Xianhuaite-(Ce), K₂CeNb₅O₁₅, a new niobium mineral from the Bayan Obo deposit, China

Xianhuaite-(Ce), K₂CeNb₅O₁₅, is a new mineral species discovered in the Bayan Obo deposit, Inner Mongolia, China. The mineral is named in honor of Professor Xianhua Li in recognition of his outstanding contributions to research on the Bayan Obo deposit. It occurs as brownish-red to pale yellow-brown tetragonal prisms or irregular granules and exhibits a Mohs hardness of 5–6, an adamantine luster, and a calculated density of 5.23 g cm⁻³. Electron microprobe analysis yields the empirical formula ^A1(K_1.30Ba_0.69Sr_0.02)_Σ2.01^A2(Ce_0.39La_0.31Nd_0.04Pr_0.02Ca_0.15Na_0.09)_Σ1.00^B(Nb_4.75Fe${}_{\mathrm{0.13}}^{\mathrm{3}+}$Ti_0.09Mg_0.04)_Σ5.01O₁₅. Xianhuaite-(Ce) crystallizes in the tetragonal system, space group P4$/$mbm, with unit-cell parameters a=12.5355(4) Å, c=3.9213(2) Å, V=616.19(5) ų, and Z=2. Its crystal structure adopts a tetragonal tungsten bronze (TTB)-type framework, characterized by corner-sharing NbO₆ octahedra forming channels hosting K, Ba, and rare earth element (REE) cations. Xianhuaite-(Ce) is associated with minerals typical of skarnization assemblages (e.g. dolomite, forsterite, phlogopite, and chlorite), as well as other Nb phases, including fersmite, columbite-(Fe), fergusonite-(Ce), and aeschynite. As the first naturally occurring mineral with a TTB-type structure, xianhuaite-(Ce) expands the known structural diversity of Nb minerals and provides new mineralogical constraints on Nb mineralization at Bayan Obo.

1 Introduction

The Bayan Obo deposit in northern China is a world-class rare earth element (REE)–niobium (Nb) deposit (Huang et al., 2024; Li et al., 2025). In addition to hosting the largest known global REE reserves, it contains significant Nb resources (She et al., 2021; Ren et al., 2023; Li et al., 2024; Chen et al., 2025). Owing to the strategic importance of REE and Nb in advanced technological and defense applications (Rooks et al., 2025), the deposit has been the focus of sustained geological and mineralogical research (Institute of Geochemistry, Chinese Academy of Sciences, 1988; Yang et al., 2024a; Yu et al., 2024).

Despite decades of investigation, fundamental mineralogical issues concerning Nb enrichment at Bayan Obo remain unresolved (Institute of Geochemistry, Chinese Academy of Sciences, 1988; Yang et al., 2024a). Nb is distributed among numerous discrete mineral species rather than being concentrated in a single dominant host. To date, at least 30 Nb or Nb-bearing minerals have been identified, reflecting exceptional mineralogical diversity and extensive elemental isomorphism (Yang et al., 2024b). Such complexity obscures the crystallochemical mechanisms governing Nb incorporation, stabilization, and redistribution, underscoring the necessity of mineral-scale investigations (Yang et al., 2024a; Yu et al., 2024). In particular, Nb or Nb-bearing minerals with previously unrecognized compositional or structural characteristics are critical for constraining the crystallographic controls on Nb mineralization.

Within this context, xianhuaite-(Ce), a compositionally and structurally distinct Nb mineral, provides new insight into Nb mineralization processes. It is the first naturally occurring mineral exhibiting a tetragonal tungsten bronze (TTB)-type structure and has the ideal chemical formula K₂CeNb₅O₁₅. The mineral and its name have been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA-CNMNC) under proposal number IMA 2024-091.

Xianhuaite-(Ce) is named in honor of Professor Xianhua Li, academician of the Chinese Academy of Sciences, in recognition of his significant contributions to studies of early Earth and planetary evolution and especially to recent advances in the geochronology, geochemistry, and resource development of the Bayan Obo deposit. The approved mineral symbol is Xhu-Ce. The holotype specimen (under catalog no. GMCTM2024012) is deposited in the Geological Museum of China (Xisi Yangrou hutong no. 15, Xicheng District, Beijing), P.R. China. The co-type specimen is deposited at the Crystal Structure Laboratory, China University of Geosciences, Beijing 100083, People's Republic of China, under catalog no. BYEB-3.

2 Occurrence and associated minerals

Xianhuaite-(Ce) was identified in two specimens collected from the East Orebody and the East Contact Zone of the Bayan Obo deposit. Overviews of the geology and mineralogy of the Bayan Obo deposit can be found in the Institute of Geochemistry, Chinese Academy of Sciences (1988), and Yang et al. (2024b), among other publications.

Sample BY-E2024-103 was collected from the East Orebody (coordinates: 41°48^′01^′′ N, 109°59^′39^′′ E). The specimen occurs as black-brown megacrystalline aggregates at the contact between aegirine-type Nb–REE–Fe ore and dolomite-type Nb–REE–Fe ore. Analytical work was conducted on fragments mechanically separated from the sample (Fig. 1a–d). The principal minerals in sample BY-E2024-103 are xianhuaite-(Ce), fersmite, and fergusonite-(Ce) (Fig. 1a–d).

Figure 1 Transmitted-light photomicrographs, backscattered electron (BSE) images, and TIMA false-color mineral maps of samples. (a) Euhedral xianhuaite-(Ce) crystals observed under transmitted light in sample BY-E2024-103. (b) Well-developed cleavage sets in xianhuaite-(Ce) from sample BY-E2024-103 (transmitted light). (c–d) Euhedral grains of xianhuaite-(Ce) and associated minerals in sample BY-E2024-103 (BSE). (e–f) Relict pyrochlore enclosed within fersmite and xianhuaite-(Ce) in sample BY-EC2024-14 (BSE). (g–h) TIMA false-color mineral maps illustrating the spatial distribution of xianhuaite-(Ce) and its paragenetic associations with other minerals in sample BY-EC2024-14.

Sample BY-EC2024-14 was collected from the East Contact Zone (coordinates: 41° 48^′47^′′ N, 110° 01^′27^′′ E) and belongs to dolomite-type Nb–REE–Fe ore. After cutting and polishing, the specimen was analyzed using a Tescan integrated mineral analyzer (TIMA) (Fig. 1e–h). Quantitative mineralogical data indicate that the sample is composed of dolomite (71.89 vol %), magnetite (12.19 vol %), fersmite (8.00 vol %), calcite (2.69 vol %), columbite-(Fe) (1.72 vol %), xianhuaite-(Ce) (0.40 vol %), aeschynite (0.37 vol %), fergusonite-(Ce) (0.28 vol %), kinoshitalite (0.21 vol %), pyrochlore (0.19 vol %), phlogopite (0.18 vol %), barite (0.15 vol %), apatite (0.10 vol %), chlorite (0.09 vol %), biotite (0.05 vol %), and forsterite (0.02 vol %), with other minerals accounting for 1.47 vol % (Fig. 1g–h). In this sample, xianhuaite-(Ce) occurs in close spatial association with magnetite, fersmite, columbite-(Fe), and fergusonite-(Ce), forming mineral aggregates (Fig. 1g–h). Anhedral pyrochlore grains are commonly preserved as inclusions within both fersmite and xianhuaite-(Ce) (Fig. 1e–f).

3 Physical and optical properties

Xianhuaite-(Ce) occurs as irregular grains or tetragonal prisms ranging in size from 0.03 × 0.03 × 0.1 to 0.4 × 0.4 × 0.6 mm. In hand specimens the mineral is dark brown to black brown, whereas thin fragments appear pale yellow brown to brownish red in transmitted light (Fig. 1a–b), with a white streak and adamantine luster. No fluorescence is observed under longwave (365 nm) ultraviolet radiation. The mineral has a Mohs hardness of 5–6 (measured using a Mohs hardness test pen) and a micro-indentation hardness of 469.5 (mean) with a range of 448.7–490.2 kg mm⁻² (VHN, 200 g load). It displays perfect cleavage on {001} (Fig. 1b), with no parting observed, and is brittle in tenacity, showing a conchoidal fracture. The calculated density, based on the empirical formula and unit-cell volume refined from single-crystal XRD data, is 5.23 g cm⁻³. Magnetic properties are none.

This translucent mineral has a calculated refractive index (N_calc) of 2.22 (determined by N= Kd + 1, with K values from Mandarino, 1981). Reflectivity ranges from R_min–R_max: 17.6–18.5 (470 nm), 17.4–18.5 (546 nm), 17.3–18.3 (589 nm), and 17.2–18.4 (650 nm). The reflectance values for xianhuaite-(Ce) were measured in air using a CRAIC 20/30PV Pro microspectrophotometer at Westlake University (Hangzhou, China), with aluminum metal (R= 90 %) as the reference material. Data were collected from six spots across six grains (×50 objective, 5×5 µm aperture), and reflectance results are shown in Fig. 2. In reflected light, the mineral appears grey.

Figure 2 Reflectance data (R, %) for xianhuaite-(Ce) in air, plotted versus wavelength in nm.

4 Chemical composition

Electron probe microanalysis (EPMA) with wavelength-dispersive spectrometry (WDS) was performed using a JXA-iHP200F electron probe microanalyzer at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. Analytical conditions were 15 kV accelerating voltage, 20 nA beam current, and 10 spot analyses on a single grain. Backscattered electron imaging (Fig. 1e) confirms the chemical homogeneity at the micrometer scale. Elemental contents and their variation ranges are detailed in Table 1.

Table 1 Chemical data (in wt %) of xianhuaite-(Ce).

Calculated based on oxygen (O) = 15 atoms per formula unit (apfu), the empirical formula of xianhuaite-(Ce) is ^A1(K_1.30Ba_0.69Sr_0.02)_Σ2.01^A2(Ce_0.39La_0.31Nd_0.04Pr_0.02Ca_0.15 Na_0.09)_Σ1.00^B(Nb_4.75Fe${}_{\mathrm{0.13}}^{\mathrm{3}+}$Ti_0.09Mg_0.04)_Σ5.01O₁₅. The simplified formula is (K,Ba)₂(Ce,La)(Nb,Fe)₅O₁₅, and the further refined ideal formula is K₂CeNb₅O₁₅. For this ideal formula, the corresponding oxide composition and weight percentages are Nb₂O₅ 72.01, Ce₂O₃ 17.78, and K₂O 10.21, with a total of 100 wt %.

5 Raman spectroscopy

Raman measurements were performed using a WITec 300R spectrometer in reflection mode at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), equipped with a 300 line mm⁻¹ grating and laser excitation at 532 nm. Spectral data were acquired over the 50–4000 cm⁻¹ range with a 45 s collection time, 20 scans, and 0.5 cm⁻¹ resolution.

The Raman spectrum of xianhuaite-(Ce) (Fig. 3) exhibits characteristic vibrational modes diagnostic of the TTB-type structure. A prominent band centered at ∼853 cm⁻¹ is assigned to NbO₆ octahedral vibrations. Additional peaks at 645 and 537 cm⁻¹ correspond to Nb–O stretching vibrations within the NbO₆ octahedra. In the low-frequency region, a peak at 175 cm⁻¹ is attributed to internal NbO₆ modes, while external modes at 135 and 81 cm⁻¹ are typical of cation motions in the tetragonal phase of TTB-type structures (Amira et al., 2010). No peaks are observed above 1000 cm⁻¹.

Figure 3 Raman spectrum of xianhuaite-(Ce).

6 Crystallography

6.1 X-ray powder diffraction

X-ray powder diffraction measurements were performed using a Rigaku XtaLAB PRO-007HF diffractometer operated in crystal rotation mode at the China University of Geosciences. The instrument was configured with a rotating anode microfocus X-ray source (MoKα radiation, λ= 0.71073 Å), generating 50 kV and 24 mA, coupled to a hybrid pixel array detector for signal acquisition. Collected powder patterns were processed to refine unit-cell parameters via Chekcell software. Simulated diffraction patterns, calculated based on the refined structure, were generated using VESTA (Momma and Izumi, 2011) to facilitate phase identification and comparison.

X-ray powder diffraction data of xianhuaite-(Ce) are presented in Table 2. There is a good match between the observed and calculated powder diffraction patterns. The unit-cell parameters refined from the powder-diffraction data are a= 12.5458(8) Å, c= 3.9279(1) Å, and V= 618.24(9) ų.

Table 2 X-ray powder diffraction data (d in Å, intensity is I/I₀) for xianhuaite-(Ce).

Lines with $I/I_{\mathrm{0}}\ge$ 3 are listed, and the seven strongest observed lines are in bold.

6.2 Single-crystal X-ray diffraction

Single-crystal X-ray diffraction data were collected using the same Rigaku XtaLAB PRO-007HF diffractometer to maintain methodological consistency. A euhedral crystal fragment (0.02 × 0.02 × 0.01 mm) was selected for analysis, with data collection covering a hemispherical reciprocal space region defined by −15 $\le h\le$ 17, −16 $\le k\le$ 16, and −5 $\le l\le$ 5. Raw intensity data underwent Lorentz-polarization correction and multi-scan absorption correction to minimize systematic errors. Structure determination was initiated with SHELXT (Sheldrick, 2015), followed by full-matrix least-squares refinement of atomic coordinates and displacement parameters using OLEX2-1.3 (Dolomanov et al., 2010).

The unit-cell parameters obtained from single-crystal XRD data are as follows: a= 12.5355 (4) Å, c= 3.9213 (2) Å, and V= 616.19 (5) ų. The crystal structure is described within the space group $P\mathrm{4}/$mbm (#127). The structure was refined to R₁= 0.0189 based on 427 independent reflections with I>2σ_I. A summary of the details for data collection and refinement procedures can be found in Table 3. Atom coordinates, site occupancies, and bond lengths are given in Tables 4–6. The bond valence sum (BVS) calculation is shown in Table 7. The crystal structure data of the xianhuaite-(Ce) is available in the Online Materials CIF file.

Table 3 Information on crystal and structural refinement for xianhuaite-(Ce).

Table 4 Site, Wyckoff position (Wyck.), site occupancy, fractional atomic coordinates, and equivalent isotropic displacement parameters (Ų) of atoms for xianhuaite-(Ce).

Table 5 Anisotropic displacement parameters (in Ų) for xianhuaite-(Ce).

Table 6 Selected geometric parameters (Å) for xianhuaite-(Ce). The bold font indicates the mean bond lengths.

Symmetry codes: (i) $-y+$1, x, z; (ii) $-x+$1/2, y–$\mathrm{1}/\mathrm{2}$, −z; (iii) $-x+\mathrm{1}/\mathrm{2}$, $y-\mathrm{1}/\mathrm{2}$, $-z+\mathrm{1}$; (iv) $-y+\mathrm{1}$, x, z−1; (v) x, y, z−1; (vi) $-y+\mathrm{1}/\mathrm{2}$, $-x+\mathrm{1}/\mathrm{2}$, z−1; (vii) $-x+\mathrm{1}$, $-y+\mathrm{1}$, $-z+\mathrm{2}$; (viii) y, $-x+\mathrm{1}$, $-z+\mathrm{2}$; (ix) $-x+\mathrm{1}$, $-y+\mathrm{1}$, $-z+\mathrm{1}$; (x) $-y+\mathrm{1}$, x, z+1; (xi) y, $-x+\mathrm{1}$, $-z+\mathrm{1}$; (xii) x, y, z+1; (xiii) −x, $-y+\mathrm{1}$, $-z+\mathrm{1}$; (xiv) $y-\mathrm{1}/\mathrm{2}$, $x+\mathrm{1}/\mathrm{2}$, $-z+\mathrm{1}$; (xv) $-y+\mathrm{1}/\mathrm{2}$, $-x+\mathrm{1}/\mathrm{2}$, z; (xvi) −y, x, z; (xvii) y, $-x+\mathrm{1}$, −z; and (xviii) x, y, $-z+\mathrm{2}$.

Table 7 Bond valence sums for xianhuaite-(Ce).

Note: the BVS was calculated from the bond valences using the ECoN21 program (Ilinca, 2022). Site occupancy – A1 site, K${}_{\mathrm{0.642}}^{+}$Ba${}_{\mathrm{0.358}}^{\mathrm{2}+}$; A2 site, Ce_0.812Ca_0.188; B1 site, Nb_0.947Fe_0.053; B2 site, Nb_1.00. The bond valence parameters of K⁺–O²⁻, Ba²⁺–O²⁻, Ca²⁺–O²⁻, Ce³⁺–O²⁻, and Fe³⁺–O²⁻ are from Gagné and Hawthorne (2015), and those of Nb⁵⁺–O²⁻ are from Tytko et al. (1999).

Figure 4 The tetragonal tungsten bronze (TTB)-type structure of xianhuaite-(Ce). (a) Two-dimensional network of corner-sharing NbO₆ octahedra with four- and five-membered rings in the ab plane. (b) K, Ba, and REE cations distributing in channels along the c axis.

Xianhuaite-(Ce) exhibits a tetragonal tungsten bronze (TTB)-type structure (Fig. 4), with NbO₆ octahedra constituting the fundamental framework. These octahedra share corners, creating a tunnel structure along the c axis. In the ab plane, the octahedra interconnect to form a two-dimensional network, generating four-membered and five-membered NbO₆ octahedra rings that provide channels for accommodating cations (Fig. 4a). The channels formed by the five-membered rings are slightly larger and, in xianhuaite-(Ce), host K and Ba cations. The channels formed by the four-membered rings accommodate REE and Ca cations. These large cations are distributed between the network layers in the ab plane, stabilizing the structure (Fig. 4b).

1. A1 sites: The A1 sites in the xianhuaite-(Ce) structure, which are identified as the largest cation positions within the channels composed of five-membered rings, are occupied by K and Ba ions, with a coordination number of 9. The bond lengths range from 2.80 to 3.19 Å. Structural refinement results indicate an occupancy of K_0.642(3)Ba_0.358(3) at the A1 sites.

2. A2 sites: The A2 sites, identified as the cation positions within the channels composed of four-membered rings, are primarily occupied by REE and Ca, with a coordination number of 12 and bond lengths ranging from 2.64 to 2.70 Å. Structural refinement results indicate an occupancy of ^REECe_0.812(4)Ca_0.188(4) at this position.

3. B sites: The B sites are octahedrally coordinated positions, further divided into two crystallographically independent sites, B1 and B2, both predominantly occupied by Nb. The atom at the B1 site exhibits a slightly higher atomic displacement parameter than that at the B2 site, suggesting potential minor isomorphic substitution by lighter elements. During structural refinement, a small amount of Fe substitution was modeled at the B1 site, resulting in an occupancy of Nb_0.947(15)Fe_0.053(15), which is also supported by the chemical composition analysis. The bond lengths at the B1 site closely approximate those of an ideal octahedron, with bond lengths ranging from 1.96 to 1.97 Å and an average bond length of 1.964 Å. The B2 octahedron shows slight distortion, with bond lengths ranging from 1.89 to 2.03 Å and an average bond length of 1.975 Å.

7 Discussion and conclusions

7.1 Genesis of xianhuaite-(Ce)

The Bayan Obo deposit has undergone a complex, multistage geological evolution, resulting in exceptional diversity of Nb and Nb-bearing mineral assemblages (Yang et al., 2023; Yang et al., 2024b). Despite decades of investigation, the mechanisms of Nb mineralization remain highly debated (Yang et al., 2024c; Yu et al., 2024), largely because Nb is distributed among numerous discrete mineral phases rather than being concentrated in a single dominant host. Consequently, elucidating the formation conditions of individual Nb and Nb-bearing minerals is essential for constraining the processes responsible for Nb enrichment. In this context, the occurrence of the new mineral xianhuaite-(Ce) in both the East Orebody (BY-E2024-103) and the East Contact Zone (BY-EC2024-14) provides additional constraints on Nb mineralization at the Bayan Obo deposit.

In sample BY-EC2024-14, Nb minerals such as xianhuaite-(Ce), fersmite, and columbite-(Fe) predominantly occur as interstitial phases between magnetite grains (Fig. 1g–h). In addition, both xianhuaite-(Ce) and fersmite contain relict pyrochlore (Fig. 1e–f), indicating inheritance from an earlier Nb mineralization stage. These textural relationships are inconsistent with primary magmatic crystallization and instead suggest precipitation from Nb-bearing fluids during a later hydrothermal overprint. This interpretation is further supported by petrographic evidence for skarn-type alteration in sample BY-EC2024-14, where the assemblage of forsterite, phlogopite, chlorite, and dolomite is characteristic of magnesian skarn systems typically formed by metasomatism associated with granite intrusion into dolomitic host rocks (Mazurov et al., 2018).

The East Contact Zone is known to have experienced variable degrees of skarnization during emplacement of Permian granites (Yang et al., 2024c). Although this thermal event did not introduce new Nb into the system, it likely facilitated remobilization of pre-existing Nb (Yang et al., 2024c). Taken together, the mineral assemblages, textural relationships, and regional geological context suggest that xianhuaite-(Ce) most plausibly formed under fluid-dominated conditions during the Permian granitic thermal event, thereby recording Nb remobilization and redistribution rather than primary Nb accumulation.

Sample BY-E2024-103 is coarse-grained and lacks diagnostic mineral assemblages that would allow direct petrographic constraints on its formation. However, regional geochronological data indicate that the Permian thermal event affected the entire Bayan Obo district (Institute of Geochemistry, Chinese Academy of Sciences, 1988; Yang et al., 2024c; Yu et al., 2024; Yao et al., 2025). On this basis, xianhuaite-(Ce) in BY-E2024-103 is tentatively interpreted as having formed during the same Permian Nb remobilization event, although direct geochronological constraints are presently lacking.

7.2 Structural significance and comparison with synthetic analogues

Xianhuaite-(Ce), ideally K₂CeNb₅O₁₅, has a synthetic analogue with the same ideal composition and belongs to the tetragonal tungsten bronze (TTB)-type structural family. Zeng et al. (2024) reported that this TTB-type series may undergo temperature-dependent transitions from P4bm to Ima2 and then to P4$/$mbm upon heating, with P4$/$mbm corresponding to the centrosymmetric high-temperature form and P4bm to a polar lower-symmetry modification of the same framework. Xianhuaite-(Ce), with the ideal formula K₂CeNb₅O₁₅, is most directly comparable to synthetic K₂CeNb₅O₁₅, which is described in P4$/$mbm at room temperature. Refinement of the present natural sample in P4bm did not improve the model, giving R1 = 0.0197 compared with R1 = 0.0189 for P4$/$mbm. The intensity statistics also favor centrosymmetry, with a mean |E${}^{\mathrm{2}}-\mathrm{1}|$ value of 0.925, closer to the expected value for centrosymmetric structures (0.968) than for non-centrosymmetric structures (0.736). Therefore, P4$/$mbm is adopted as the room-temperature average structure of xianhuaite-(Ce).

Xianhuaite-(Ce) represents a new Nb mineral with a TTB-type structure, a structural topology not previously documented among natural Nb minerals. Its occurrence at Bayan Obo records a late-stage Nb reorganization process rather than primary Nb accumulation. The recognition of xianhuaite-(Ce) broadens the structural diversity of natural Nb minerals and provides additional constraints on Nb behavior during the multistage evolution of the Bayan Obo deposit.