This paper describes piilonenite-(Nd), NaNd(CO3)2⋅3H₂O, the second Nd-dominant mineral after bainbridgeite-(NdCe), Na₂Ba₂NdCe(CO3)6⋅3H₂O (Lykova et al., 2023b), found at Mont Saint-Hilaire, Quebec, Canada.

Piilonenite-(Nd) is in named in honour of Paula Piilonen (born 1974), a Canadian mineralogist and research scientist at the Canadian Museum of Nature and a former president of the Mineralogical Association of Canada. Piilonen specializes in the mineralogy of alkaline rocks. She is the senior author of the descriptions of two new mineral species from Mont Saint-Hilaire. The parenthesized suffix (Nd) is added in accordance with the nomenclature for rare earth (yttrium and lanthanides) mineral species established by Levinson (1966).

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 no. IMA 2025-031. The holotype of piilonenite-(Nd) was deposited in the collection of the Canadian Museum of Nature, Ottawa, Canada. The catalogue number is CMNMC 93393.

Piilonenite-(Nd) was thought to be the unknown phase UK119 from Mont Saint-Hilaire, which was previously investigated by Robert Gault (electron microprobe analyses, 1999), Joel Grice (attempted to solve the structure, 2000), Ralph Rowe (powder X-ray diffraction, 2007), and Igor Pekov (attempted to solve the structure, 2009–2013) (Horváth et al., 2019). However, our examination revealed that “UK119” is, in fact, at least two different phases: one of them is piilonenite-(Nd), and another one is an unrelated Na–Ce carbonate. These were found together at the same time (László Horváth, personal communication, 2025), which led to the confusion. Any information available to date on UK119 could refer to either or both of the minerals.

Piilonenite-(Nd) was found at the Poudrette quarry (Demix quarry) in the famous alkaline agpaitic igneous complex Mont Saint-Hilaire, Quebec, Canada. The material was collected by Elsa Pfenninger-Horváth and László Horváth on 18 August 2000 (László Horváth, personal communication, 2025) in apophyses (3–6 cm wide) of the Poudrette pegmatite cutting the hornfels xenolith on level 8 of the quarry.

The Poudrette pegmatite is by far the largest pegmatite observed in the quarry. It is unusual for the extreme diversity of its minerals and the fact that all of the observed parts of the pegmatite and its extensive apophyses (offshoots) were enclosed in the enormous hornfels xenolith. These vein-like apophyses radiate outward from the main body and can be traced for tens of metres. These contain many of the same minerals as found in the main pegmatite. The mineral assemblages differ in various parts of the pegmatite, varying from silicate- to carbonate-dominant compositions (Horváth et al., 2019, and the references therein).

Piilonenite-(Nd) occurs as thin, bladed crystals up to 600 µm in size (Fig. 1), together with siderite, calcite, microcline, sphalerite, garronite-Na, quartz, rutile, and aegirine.

Piilonenite-(Nd) is colourless or white (Fig. 1). The streak is white; the lustre is vitreous. The mineral has perfect cleavage on {010}; its fracture is uneven. The Mohs hardness could not be determined as the crystals are very thin and flaky. The mineral is non-fluorescent under ultraviolet light. The density calculated using the empirical formula and unit-cell volume refined from the single-crystal X-ray diffraction (XRD) data is 3.21 gcm-3.

Piilonenite-(Nd) is optically biaxial (+), α=1.546(3), β=1.616(3), γ=1.638(3), 2V (meas.) = 56(3)° (from a spindle-stage extinction curve), and 2 V (calc.) = 56°.

Electron microprobe analyses (EMPA) for piilonenite-(Nd) were obtained using a JEOL 8230 SuperProbe electron microscope equipped with five wavelength dispersive (WDS) spectrometers (University of Ottawa – Canadian Museum of Nature MicroAnalysis Laboratory, Canada) using an accelerating voltage of 20 kV, a beam current of 10 nA, and a beam diameter of 30 µm. Piilonenite-(Nd) is unstable under an electron beam, and so a larger beam diameter was used to minimize element migration. The following reference materials were used: NaInSi₂O₆ (NaKα), YAG (YLα), LaPO₄ (LaLα), CePO₄ (CeLα), PrPO₄ (PrLβ), NdPO₄ (NdLα), SmPO₄ (SmLα), EuPO₄ (EuLα), GdPO₄ (GdLα), TbPO₄ (TbLα), DyPO₄ (DyLβ), and HoPO₄ (HoLβ). The intensity data were corrected for time-dependent intensity (TDI) loss (or gain) using a self-calibrated correction for NaKα, YLα, and LaLα. H₂O and CO₂ contents were not analysed due to the paucity of the available material.

The Fourier transform infrared (FTIR) spectrum of piilonenite-(Nd) 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 piilonenite-(Nd) 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 256 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 500 K detector and I µS microfocus X-ray source (λCuKα1=1.54060 Å) running at 50 KV and 1000 µA. The detector distance was set at 174.33 mm. The Kα2 contribution was 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 piilonenite-(Nd) ∼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.

Synchrotron single-crystal X-ray diffraction (SC-XRD) studies were carried out at the Lawrence Berkeley National Laboratory (USA) on Advanced Light Source beamline 12.2.1 with a Bruker D8 with PHOTONII detector using synchrotron radiation (λ=0.7288).

Piilonenite-(Nd) has no closely related minerals or synthetic compounds. There are four water-bearing carbonate minerals, with only sodium and rare earth elements being the dominant cations currently known. Thomasclarkite-(Y), NaY(HCO3)(OH)3⋅4H₂O, is a bicarbonate mineral (Grice and Gault, 1998). Shomiokite-(Y), Na₃Y(CO3)3⋅3H₂O (Khomyakov et al., 1992), and lecoqite-(Y), Na₃Y(CO3)3⋅6H₂O (Pekov et al., 2010), have a very different stoichiometry (Na : REE = 3:1). Adamsite-(Y), NaY(CO₃)2⋅6H₂O, has a similar stoichiometry but a higher water content. It also has a layered structure and two types of carbonate groups, namely flat-lying and standing on edge, but the layers have different configurations, with two adjacent Na layers that are connected via hydrogen bonds only, unlike in the structure of piilonenite-(Nd) (Fig. 7; Grice et al., 2000).

In the vast majority of cases, rare earth elements (REEs; yttrium and lanthanides) are found together in minerals due to their similar chemical properties. This is the basis of the currently used nomenclature for rare earth minerals proposed by Levinson (1966) and later revised by Bayliss and Levinson (1988), which uses a suffix in parentheses to indicate the dominant REE at a site of the mineral structure. Cerium, with over 160 known minerals, and yttrium, with almost 140 minerals, have the largest number of their own minerals. Other REEs usually disperse in them, with larger light REEs going preferentially in Ce-dominant phases, whereas smaller heavy REEs disperse in Y-dominant phases.

Due to its size, Nd³⁺ occupies an intermediate position between Ce³⁺ and Y³⁺ and thus disperses in both Ce- and Y-dominant minerals and rarely forms its own phases. Only 37 Nd-dominant minerals are listed on the official IMA list of minerals (April 2026; IMA-CNMNC, 2026), excluding piilonenite-(Nd).

This is why agpaitic complexes, generally characterized by rich late-stage pegmatite and hydrothermal rare earth mineralization, are not known for diverse Nd minerals, and the few Nd-dominant phases described in such environments are usually products of very specific local conditions.

Rhabdophane-(Nd), Nd(PO4)⋅H₂O, and a prospective neodymium analogue of abenakiite-(Ce), Na₂₆Ce₆(Si₆O18)(PO4)6(CO3)6(SO2)O, were described from the Shkatulka pegmatite, Mt. Alluaiv, Lovozero Massif, Russia (Chukanov et al., 2005). They were found in polymictic pseudomorphs after steenstrupine-(Ce) containing solid bituminous matters and were characterized by both a high content of rare elements and a high degree of their separation in different phases. Carlgieseckeite-(Nd), NaNdCa₃(PO4)3F, a belovite-group mineral, and its Ce–Ba-analogue kuannersuite-(Ce), NaCeBa₃(PO4)3F_0.5Cl_0.5, were found in cavities of the same albite vein in the Ilimaussaq complex, Greenland (Pekov et al., 2012). The authors of the description hypothesized that, due to an Sr-poor environment, the Sr-dominant member of the group belovite, NaCeSr₃(PO4)3F, the most widespread mineral for this structural type, was not formed, which allowed for the formation of Ca- and Ba-dominant minerals. Ba²⁺ is significantly larger than Ca²⁺ and increases the unit-cell volume, which the favoured entry of larger REEs (Ce and La) in kuannersuite, whereas smaller Nd atoms preferentially concentrated in carlgieseckeite. Tundrite-(Nd), Na₂Nd₂TiO₂(SiO4)(CO3)2, was also described from the Ilimaussaq complex, Greenland (Semenov et al., 1967), although the mineral was accepted based on a single chemical analysis without proper investigation and description of the material, and the find has not been replicated yet.

The Nd-dominant phases described above were formed instead of more common Ce-dominant phases of the same structural type when specific local environments allowed for the separation and concentration of Nd.

Piilonenite-(Nd) and two Nd-dominant mckelveyite-group minerals we described recently – bainbridgeite-(NdCe), Na₂Ba₂NdCe(CO3)6⋅3H₂O (IMA2023-018; Lykova et al., 2023b), also from Mont Saint-Hilaire, and mckelveyite-(Nd), NaCaBa₃Nd(CO3)6⋅3H₂O (IMA2023-017; Lykova et al., 2023a), from the Khibiny Massif, Russia – follow a different pattern. Bainbridgeite-(NdCe) and mckelveyite-(Nd) have more common Y-dominant analogues but not Ce-dominant ones, and there are Y-rich areas (to 0.53 atoms per formula unit of Y) in piilonenite-(Nd) crystals, indicating that the crystal chemically driven affinity of Nd for Y rather than Ce guided the formation of these minerals. In very late-stage hydrothermal processes, with depletion of Y from the solution, Nd can become the prevalent REE in a phase, which resulted in the formation of piilonenite-(Nd); bainbridgeite-(NdCe); mckelveyite-(Nd); and, potentially, other Nd-dominant phases.