Mckelveyite group minerals – Part 3: Bainbridgeite-(YCe), Na 2 Ba 2 YCe(CO 3 ) 6 · 3H 2 O, a new species from Mont Saint-Hilaire, Canada

. The new mckelveyite group mineral bainbridgeite-(YCe), ideally Na 2 Ba 2 YCe(CO 3 ) 6 · 3H 2 O, was found at Mont Saint-Hilaire, Quebec, Canada. Bainbridgeite-(YCe) occurs as pseudotrigonal and pseu-dohexagonal hemimorphic crystals that show platy, columnar, tabular, cone-shaped, barrel-shaped, saucer-shaped, or spindle-shaped habit. They often form stacked or parallel growth aggregates, rosettes, and groups of radiating crystals. The crystals are usually less than 1 mm in size. Bainbridgeite-(YCe) varies in colour from pale yellow to yellow, grey to almost black, bluish grey, green-grey, or white. The streak is white; the lustre is vitreous. The mineral has no cleavage. The Mohs hardness is 3. D calc is 3.49 g cm − 3 . Bainbridgeite-(YCe) is optically biaxial ( + ), α = 1.572(2), β = 1.586(2), γ = 1.628(2), 2 V (

The mineral with chemical formula Na 2 Ba 2 YCe(CO 3 ) 6 • 3H 2 O was first found and marked as an unknown phase UK109 from Mont Saint-Hilaire, Quebec, Canada, by Andrew M. McDonald in 1998 (Horvath et al., 2019).The poor quality of UK109 crystals, typical for the mckelveyite group minerals, presented a major challenge Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC. to studying the phase, and thus the mineral has not been formally described until now.
Bainbridgeite-(YCe) is named in honour of Michael Bainbridge (born 1974), a Canadian mineralogist, prominent mineral collector, photographer, and writer.Michael has been an invaluable contributor to the Canadian mineralogical community and a tireless promoter of mineralogy.He is the author of the Minerals of Grenville Province and The Pinch Collection at the Canadian Museum of Nature books, as well as a major contributor to the latest monograph on the minerals of Mont Saint-Hilaire by Horvath et al. (2019).In January 2023 Michael joined the Canadian Museum of Nature collections staff.
The parenthesised Levinson suffix -(YCe) was added in accordance with the nomenclature for rare-earth and Y mineral species (Levinson, 1966;Bayliss and Levinson, 1988) and the recently approved nomenclature of the mckelveyite group; the first symbol represents the dominant cations at one of the A sites and the second symbol at one of the B sites (Lykova et al., 2023a).
Both the new mineral and the name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA CNMNC), proposal IMA 2020-065.The holotype of bainbridgeite-(YCe) is the specimen with the catalogue number CMNMC 46324 from the collection of the Canadian Mu-  seum of Nature, Canada, originally labelled as "mckelveyite-(Y)".
Bainbridgeite-(YCe) crystals are hemimorphic pseudotrigonal and pseudohexagonal and show a wide range of habits, including platy, columnar, tabular, cone-shaped, barrel-shaped, saucer-shaped, and spindle-shaped (Fig. 1).The crystal faces are often curved, split, and/or stepped.They commonly form stacked or parallel growth aggregates, rosettes, and groups of radiating crystals.The crystals are usually less than 1 mm in size with some reaching 1.5-2 mm.Aggregates can reach 3 mm in size.Generally, bainbridgeite-(YCe) is indistinguishable in hand specimens from other mckelveyite group minerals, although dark-grey to almostblack mckelveyite-like phases are likely to correspond to bainbridgeite-(YCe) (Fig. 1b-d).
The bainbridgeite-(YCe) type material was found by Elsa Pfenninger- Horvath andLaszlo Horvath on 25 and26 February 1978 (Laszlo Horvath, personal communication, 2020).At the time there were two independently operating quarries, separated by a dividing wall: Demix and Poudrette.The material was collected in the former Demix quarry, which is now the western half of the unified quarries.Bainbridgeite-(YCe) was found in a heavily altered body mostly consisting of calcite and albite in nepheline syenite.This geological environment can be classified as a "carbonate pegmatite" (Normand and Tarassoff, 2006), also known as a "carbonate vug" (Chao et al., 1967) based on the absence of minerals essential in the alkaline pegmatites and the lack of pyroxene and amphibole group minerals, as well as the abundance of calcite.No systematic study has ever been carried out on "carbonate pegmatites", and little is known about their origin (Normand and Tarassoff, 2006;Horvath et al., 2019).Bainbridgeite-(YCe) forms barrel-shaped short prismatic crystals up to 2 mm in length (Figs.1a, 2).The crystals are found sparsely distributed on albite encrusted with scaly brown stilpnomelane.

Physical and optical properties
Bainbridgeite-(YCe) varies in colour from pale yellow to yellow, orange-yellow, orange, grey to almost black, bluish grey, green-grey, green, or even white (Fig. 1).Grey and black colours are most common.The streak is white; the lustre is vitreous.The mineral has no cleavage, and its fracture is uneven.The Mohs hardness is 3.The mineral is non-fluorescent under ultraviolet light.The density calculated using the empirical formula and unit-cell volume refined from the singlecrystal XRD data is 3.49 g cm −3 .

Experimental methods
Electron microprobe analyses (EMPAs) for bainbridgeite-(YCe) were obtained using a JEOL 8230 SuperProbe electron microscope equipped with five WDS spectrometers (University of Ottawa -Canadian Museum of Nature Mi-croAnalysis Laboratory, Canada) using an acceleration voltage of 20 kV, a beam current of 10 nA, and a beam diameter of 20-50 µm depending on the grain size.Bainbridgeite-(YCe) is unstable under an electron beam, and so a larger beam diameter was used to minimise element migration.
The Fourier transform infrared (FTIR) spectrum of bainbridgeite-(YCe) was obtained using a Bruker Hyperion 2000 microscope interfaced to a Tensor 27 spectrometer with a wide-band mercury cadmium telluride (MCT) detector (Canadian Conservation Institute, Canada).A small fragment of bainbridgeite-(YCe) was mounted on a low-pressure diamond anvil microsample cell and analysed in transmission mode.The spectrum was collected between 4000-400 cm −1 with the co-addition of 150 scans at a 4 cm −1 resolution.
Powder X-ray diffraction (PXRD) data were collected at the Canadian Museum of Nature, Canada, using a Bruker D8 Discover microdiffractometer equipped with a DECTRIS EIGER2 R 500K detector and IµS microfocus X-ray source (λ CuKα1 = 1.54060Å) with the Kα 2 contribution removed using the "Strip Kα2" tool in Bruker Diffrac.EVA V4.3.The instrument was calibrated using a statistical calibration method (Rowe, 2009).A powder ball of bainbridgeite-(YCe) ∼ 200 µm in diameter, mounted on a fibre pin mount, was analysed with continuous Phi rotation and 10 • rocking motion along the Psi axis of the Centric Eulerian Cradle stage.
Single-crystal X-ray diffraction (SXRD) studies were carried out at the Natural History Museum, University of Oslo, Norway, using a Rigaku XtaLAB Synergy-S diffractometer equipped with a HyPix 6000HE detector (λ MoKα = 0.71073 Å) operating at 50 kV and 1 mA.The data were processed, including absorption correction, using Rigaku's CrysAlis Pro software.

Chemical data
Representative EMPAs for bainbridgeite-(YCe) collected on several specimens from Mont Saint-Hilaire are given in Table 1.The contents of Zr, Lu, and Hf are below the detection limit.
The empirical formula of the holotype calculated on the basis of six cations, excluding H

Infrared spectroscopy
The IR spectrum of bainbridgeite-(YCe) (Fig. 3) shows IR bands of O-H stretching (in the range from 3270 to 3360 cm −1 ) and H-O-H bending (at 1672 cm −1 ) vibrations of H 2 O molecules and C-O stretching (at 1377 and 1514 cm −1 ) vibrations of CO 2− 3 group molecules.The band at 1064 cm −1 can be assigned to the non-degenerate mode of C-O stretching vibrations, indicating polarisation of CO 2− 3 groups, as this mode would have been inactive, if symmetric non-polarised carbonate groups (with a three-fold axis) were present in the IR spectrum.The band assignment was made in accordance with Chukanov and Chervonnyi (2016).The single-crystal X-ray diffraction data were indexed in the P 1 space group with the following unit-cell parameters: a = 9.0874(4) Å, b = 9.0877(4) Å, c = 6.8826(3)Å, α = 102.912(4)• , β = 116.208(4)• , γ = 60.322(4)• , and V = 443.06(4)Å 3 .The structure was solved and refined to R1 = 0.040 on the basis of 4196 independent reflections with I >2σ (I ) using the SHELXL 2018/3 program package (Sheldrick, 2015).Crystal data, data collection information, and structure refinement details are given in Table 3, atom coordinates, equivalent displacement parameters, site composition, and bond-valence sums (BVSs) in Table 4, and selected interatomic distances in Table 5.The studied crystal demonstrated twinning by merohedry Class I (Nespolo and Ferraris, 2000), with twin domains ratio of 21 : 79.The crystallographic information file (CIF) for bainbridgeite-(YCe) is available as a file in the Supplement.It was also deposited in the Inorganic Crystal Structure Database (ICSD; no.CSD 2297132).SXRD studies of the mckelveyite group minerals are very challenging because of the poor quality of their crystals resulting in multiple split reflections and streaks.Bainbridgeite-(YCe) is no exception, and the structure could only be refined to R 1 = 0.040.
Bainbridgeite-(YCe) is strongly pseudotrigonal.There are six independent large cation sites in the structure (Fig. 4) forming two alternating layers parallel to the ab plane (Fig. 5).Na, Ba, Y, Ce, Sr, and Dy were distributed among these sites based on the EMPA, refined site-scattering factors (e ref , in electrons per site), and charge balance taking into account bond-valence sums (BVSs) and interatomic distances (Tables 4-6).Ca atoms were not included in the refinement; lighter lanthanoids (Ln, La-Lu) were formally refined as Ce atoms, heavier Ln -as Dy atoms.One of the layers is formed by the Ba1, Ba2, and Ce3 sites that have 10-fold coordination.The large Ba1-centred polyhedron with the < Ba1-O > distance of 2.80 Å is occupied by Ba atoms.The refinement showed that the remaining Ba, Ce, and Sr atoms are distributed between Ba2 and Ce3 sites.Their occupancies were refined as Ba 0.763(10) Sr 0.237(10) (e ref = 51.7)for the Ba2 site and Ce 0.913(9) Sr 0.087(10) (e ref = 56.3)for the Ce3 site (Table 6).Based on the e ref values, Sr atoms prefer the slightly larger Ba2-centred polyhedron with the < Ba2-O > distance of 2.70 Å; thus, the slightly smaller Ce3-centred polyhedron with the < Ce3-O > distance of 2.68 Å should be occupied predominantly by Ce atoms as it is too small for Ba atoms.
H 2 O molecules are bonded to Ba2-and Na5-centred polyhedra that share a face.
On the other hand, bainbridgeite-(YCe) and mckelveyite-(Y) occur at Mont Saint-Hilaire as distinct mineral species, and we have not found phases with an intermediate composition.In all studied samples, the content of Ca in bainbridgeite-(YCe) does not exceed 0.23 apfu.Similar relationships were described between alicewilsonite-(YCe) and donnayite-(Y) from Mont Saint-Hilaire (Lykova et al., 2023b).Bainbridgeite-(YCe) and mckelveyite-(Y) are also rarely found in one crystal, although epitactic overgrowth of bainbridgeite-(YCe) on mckelveyite-(Y) has been observed.

Unlike in donnayite-(Y) and mckelveyite-(Y)
, where Sr and Ba atoms are distributed evenly among three largecation sites (Demartin et al., 2008;Lykova et al., 2023a), in bainbridgeite-(YCe) Ba and Sr atoms show a preference for different sites.Therefore, a mineral with the general formula Na 2 SrBaYCe(CO 3 ) 6 • 3H 2 O with Ba, Sr, and Ce atoms ordered between three different cation sites could exist in nature.
A higher content of heavier Ln (up to 0.7 apfu), especially Nd, Sm, Gd, and Dy, and a lower Y content (as low as 0.2 apfu) were observed in some bainbridgeite-(YCe) samples (Table 1).The Ce and La contents in these phases remain relatively stable confirming the Y 3+ ↔ HREE 3+ substitution scheme at the Y6 site and showing that Nd and Sm atoms can also selectively concentrate at that site, as it was also observed in alicewilsonite-(YCe) (Lykova et al., 2023a).A substantial correlation (R 2 = 0.82) observed between Y and heavier Ln contents in bainbridgeite-(YCe) based on 36 analyses (Fig. 7) confirms that the primary mechanism of entry of heavier and smaller Ln atoms into the structure is preferential concentration at the Y6 site.
Th is a common minor admixture in bainbridgeite-(YCe).Its entry could follow the Ca 2+ + Y 3+ ↔ Na + + Th 4+ substitution mechanism, but, as Th 4+ cations are significantly larger than Y 3+ , the degree of tolerance of the bainbridgeite-(YCe) structure to Th cations is unclear.
The typical association of bainbridgeite-(YCe) and bastnaesite group minerals, which are often found in close proximity to one another and are sometimes intergrown, indicates that these minerals are often formed concurrently with late-stage alteration products of primary REEbearing carbonates.Burbankite group minerals, for example remondite-(Ce), Na 3 (Ce,Ca,Na) 3 (CO 3 ) 5 , or petersenite-(Ce), Na 4 (Ce,La,Nd) 2 (CO 3 ) 5 , could be one such carbonate.This hypothesis is confirmed by the observation of pseudomorphs of bastnaesite after a burbankite group mineral with multiple rosettes of bainbridgeite-(YCe) forming girdles around the pseudomorphs (Fig. 8).
Data availability.Crystallographic data for bainbridgeite-(YCe) and its PXRD pattern in the xy format are available in the Supplement.
Author contributions.IL conceptualised the project.RR collected powder X-ray diffraction data and sub-sampled the specimens.EM-PAs were obtained by GP.HF collected X-ray single-crystal diffraction data.KH obtained the IR spectrum.IL processed the data and interpreted the results.The manuscript was written by IL with contributions from all co-authors.
Competing interests.The contact author has declared that none of the authors has any competing interests.
Disclaimer.Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper.While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors.

Figure 4 .
Figure 4. General view of the crystal structure of bainbridgeite-(YCe).Sky-blue spheres are H 2 O molecules.The unit cell is outlined.

Figure 5 .
Figure 5.A view along [010] of the crystal structure of bainbridgeite-(YCe).Ba-, Ce-, Y-, and Na-centred polyhedra are shown in green, blue, purple, and yellow, respectively.Carbonate groups are black triangles.The unit cell is outlined.
1 Calculated from the crystal structure determination; only reflections with intensities > 1 are given.
* / −1.30 * Located 0.77 Å away from the Ce(3) site.There are several 1.7-2.1 e Å −3 peaks located 0.7-0.8Å away from large cations sites that indicate a relatively poor overall quality of the data (caused by a poor quality of bainbridgeite-(YCe) crystals) leading to spurious peaks of residual electron density.

Table 6 .
Refined site-scattering factors and assigned occupancies for Ba2, Ca3, and Y6 sites in the structure of bainbridgeite-(YCe) a .
bThe occupancy of Na was fixed.