Plumbogaidonnayite, PbZrSi₃O₉⋅2H₂O, a new Pb-member of the gaidonnayite group from the Saima alkaline complex, Liaoning Province, China

Abstract

Plumbogaidonnayite, ideally PbZrSi₃O₉⋅2H₂O, is a new gaidonnayite-group mineral discovered as a secondary product derived from the alteration of eudialyte from the Saima alkaline complex, China. It occurs as aggregates (up to 1 mm) composed of subhedral to anhedral or platy crystals (individually 5–50 μm), associated closely with microcline, natrolite, aegirine, gaidonnayite, georgechaoite, zircon, bobtraillite and britholite-(Ce) in eudialyte pseudomorphs. The crystals are transparent, colourless or light brown with a vitreous lustre. Plumbogaidonnayite is brittle with conchoidal fracture, and it has a Mohs hardness of ~5 and a calculated density of 3.264 g/cm³. It is optically biaxial (+) with α = 1.61(3), β = 1.63(3) and γ = 1.66(4). The calculated 2V is 80°, with the optical orientations X, Y and Z parallel to the crystallographic a, b and c axes, respectively. The empirical formula is (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 calculated on the basis of nine oxygen atoms per formula unit and assuming the occurrence of two H₂O groups. Plumbogaidonnayite is orthorhombic, P2₁nb, a = 11.7690(4) Å, b = 12.9867(3) Å, c = 6.66165(16) Å, V = 1018.17(5) ų and Z = 4. The nine strongest lines of its powder XRD pattern [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). Plumbogaidonnayite has a similar spiral chain framework structure with gaidonnayite and georgechaoite, which is composed of SiO₄ tetrahedra and ZrO₆ octahedra, but with disordered extra-framework sites (cations and H₂O groups) characterised by the substitution of 2Na⁺ (K⁺)→Pb²⁺ (Ca²⁺) + □ (vacancy). The discovery of plumbogaidonnayite adds a new perspective on the cation ordering and heterovalent substitution mechanism in gaidonnayite-group minerals.

Introduction

Microporous materials with zeolitic structure, specifically titano- and zirconosilicates with complex octahedral–tetrahedral frameworks, have been studied extensively owing to their industrial properties (e.g. ion-exchange, sorption and catalysis) in modern technologies (Mumpton, 1999; Kuznicki et al., 2001; Celestian et al., 2019). Gaidonnayite-group minerals are hydrous zirconosilicates with a microporous heteropolyhedral framework, though only two natural types of alkali metal-dominant members, namely gaidonnayite (Na₂ZrSi₃O₉⋅2H₂O) and georgechaoite (KNaZrSi₃O₉⋅2H₂O), have been reported (Chao and Watkinson, 1974; Boggs and Ghose, 1985).

The new mineral plumbogaidonnayite PbZrSr₃O₉⋅2H₂O, the first divalent cation-dominant Pb member of the gaidonnayite group, was discovered in lujavrite from the Saima alkaline complex, Liaoning Province, China. This complex is also the type locality for fengchengite (IMA2007-018a), hezuolinite (IMA2010-045), and recently approved fluorsigaiite (IMA2021-87a) and gysinite-(La) (IMA2022-008, Yang et al., 2012; Shen et al., 2017; Wu et al., 2022, 2023a). The prefix ‘plumbo’ was added to indicate its Pb-dominant compositional signature on the suggestion of the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC, Hatert and Burke, 2008). This new species with the official name plumbogaidonnayite and symbol ‘Pgdn’ has been approved by the IMA-CNMNC (IMA2022-095, Wu et al., 2023b). The type material (catalogue number M16139) has been deposited at the Geological Museum of China (No. 15 Yangrouhutong, Xisi, Beijing 100031, China). This paper presents the mineral paragenesis, chemical composition and crystal structure of plumbogaidonnayite, and compares its characteristics with other members of the gaidonnayite group.

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., 2016; Ma and Liu, 2023). 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., 2016). 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., 2015, 2018). The geological, mineralogical and geochronological features of the Saima complex have been reported extensively in recent work (e.g. Wu et al., 2010, 2016; Zhu et al., 2016, 2017).

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, 2005; Pekov et al., 2013, 2014). 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., 2016).

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 (2021): 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, 1981).

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., 2019, 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., 2000; Kovalskaya et al., 2023). 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, 1985). 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, 1998). 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., 2022; Kovalskaya et al., 2023).

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, 1997).

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, 2011).

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, 2015a, 2015b) 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 (1983).

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 (1991).

^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, 2011). (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., 2023c).

Plumbogaidonnayite is a new Pb-member of the gaidonnayite-group minerals containing a similar zirconosilicate framework to gaidonnayite and georgechaoite (Chao, 1985; Ghose and Thakur, 1985). 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.

Si–O tetrahedra

In plumbogaidonnayite, three crystallographically distinct Si1, Si2 and Si3 sites in SiO₄ tetrahedra are fully occupied by Si with average Si–O bond distances of 1.628 (Si1–O), 1.615 (Si2–O) and 1.614 (Si3–O) Å. These tetrahedra form a basic repeating sinusoidal six [SiO₄] tetrahedra single [Si₆O₁₈]^12– chain by corner-sharing (Fig. 4a). The Si–O–Si angles which involve bridging oxygen range from 133.0 to 135.2° with an average of 134.4°. The average bridging Si–O distance (1.629 Å) is longer than that of non-bridging Si–O bonds (1.608 Å), and the average O–Si–O angle involving the bridging bonds (106.5°) is smaller than that with the non-bridging bonds (113.1°). These so-called ‘²T₆ chains’ and similar trends of the Si–O bond and O–Si–O angle also exist in gaidonnayite, georgechaoite, stokesite and some synthetic materials (Day and Hawthorne, 2020 and references therein). The calculated bond-valence sum (BVS) for Si1 (4.04 valence units), Si2 (4.20 vu) and Si3 (4.21 vu) are close to the ideal values within error (Table 7).

Zr–O octahedra

The Zr–O bond distances in relatively regular octahedra range from 2.056 to 2.154 Å with an average length of 2.094 Å. A single ZrO₆ octahedron is corner-linked with three different [Si₆O₁₈]^12– chains by sharing two oxygen atoms within each chain. These ZrO₆ octahedra and SiO₄ tetrahedra form 7-member rings and 3-member rings from the view nearly along the c axis (Fig. 4b), as is also present in other gaidonnayite-group minerals. The ZrO₆ octahedron in plumbogaidonnayite tends to link with disordered Pb (Pb1, Pb2 and Pb3) atoms via face- and edge-sharing, and with Ca (Ca1, K1 and Pb4) atoms via corner-sharing, whereas in the gaidonnayite and georgechaoite structure it shares O–O edges with Na(K)–O octahedra (Chao, 1985; Ghose and Thakur, 1985).

Pb–O polyhedra

The Pb in plumbogaidonnayite tends to occupy the Na2 site of the two Na sites in gaidonnayite, and it splits into three disordered sites, Pb1, Pb2 and Pb3 with occupancies of 0.461(9), 0.099(7) and 0.057(9), respectively (Fig. 4c). Distances for Pb1–Pb2, Pb1–Pb3 and Pb2–Pb3 are 0.935(16), 0.88(5) and 1.23(5) Å, respectively. Pb1 is coordinated to three oxygen atoms (O1, O7 and O8) and two H₂O groups (O10 and O11), with three moderate Pb1–O bond lengths ranging from 2.396(14) to 2.661(14) Å and two Pb1–H₂O lengths of 2.30(4) and 2.67(3) Å, respectively. In contrast, Na–H₂O bonds are normally shorter than other Na–O bonds in Na–O octahedra in gaidonnayite and georgechaoite. The Pb1–O polyhedron shares a face (O1–O7–O8) with an adjacent ZrO₆ octahedron and a corner (O1) with an adjacent Si1O₄ tetrahedron. Pb2 is coordinated to four oxygen atoms (O1, O5, O7 and O9), and the Pb2–O polyhedron shares two edges (O5–O9 and O5–O7) with ZrO₆ octahedron and Si2O₄ tetrahedron, and two corners (O1 and O9) with Si1O₄ and Si3O₄ tetrahedra, respectively. The short Pb2–O10 distance of 2.09(3) Å may be a result of the statistically average positions of the extra-framework atoms, or indicates the mutually exclusive occupancy of these two positions. Pb3 is coordinated to three oxygen atoms (O6, O7 and O9) and one H₂O molecule (O11) with an average length of 2.84 Å. The Pb3–O polyhedron shares oxygen (O6, O7 and O9) corners with adjacent ZrO₆ octahedron and SiO₄ (Si2 and Si3) tetrahedra.

Ca–O polyhedra

The Ca in plumbogaidonnayite tends to occupy the Na1 site of the two Na sites in gaidonnayite, which splits into different Ca1, K1 and Pb4 subsites with occupancies of 0.18(4), 0.14(5) and 0.040(11), respectively (Fig. 4c). Distances for Ca1–K1, Ca1–Pb4 and K1–Pb4 bonds are 1.81(15), 0.87(7) and 0.95(16) Å, respectively. Ca1 is bound to one oxygen atom (O6) and one H₂O group (O10) with distances of 2.79(5) Å and 2.83(5) Å. Four K1–O (O1, O3, O4 and O8) bonds range from 2.69(11) to 3.13(10) Å. The K1–O polyhedron is corner-linked with Si1O₄, Si3O₄ tetrahedra and a ZrO₆ octahedron, and also shares an O3–O4 edge with a Si1O₄ tetrahedron and an O1–O8 edge with a ZrO₆ octahedron, respectively. The short K1–O11 distance (1.47(14) Å) may indicate that these two atoms cannot be occupied simultaneously, or the possibility of H₂O groups at these disordered extra-framework cations. The Pb4–O polyhedron is corner-linked (O8) with a Si3O₄ tetrahedron and a ZrO₆ octahedron. The Pb4–O8 bond distance (2.78(7) Å) is longer than Pb4–H₂O (O10 and O11) bonds (2.62(6) Å and 2.40(9) Å, respectively).

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., 2015; Borst et al., 2016; Mikhailova et al., 2022), 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., 2015, 2018), 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., 2022, 2023a).

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., 2002; Aksenov et al., 2016; Celestian et al., 2019). 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., 2019), 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., 2002; Pekov et al., 2013, 2014; Aksenov et al., 2016). 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, 1978; Ilyushin et al., 1981). Nevertheless, the discovery of plumbogaidonnayite draws attention to the heterovalent substitution and structural disordering in gaidonnayite-group minerals.