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

The new mineral piilonenite-(Nd), ideally NaNd(CO 3 ) 2 ⋅3H 2 O, 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}. D calc 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 Na 2 O 9.29, Y 2 O 3 1.71, La 2 O 3 0.93, Ce 2 O 3 8.32, Pr 2 O 3 2.75, Nd 2 O 3 23.90, Sm 2 O 3 6.85, Eu 2 O 3 0.27, Gd 2 O 3 3.64, Tb 2 O 3 0.12, Dy 2 O 3 0.90, Ho 2 O 3 0.13, Er 2 O 3 0.12, Tm 2 O 3 0.07, CO 2 26.32, H 2 O 16.16, total 101.48. The empirical formula of the holotype calculated on the basis of two cations is as follows: Na 1.00 (Nd 0.48 Ce 0.17 Sm 0.13 Gd 0.07 Pr 0.06 Y 0.05 La 0.02 Dy 0.02 ) ∑1.00 (CO 3 ) 2 (H 2 O) 3 . The IR spectrum shows IR bands of O–H-stretching and H–O–H bending vibrations of H 2 O molecules and C–O-stretching vibrations of CO32--group molecules. The mineral is orthorhombic, P 2 1 2 1 2 1 , a =6.7914(11) Å, b =17.135(3) Å, c =6.4360(10) Å, and V =749.0(2) Å 3 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 ( R 1 =0.092), is unique. There are two alternating layers: (1) vertex-sharing Nd-centred polyhedra and “flat-lying” carbonate (CO 3 ) 2− groups and (2) chains of vertex-sharing Na-centred polyhedra parallel to (100) and “standing-on-edge” carbonate (CO 3 ) 2− groups.