Calcioancylite-(La), (La,Ca)₂(CO₃)₂(OH,H₂O)₂, a new member of the ancylite group from Gejiu nepheline syenite, Yunnan Province, China

Abstract

Calcioancylite-(La), ideally (La,Ca)₂(CO₃)₂(OH,H₂O)₂, has been discovered from nepheline syenite of the Gejiu alkaline complex in the Honghe Hani and Yi Autonomous Prefecture, Yunnan Province, China. The mineral occurs as aggregates of subhedral grains, and the size of single crystals varies between 5–20 μm. Calcioancylite-(La) is colourless to pale pinkish grey and has transparent to translucent lustre. It is brittle with a Mohs hardness of 4. The calculated density is 4.324 g/cm³. The mineral is biaxial (−), with α =1.662, β = 1.730, γ = 1.771, 2V_meas. = 70°(1) and 2V_calc. = 73°. Electron microprobe analysis for holotype material yielded an empirical formula of (La_0.58Ce_0.55Pr_0.14Nd_0.10Ca_0.39Sr_0.20K_0.04)_Σ2.00(CO₃)₂[(OH)_1.25F_0.06⋅0.69H₂O]_Σ2.00. Calcioancylite-(La) is orthorhombic, with space group Pmcn, a = 5.0253(3) Å, b = 8.5152(6) Å, c = 7.2717(6) Å, V = 311.17(4) ų and Z = 2. By using single-crystal X-ray diffraction, the crystal structure has been determined and refined to a final R₁ = 0.0652 on the basis of 347 independent reflections (I > 2σ). The seven strongest powder X-ray diffraction lines [d in Å (I) (hkl)] are: 2.334 (100) (013), 2.970 (80) (121), 4.334 (75) (110), 3.678 (68) (111), 2.517 (55) (200), 2.647 (47) (031) and 2.077 (44) (221). Calcioancylite-(La) is the La-analogue of calcioancylite-(Ce) and is a new member of ancylite-group minerals. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2021-090).

Introduction

Ancylite-group minerals are hydrous carbonates that contain rare earth elements (REE), Ca, Sr and Pb as major cations. The general chemical formula for the ancylite-group minerals is: (REE)_x(M ²⁺)_2–x(CO₃)₂(OH)_x⋅(2–x)H₂O (Z = 2), where 1 < x ≤ 2 (Dal Negro et al., 1975; Sarp and Bertrand 1985; Bulakh et al., 1998; Miyawaki et al., 2000). The crystal structure of ancylite-(Ce) was first solved by Dal Negro et al. (1975). Larsen et al. (2002) performed the structure refinement for two species of ancylite-(Ce) within the space groups Pmcn and Pmc2 ₁. Belovitskaya et al. (2013) studied the crystal structure of calcioancylite-(Ce) by the Rietveld method and showed that most minerals of the ancylite group were regarded as orthorhombic, and that the whole structure of ancylite-group minerals can be derived from orthorhombic carbonates by adding hydroxyl groups that are positioned on the mirror planes and bonded to the heavy cations. However, the ancylite group of minerals has been controversial on the issue of crystallographic system. Some previous studies have shown that ancylite minerals are monoclinic, but metrically nearly orthorhombic (Szymanski and Chao 1986; Orlandi et al. 1990).

During a systematic investigation of rare minerals from the Gejiu nepheline syenite, some La-rich carbonate phases, corresponding to the formula (La,Ca)₂(CO₃)₂(OH,H₂O)₂ were identified. Further detailed chemical and crystallographic studies confirmed the first finding of the La-dominant member of the calcioancylite series, thus allowing the proposal of the new mineral species calcioancylite-(La). The new mineral and its name (symbol Canc-La) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2021-090, Wang et al., 2022). The type material is deposited at the Geological Museum of China, No. 16, Yangrou Hutong, Xisi, Beijing 100031, People's Republic of China, under catalogue number M16129. In this paper, the description of calcioancylite-(La) is reported including a detailed description of the mineral paragenesis, Raman spectroscopy, chemical composition, and crystal-structure refinements for this new member of the ancylite group.

Occurrence and physical properties

The new mineral calcioancylite-(La) was found from the Gejiu alkaline intrusive complex, Honghe Hani and Yi Autonomous Prefecture, Yunnan Province, China (23°29′40″N, 103°4′41″E). The intrusion forms an irregular ellipse body that is generally elongated in the S–N direction, covering ~28 km². The Gejiu alkaline complex yielded emplacement ages of 80–82 Ma and intruded into Triassic sandstone and carboniferous rocks (Zhang, 2013; Huang et al., 2018; Wang et al., 2021). The pinkish feldspathoid syenite is a massive, medium- to coarse-grained rock and contains the feldspathoid minerals nepheline and sodalite as well as dark alkaline minerals.

Calcioancylite-(La) is found in association with calcioancylite-(Ce), ancylite-(La), ancylite-(Ce), britholite-(Ce), fluorcalciobritholite, La-dominant fluorcalciobritholite, moxuanxueite, bobtraillite, catapleiite, baddeleyite, jadeite, zircon, magnetite, andradite, orthoclase and albite (Fig. 1). As it is always veined and/or occurs on the edges of other zirconium silicates, the crystallisation of calcioancylite-(La) may be related to the late carbonation. Calcioancylite-(La) forms aggregates of subhedral grains with pseudo-octahedral dipyramidal crystal habit {111} with {110}, elongated along the b axis. The veinlets and aggregates are between 10 to 200 μm in size, and the size of individual crystals varies from 5 to 20 μm. Calcioancylite-(La) is colourless to pale pinkish grey with a white streak and vitreous lustre. The Mohs hardness is 4 and the mean Micro-indentation hardness is VHN₅₀ = 175 kg/mm² (range from 165 to 185 kg/mm²). It is brittle with an irregular fracture. The density of 4.324 g/cm³ was calculated based on the empirical formula and unit cell volume refined from single-crystal X-ray diffraction (XRD) data. Optically, it is biaxial (−), with α =1.662, β = 1.730, γ = 1.771, 2V_meas. = 70°(1), 2V_calc. = 73° and dispersion r < v.

Fig. 1. SEM images of calcioancylite-(La) in aggregates of other minerals. Mineral symbols are from Warr (2021): Canc-La – calcioancylite-(La); Canc-Ce – calcioancylite-(Ce); Bri-Ce – britholite-(Ce); Adr – andradite; Mox –moxuanxueite; Bta – bobtraillite; Ctp – catapleiite; Jd – jadeite; Ab – albite; Flr – fluorite; Or – orthoclase; Mag – magnetite; and Zrn – zircon. Specimen # 18CL18-5.

Raman spectroscopy

The Raman spectrum of calcioancylite-(La) (Fig. 2) was recorded on a LabRAM HR Raman microscope with a 532nm laser (20 mW, 1 μm) in the spectral range from 100 to 4000 cm⁻¹ at the Raman Laboratory of the Tianjin Center, China Geological Survey. The Raman spectrum was collected in situ on the crystal used for the single-crystal XRD study from the polished thin section with a 50× objective.

Fig. 2. The Raman spectra of calcioancylite-(La).

Chemical data

The chemical composition was determined using a JXA-8100 electron microprobe microanalyser (EPMA) at the Beijing Research Institute of Uranium. Experimental conditions were wavelength dispersive spectroscopy mode, accelerating voltage = 15 kV, beam current = 10 nA and beam diameter = 1 μm. The standards include: phlogopite for K and Fe; plagioclase for Ca; celestine for Sr; albite for Al; monazite for La, Ce, Pr, Nd and Sm; and fluorapatite for F. H₂O was calculated by stoichiometry in order to achieve the charge balance, and the hydrous nature was confirmed by the presence of the OH stretching vibration absorption in the Raman spectrum. The chemical analysis results are reported in Table 1.

Table 1. Compositional data for calcioancylite-(La).

Note: S.D. = standard deviation, and bdl = below detection limits.

*Calculated by C = 2 apfu; ^†the number of OH group was calculated from the stoichiometry; the number of water molecules was calculated from the difference of (OH + F + H₂O) = 2 apfu. The ideal boundary formula for I and II is (LaCa)_Σ2(CO₃)₂(OH⋅H₂O) and (La_1.5Ca_0.5)_Σ2(CO₃)₂(OH_1.5⋅0.5H₂O), respectively.

X-ray crystallography and structure determination

The crystal for XRD analysis was extracted from the polished thin section by using an FEI Helios NanoLab 600i dual beam system equipped with a Focused Ion beam (FIB). The phases were re-checked by scanning electron microscope (SEM). Both powder and single-crystal X-ray studies of calcioancylite-(La) were carried out using a Rigaku XtaLAB Synergy diffractometer (CuKα radiation). The powder X-ray diffraction data were recorded using the Gandolfi technique in powder mode at 50 kV and 1 mA. The pattern was indexed on the basis of the powder spectra and calculated on the basis of cell parameters determined by single-crystal XRD using CHEKCELL software (Laugier and Bochu, 2004). The refined lattice parameters yielded from the powder patterns are a = 5.0332(20) Å, b = 8.5238(33) Å, c = 7.2799(28) Å and V = 312.32(36) ų. The observed and indexed powder diffraction data for calcioancylite-(La) are listed in Table 2.

Table 2. Powder X-ray diffraction data (d in Å) for calcioancylite-(La).

Note: the strongest lines are in bold, and the d _calc were refined on the basis of single-crystal data using CHEKCELL software (Laugier and Bochu, 2004).

Single-crystal X-ray studies were performed using a Rigaku XtaLAB Synergy diffractometer equipped with a Hybrid Pixel Array Detector and CuKα radiation at 50 kV and 1 mA from a colourless platy fragment of calcioancylite-(La) (~16 × 7 × 5 μm) at the Laboratory of Crystal Structure, Central South University, China. The intensity data were corrected for X-ray absorption using the multi-scan method. Empirical absorption correction was performed using CrysAlisPro software spherical harmonics (Rigaku Oxford Diffraction, 2015), which was implemented in the SCALE3 ABSPACK scaling algorithm. The crystal structure was determined and refined using the SHELX (Sheldrick, 2015) and Olex2 software (Dolomanov et al., 2009). The refined unit-cell parameters are a = 5.0253(3) Å, b = 8.5152(6) Å, c = 7.2717(6) Å, V = 311.17(4) ų and space group Pmcn. We also tried to refine the structure under a monoclinic system. However, the attempt yielded a worse refinement result. The following neutral scattering curves, taken from the International Tables for Crystallography (Wilson, 1992), were initially used: La vs. Ca at the M site, and C at the C site. The final anisotropic full-matrix least-squares refinement on F ² with 347 unique reflections (I > 2σ(I)) converged at R ₁ = 6.52% and 6.79% for all 360 data. The goodness-of-fit was 1.258. The details of the data collection and the final structure refinement are provided in Table 3. Site occupancies, atomic coordinates, and displacement parameters are given in Table 4, and selected bond distances are in Table 5. The bond-valence sums (BVS), calculated using the bond-valence parameters of Brese and O'Keeffe (1991), are shown in Table 6. A view of the structure is presented in Fig. 3. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 3. Information on structural refinement for calcioancylite-(La).

* w = 1/[σ²(${\it F}_{\rm o}^ 2$)+(0.0428P)²+16.9132P], where P = (${\it F}_{\rm o}^ 2$+2${\it F}_{\rm c}^ 2$)/3

Table 4. Atomic coordinates and equivalent isotropic displacement parameters (in Ų) for calcioancylite-(La).

* M = La_0.66(4)Ca_0.34(4)

Table 5. Selected bond distances (Å) and angles (°) of calcioancylite-(La).

Table 6. Bond-valence analysis (vu) for calcioancylite-(La).

Notes: Bond-valence sums were calculated with the site-occupancy factors given in Table 4.

M site = ⅔La + ⅓Ca. The theoretical vu of O3 is calculated based on ⅔OH + ⅓H₂O for charge balance. Calculations were done using the equation and constants of Brown (1977), S = exp[(R ₀–d ₀)/b].

Fig. 3. The crystal structure model of calcioancylite-(La) viewed along (a) [001] and (b) [010].

Results and discussion

Raman spectroscopy

The characteristics of calcioancylite-(La) are comparable to that of calcioancylite-(Ce) from Mont Saint-Hilaire, Rouville County, Quebec, Canada (RRUFF R130005, https://rruff.info/) and ancylite-(Ce) from Bear Lodge, Wyoming, USA (Chakhmouradian et al., 2017). In general, the major Raman bands of calcioancylite-(La) at 1085 cm⁻¹ and a shoulder band at 1076 cm⁻¹ can be confidently identified as symmetric C–O stretching modes (ν ₁). The bands at 700 cm⁻¹ and 728 cm⁻¹ are assigned to O–C–O in-plane bending (ν ₄) modes. A very low-intensity band at 1449 cm⁻¹, which is commonly observed in bastnäsite-group minerals can be assigned to the ν ₃ (CO₃)²⁻ asymmetric stretching mode (Frost and Dickfos, 2007). The bands observed at 3529 cm⁻¹ and 3604 cm⁻¹ are assigned to H₂O molecules and hydroxyl groups, respectively. A number of low wavenumber bands are observed at 127 cm⁻¹, 370 cm⁻¹ and between 220 and 245 cm⁻¹ could be described as lattice translation modes.

Chemical formula

The empirical formula of calcioancylite-(La) (average of 5 EPMA spot analyses) calculated on the basis of C = 2 atoms per formula unit, (OH + F + H₂O) = 2 atoms per formula group [apfu] is (La_0.58Ce_0.55Pr_0.14Nd_0.10Ca_0.39Sr_0.20K_0.04)_Σ2.00(CO₃)₂(OH_1.25F_0.06⋅0.69H₂O)_Σ2.00. Note that the unusual presence of Al in the EPMA data (average: 1.34 wt.% Al₂O₃) could be caused by sub-microscopic ingrowths or thin films of Al hydroxides (i.e. gibbsite, boehmite, nordstrandite or amorphous Al(OH)₃). These phases are commonly associated with ancylites in late, low-temperature hydrothermal alkaline rocks. The ‘alien’ nature of Al in calcioancylite-(La) is indirectly but evidently confirmed by the wide variation of its content in the EPMA data, ranging from 0.41 to 2.69 wt.% Al₂O₃. As the presence of Al has not been reported for ancylites in the literature so far and as Al³⁺ strongly differs in ionic radius and electron shell structure from large cations such as REE, Ca, and Sr, the empirical formula was calculated without Al. The ideal compositions for the ideal boundary formula (LaCa)_Σ2(CO₃)₂(OH⋅H₂O) and (La_1.5Ca_0.5)_Σ2(CO₃)₂(OH_1.5⋅0.5H₂O) are shown in Table 1 for comparison.

Crystal structure description

The structure of calcioancylite-(La) is identical to that of the calcioancylite-(Ce) reported by Pekov et al. (1997) and other members of the ancylite group (Dal Negro et al., 1975; Sarp and Bertrand 1985; Miyawaki et al., 2000, 2003), in which the REE ³⁺ and divalent cations are situated at the centre of a ten-vertex polyhedron formed by O(1), O(2) and O(3) oxygen atoms. The O(1) and O(2) positions are occupied by oxygen atoms, whereas the O(3) position is statistically filled with (OH) groups and H₂O molecules. The chains of this ten-vertex polyhedron are stretched along the c axis and connected by shared O(2)–O(3)–O(2) faces. The chains are interconnected into a three-dimensional framework via (CO₃) triangles. In calcioancylite-(La), the REE share their positions with relatively light Ca and Sr atoms, with La > Ce and Ca > Sr and are bonded to eight oxygen atoms (O(1)^×2, O(2)^×6) belonging to the CO₃ group and two hydroxyls and water molecules (O(3)^×2). The occupancy of the cationic position M was 0.66(4) and 0.34(4) for La and Ca cations, respectively. The resulting occupancy of La_0.66Ca_0.34 yields a site scattering of 44.4 e ^– which is consistent with the calculated value of 47.7 e ^– based on the empirical formula. The M–O distances range from 2.576 to 2.765 Å for the oxygen atoms, whereas the shorter M–O distances involving the hydroxyls vary between 2.465 and 2.496 Å. The mean M–O bond distance of 2.609 Å is comparable to the mean M–O distance of ancylite-(Ce) (2.61 Å) (Dal Negro et al., 1975), and is slightly longer than the mean M–O bond length in calcioancylite-(Nd) (2.585 Å), which contained no strontium at the M site (Orlandi et al., 1990). The C–O bond distances are consistent with values reported in the literature. The bond valences were calculated from the interatomic distances following the procedure of Brown and Altermatt (1985). The bond-valence sums for the M site and C positions are 2.711 and 3.966 valence units (vu), respectively, which agrees with the expected values given that various cations occur at the same sites. The BVS for the O(1), O(2) and O(3) positions are 1.919, 2.007 and 0.744 vu, respectively. The lower BVS for O(3) implies a mixed occupancy by (OH) group and water molecules at the O(3) site, consistent with the chemical data and Raman spectroscopy results. As shown in Table 6, all calculated BVS values are comparable to ideal values, and basically match the charge-balance requirement.

Relation between the M site and additional anion site

The chemical formula of ancylite-group minerals can be defined as: $( {\it REE}_{\it x}^{{\rm 3+}}{\it M}_{{\rm 2\ndash \it x}}^{{\rm 2+ }}$)(CO₃)₂[(OH)_x⋅(2–x)H₂O] (M = Ca, Sr and Pb; Z = 2). REE ³⁺ and M ²⁺ occupy the same crystallographic site and the formula is charge-balanced through the following substitution mechanism: REE ³⁺ + OH⁻ → M ²⁺ + H₂O. The excess positive charge of the REE ³⁺ is compensated by the incorporation of (OH)⁻, and the number of hydroxyl ion and water molecule are equivalent to those of REE ³⁺ and M²⁺, respectively. In our case, combined structure refinement and bond-valence calculations verify that the substitution mechanism for the material studied is (⅔REE ³⁺ + ⅓M ²⁺) + (⅔OH⁻ + ⅓H₂O). According to the predominant cation on the M ²⁺ site, the ancylite-group minerals can be subdivided into ancylite, calcioancylite, gysinite and kozoite species (Hatert and Burke, 2008; Belovitskaya et al., 2013). Additionally, the primary REE component is also identified by a hyphenated suffix that appears in parentheses on each distinct mineral species, e.g. ancylite-(Ce) (Levinson, 1966; Bayliss and Levinson, 1988). On the basis of the composition, the minerals of the ancylite group can be described as solid solutions among ancylite, calcioancylite, gysinite and kozoite (Sarp and Bertrand, 1985; Miyawaki et al., 2000). Miyawaki et al. (2000) constructed a ternary composition diagram for the ancylite-group minerals and plotted available published mineral analyses. The results indicate that the value of x in the general formula of nearly all published mineral analysis exceeds 1. The sum of (REE ³⁺ + M ²⁺) is usually ~2 apfu, which indicates that the ancylite-group minerals are probably not intermediate solid solutions between REE ³⁺(CO₃)(OH) and M ²⁺(CO₃)⋅H₂O (Sarp and Bertrand, 1985; Miyawaki et al., 2000). Without quantitative chemical analysis and clear boundaries of mineral species, members of solid-solution series cannot be identified precisely. On the issue of boundaries, Miyawaki et al. (2000) discussed the value of x in the general formula exceeding 1.5 that is the boundary between kozoite and calcioancylite. The ratios between the divalent elements and REE range from 1:1 to 1:3, indicating 1 < x < 1.5, in the ancylite–calcioancylite–gysinite isomorphous series (Larsen et al., 2002).

Conclusions

Although the ancylite-mineral group has only been informally defined, the chemical formula of ancylite-group minerals can be defined as (${\it REE}_{\it x}^{{\rm 3+}}{\it M}_{{\rm 2\ndash \it x}}^{{\rm 2+ \ }}$)(CO₃)₂[(OH)_x⋅(2–x)H₂O] (1 < x ≤ 2 and Z = 2). It is worth noting that significant differences are present in the published general formulas within valid mineral species belonging to the ancylite group (e.g. calcioancylite-(Ce): (Ce,Ca,Sr)(CO₃)(OH,H₂O), Belovitskaya et al., 2013; Calcioancylite-(Nd): Nd_2.8Ca_1.2(CO₃)₄(OH)₃⋅H₂O, Orlandi et al., 1990). Without better understanding of this group of minerals, it is not easy to understand whether rare earth elements and divalent cations occupy the same crystallographic sites or independent sites. For example: chemically, some REE minerals can be written as Ce₂Sr(CO₃)₃(OH)₂⋅H₂O or Ce₃Sr(CO₃)₄(OH)₃⋅H₂O. Thus, the ratio of Ce to Sr is 2:1 or 3:1, which is obviously different from the simplified formula of ancylite-(Ce) in the IMA list (Pasero, 2023). The two mineral formulas apparently represent two potential new mineral species but are actually intermediate between the end-members (Ce,Sr)₂(CO₃)₂(OH,H₂O)₂ [ancylite-(Ce)] and Ce₂(CO₃)₂(OH)₂ [potential Ce-analogue of kozoite-(Nd)]. Considering that the ancylite group has not yet been formally approved by the IMA, an ancylite-group nomenclature proposal is in preparation to clarify the above aspects. The well-defined mineral boundaries and a unified ideal formula, enabling the full description of the chemical variability of the informal ancylite group, will allow important chemical information to be imparted, not only to mineralogists but also to petrologists and ore geologists.