The story of the new mineral begins with a specimen collected on 18 August 2000 by Elsa Pfenninger-Horváth and László Horváth in narrow apophyses — vein-like offshoots three to six centimetres wide — of the Poudrette pegmatite on level 8 of the quarry. The Poudrette pegmatite is the largest pegmatite ever observed at Mont Saint-Hilaire, an agpaitic alkaline igneous complex famed among collectors for producing hundreds of mineral species. Its apophyses radiate outward from the main body for tens of metres, entirely enclosed within an enormous hornfels xenolith, a baked block of country rock. Mineral assemblages shift dramatically across the pegmatite, ranging from silicate-dominated to carbonate-dominated zones, and it was in one of these late-stage carbonate-rich pockets that the thin, bladed crystals of piilonenite-(Nd) formed alongside siderite, calcite, microcline, sphalerite, garronite-Na, quartz, rutile and aegirine.

For years the material hid in plain sight under a placeholder identity. It was thought to be the unknown phase UK119 from Mont Saint-Hilaire, a mystery mineral probed repeatedly without success: Robert Gault analysed it with the electron microprobe in 1999, Joel Grice attempted to solve its crystal structure in 2000, Ralph Rowe collected powder X-ray diffraction data in 2007, and Igor Pekov made further structural attempts between 2009 and 2013. The new examination revealed that UK119 was never a single phase at all. At least two different minerals — piilonenite-(Nd) and an unrelated sodium–cerium carbonate — were mixed together in the same material, having formed at the same time, a coincidence that confounded every earlier attempt. Any information previously attributed to UK119 could therefore refer to either or both of the two phases.

Physically, piilonenite-(Nd) is unassuming. It forms thin, bladed crystals up to 600 micrometres across, colourless to white, with a white streak, vitreous lustre, perfect cleavage on {010} and an uneven fracture. The crystals are so thin and flaky that a Mohs hardness could not be measured, and the mineral shows no fluorescence under ultraviolet light. Its calculated density is 3.21 grams per cubic centimetre. Optically it is biaxial positive, with refractive indices α = 1.546, β = 1.616 and γ = 1.638 and a measured axial angle of 56 degrees. The mineral dissolves slowly in aqueous hydrochloric acid at room temperature with gentle effervescence — the tell-tale fizz of a carbonate releasing carbon dioxide.

Chemical analysis demanded care. Because the mineral is unstable under the electron beam, the team used a JEOL 8230 SuperProbe with a widened 30-micrometre beam to minimise element migration, applying time-dependent intensity corrections for sodium, yttrium and lanthanum. Seven analyses yielded an average composition dominated by neodymium oxide at 23.90 weight percent, with significant cerium, samarium, praseodymium and gadolinium, plus a small yttrium contribution. The empirical formula, calculated on the basis of two cations, is Na1.00(Nd0.48Ce0.17Sm0.13Gd0.07Pr0.06Y0.05La0.02Dy0.02)(CO3)2(H2O)3 — confirming neodymium as the dominant rare earth element. Water and carbon dioxide contents were calculated from stoichiometry because so little material was available for direct measurement. Intriguingly, several areas within the crystals showed extraordinarily high yttrium, up to 18.4 weight percent Y2O3, hinting at a possible yttrium-dominant analogue of piilonenite lurking within the same tiny crystals.

Infrared spectroscopy filled in the structural picture that chemistry alone could not. The Fourier transform infrared spectrum, collected on a microsample pressed in a diamond anvil cell, shows O–H stretching bands between 3105 and 3230 wavenumbers and an H–O–H bending band at 1694 wavenumbers, confirming molecular water, along with C–O stretching vibrations of carbonate groups between 1356 and 1460 wavenumbers. A shoulder at 3420 wavenumbers points to hydroxyl anions, while weak bands at 1061 and 1091 wavenumbers reveal that some carbonate groups are polarised — a mode that would be forbidden in perfectly symmetric, threefold-axis carbonate groups. These spectroscopic fingerprints corroborated the presence of water molecules at three distinct crystallographic sites, later confirmed by bond-valence calculations based on hydrogen-bond distances.

The crystal structure itself proved the hardest prize. Previous structural attempts by Grice and Pekov had failed because the crystals diffract extremely poorly, bent as they are. Laboratory single-crystal diffractometers got the team nowhere, so they turned to synchrotron radiation at the Advanced Light Source in Berkeley, using beamline 12.2.1. Even there the data were poor, with split reflections and streaking across the diffraction pattern. Nevertheless, the team solved the structure in the orthorhombic space group P212121, with unit-cell parameters a = 6.7914 Å, b = 17.135 Å and c = 6.4360 Å, refining against 1364 independent reflections to an R1 of 0.092. The studied crystal proved to be an inversion twin, with domains in a 54:46 ratio. The resulting model matched the powder diffraction pattern well under Rietveld refinement, and the crystallographic data were deposited in the Inorganic Crystal Structure Database.

That structure is genuinely unique in mineralogy. It is layered on (010), built from two alternating types of sheets. One layer consists of vertex-sharing neodymium-centred polyhedra combined with carbonate groups lying flat in the layer. The other contains chains of strongly distorted, vertex-sharing sodium-centred octahedra running parallel to (100), flanked by carbonate groups standing on edge. Water molecules occupy three sites: one connects the sodium polyhedra into chains, while the other two tie the neodymium and sodium layers together through hydrogen bonds. Among the handful of known water-bearing carbonates dominated only by sodium and rare earth elements — thomasclarkite-(Y), shomiokite-(Y), lecoqite-(Y) and adamsite-(Y) — none shares this architecture. Adamsite-(Y) comes closest, with flat-lying and standing carbonate groups, but its layers are configured differently and its sodium layers are joined only by hydrogen bonds.

The deeper significance lies in why neodymium minerals are so scarce. Rare earth elements travel together in nature because of their similar chemistry, which is why the Levinson nomenclature system tags rare earth minerals with their dominant element in parentheses. Cerium, the largest of the light rare earths, boasts more than 160 minerals of its own, and yttrium nearly 140; neodymium, intermediate in size between cerium and yttrium, tends to disperse into both groups rather than concentrate. Only 37 neodymium-dominant minerals appear on the official IMA list as of April 2026, and those described from agpaitic complexes typically formed under highly specific local conditions that separated neodymium from its neighbours. Piilonenite-(Nd) follows a different pattern. Together with the recently described mckelveyite-group minerals bainbridgeite-(NdCe) from Mont Saint-Hilaire and mckelveyite-(Nd) from Russia’s Khibiny Massif, it shows a chemical affinity of neodymium for yttrium rather than cerium. The authors propose that in very late-stage hydrothermal fluids, as yttrium is depleted from solution, neodymium can become the prevalent rare earth element — allowing piilonenite-(Nd) and its kin to crystallise where cerium phases would normally dominate.

The mineral is named in honour of Paula Piilonen, a Canadian mineralogist at the Canadian Museum of Nature, former president of the Mineralogical Association of Canada and senior author of two earlier new-species descriptions from Mont Saint-Hilaire, with the suffix (Nd) following the Levinson convention. The holotype specimen, catalogue number CMNMC 93393, is preserved in the Canadian Museum of Nature collection in Ottawa. For a quarry that has already yielded dozens of first descriptions, piilonenite-(Nd) is a fitting addition — a mineral that spent a quarter of a century mistaken for something else, resisted four separate structural assaults, and finally surrendered its secrets to synchrotron light, revealing a crystal architecture no one had ever seen before.

Subject of Research: Description of the new neodymium-dominant carbonate mineral piilonenite-(Nd) from Mont Saint-Hilaire, Quebec

Article Title: Piilonenite-(Nd), NaNd(CO3)2 ⋅ 3H2O, a new neodymium-dominant carbonate mineral from Mont Saint-Hilaire, Quebec, Canada

Article References: Lykova, I., Rowe, R., Teat, S. J., Poirier, G., & Barnes, S. (2026). Piilonenite-(Nd), NaNd(CO 3 ) 2 ⋅ 3H 2 O, a new neodymium-dominant carbonate mineral from Mont Saint-Hilaire, Quebec, Canada. European Journal of Mineralogy, 38(3), 337-345. https://doi.org/10.5194/ejm-38-337-2026

Image Credits: AI Generated

DOI: 10.5194/ejm-38-337-2026

Keywords: piilonenite-(Nd), neodymium, new mineral species, Mont Saint-Hilaire, carbonate mineral, crystal structure, rare earth elements, agpaitic complex, synchrotron diffraction, Poudrette pegmatite, mineralogy, IMA approval