Piilonenite-(Nd), NaNd(CO ₃ ) ₂  ⋅ 3H ₂ O, a new neodymium-dominant carbonate mineral from Mont Saint-Hilaire, Quebec, Canada

Abstract. The new mineral piilonenite-(Nd), ideally NaNd(CO3)2⋅3H2O, was found at Mont Saint-Hilaire, Quebec, Canada, in apophyses of the Poudrette pegmatite. It occurs as thin, bladed crystals up to 600 µm in size. Piilonenite-(Nd) is colourless or white. The streak is white; the lustre is vitreous. Cleavage is perfect on {010}. Dcalc is 3.21 g cm−3. Piilonenite-(Nd) is optically biaxial (+), α=1.546(3), β=1.616(3), γ=1.638(3), 2 V (meas.) = 56(3)°, and 2 V (calc.) = 56° (589 nm). The composition (wt. %, average of seven analyses) is Na2O 9.29, Y2O3 1.71, La2O3 0.93, Ce2O3 8.32, Pr2O3 2.75, Nd2O3 23.90, Sm2O3 6.85, Eu2O3 0.27, Gd2O3 3.64, Tb2O3 0.12, Dy2O3 0.90, Ho2O3 0.13, Er2O3 0.12, Tm2O3 0.07, CO2 26.32, H2O 16.16, total 101.48. The empirical formula of the holotype calculated on the basis of two cations is as follows: Na1.00(Nd0.48Ce0.17Sm0.13Gd0.07Pr0.06Y0.05La0.02Dy0.02)∑1.00(CO3)2(H2O)3. The IR spectrum shows IR bands of O–H-stretching and H–O–H bending vibrations of H2O molecules and C–O-stretching vibrations of CO32--group molecules. The mineral is orthorhombic, P212121, a=6.7914(11) Å, b=17.135(3) Å, c=6.4360(10) Å, and V=749.0(2) Å3 and Z=4. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are as follows: 8.59(100)(020), 4.690(14)(101), 4.291(30)(040), 3.417(10)(200), and 3.175(17)(012, 220, 141). The crystal structure, solved and refined from synchrotron data (R1=0.092), is unique. There are two alternating layers: (1) vertex-sharing Nd-centred polyhedra and “flat-lying” carbonate (CO3)2− groups and (2) chains of vertex-sharing Na-centred polyhedra parallel to (100) and “standing-on-edge” carbonate (CO3)2− groups.