a lack of Pb-rich metasomatic fluids. In contrast, late Sr–Pb-rich fluid activity was pervasive in the Saima alkaline complex (Wu et al., Reference Wu, Wang, Yang, Wu, Zhang, Gu and Zhang2015, Reference Wu, Wang, Liu, Guo and Song2018), which resulted in the replacement of primary zirconosilicates (e.g. wadeite and eudialyte) by a variety of hydrothermal minerals including plumbogaidonnayite, calcite and strontianite, as well as the formation of the newly approved fluorsigaiite and gysinite-(La) (Wu et al., Reference Wu, Gu, Rao, Wang, Xing, Zhong, Wan and Bonnetti2022, Reference Wu, Gu, Rao, Wang, Xing, Wan, Zhong and Bonnetti2023a).

Ion exchange occurs widely in gaidonnayite-group minerals and similar zirconosilicates (e.g. catapleiite- and hilairite-group minerals) under natural and experimental conditions (Pushcharovskii et al., Reference Pushcharovskii, Pekov, Pasero, Gobechiya, Merlino and Zubkova2002; Aksenov et al., Reference Aksenov, Portnov, Chukanov, Rastsvetaeva, Nelyubina, Kononkova and Akimenko2016; Celestian et al., Reference Celestian, Lively and Xu2019). Recent experimental work has demonstrated that Cs⁺ could exchange at the extra-framework cation (Na) site into the gaidonnayite structure from room temperature to 95°C (Celestian et al., Reference Celestian, Lively and Xu2019), which implies that plumbogaidonnayite could crystallise from eudialyte alteration or its alteration product (e.g. gaidonnayite and georgechaoite) in a naturally low-temperature fluid environment. In comparison with isovalent substitution, heterovalent ion exchange in isomorphism would not only influence the main Raman vibrational features and unit-cell parameters, but also tends to cause more vacancies at the extra-framework cation sites and decrease the symmetry, as demonstrated by Na⁺ → Ca²⁺ exchanges in calciocatapleiite and calciohilairite, Na⁺ → Zn²⁺ exchanges in vigrishinite and zvyaginite, and Na⁺ → Pb²⁺ exchange in plumbogaidonnayite (Pushcharovskii et al., Reference Pushcharovskii, Pekov, Pasero, Gobechiya, Merlino and Zubkova2002; Pekov et al., Reference Pekov, Britvin, Zubkova, Chukanov, Bryzgalov, Lykova, Belakovskiy and Pushcharovsky2013, Reference Pekov, Lykova, Chukanov, Yapaskurt, Belakovskiy, Zolotarev and Zubkova2014; Aksenov et al., Reference Aksenov, Portnov, Chukanov, Rastsvetaeva, Nelyubina, Kononkova and Akimenko2016). However, although Ca occupies the Na1 site over other cations (except vacancies) during our plumbogaidonnayite structure refinement, the Ca member of gaidonnayite has never been discovered in natural samples, probably due to compositional similarity to calciocatapleiite (Mandarino and Sturman, Reference Mandarino and Sturman1978; Ilyushin et al., Reference Ilyushin, Voronkov, Ilyukhin, Nevskii and Belov1981). Nevertheless, the discovery of plumbogaidonnayite draws attention to the heterovalent substitution and structural disordering in gaidonnayite-group minerals.