Mckelveyite group minerals – Part 5: Bainbridgeite-(NdCe), Na ₂ Ba ₂ NdCe(CO ₃ ) ₆ ●3H ₂ O, a new neodymium and cerium-dominant species from Mont Saint-Hilaire, Canada

Abstract. The new mckelveyite group mineral bainbridgeite-(NdCe), ideally Na2Ba2NdCe(CO3)6⚫3H2O, was found at Mont Saint-Hilaire, Quebec, Canada. It occurs in two distinct environments: (1) in a “carbonate pegmatite” as pale-yellow or orange barrel-shaped short prismatic crystals up to 2 mm in size, where bainbridgeite-(NdCe) forms thin rims while the core is composed of bainbridgeite-(YCe), and (2) in a hornfels rock as white or pale-grey tabular saucer-shaped crystals up to 0.5 mm in size. It has a white streak, a vitreous lustre, and no cleavage. Dcalc is 3.49 g cm−3. Bainbridgeite-(NdCe) is optically biaxial (+), α=1.577(3), β=1.592(3), γ=1.657(3), 2V (meas.) = 40(3)°, and 2V (calc.) = 52° (589 nm). The IR spectrum is reported. The composition (wt. %, average of seven analyses) is Na2O 6.85, CaO 1.16, SrO 4.83, BaO 24.02, Y2O3 1.84, La2O3 6.07, Ce2O3 8.82, Pr2O3 0.76, Nd2O3 6.24, Sm2O3 3.34, Eu2O3 0.46, Gd2O3 2.74, Tb2O3 0.21, Dy2O3 0.53, ThO2 0.24, CO2 27.33, H2O 5.73, total 101.17. The empirical formula of the holotype from the “carbonate pegmatite” calculated on the basis of six cations is as follows: Na2.09Ca0.19Sr0.44Ba1.48Y0.15La0.35Ce0.51Pr0.04Nd0.35Sm0.18Eu0.03Gd0.14Tb0.01Dy0.03Th0.01(CO3)5.86(H2O)3.00. The mineral is triclinic, P1, a=9.0525(3) Å, b=9.1178(2) Å, c=6.85180(19) Å, α=102.575(3)°, β=116.272(4)°, γ=59.788(4)° and V=438.23(3) Å3, and Z=1. The strongest reflections of the powder X-ray diffraction pattern [d, Å(I)(hkl)] are as follows: 6.17(50)(001, 1‾1‾1, 1‾01), 4.407(100)(11‾0, 2‾1‾1, 120), 4.077(30)(1‾2‾1, 1‾11, 210), 3.241(32)(11‾1, 2‾1‾2, 121), 2.870(88)(1‾2‾2, 1‾12, 211), 2.621(39)(3‾01, 030, 3‾3‾1), 2.253(22)(2‾21, 2‾4‾1, 4‾2‾1), and 1.9978(25)(03‾2, 3‾3‾3, 3‾03, 301, 032, 331). The crystal structure, solved and refined from single-crystal X-ray diffraction data (R1=0.036), is of the weloganite type. Bainbridgeite-(NdCe) is the first known mineral with Ce and Nd atoms preferentially concentrated at two different sites of the structure.