Mckelveyite group minerals – Part 2: Alicewilsonite-(YCe), Na2Sr2YCe(CO3)6  ⋅  3H2O, a new species

Abstract. The new mckelveyite group mineral alicewilsonite-(YCe), ideally Na2Sr2YCe(CO3)6 ⋅ 3H2O, was found at Mont Saint-Hilaire, Quebec, Canada, and subsequently at the Saint-Amable sill, Quebec, Canada, and the Khibiny Massif, Kola Peninsula, Russia. Alicewilsonite-(YCe) crystals are commonly hemimorphic pseudotrigonal and pseudohexagonal and show barrel-shaped, saucer-shaped, spindle-shaped, cone-shaped, columnar, tabular, and platy habits. They are usually up to 2–3 mm in size with some larger crystals reaching 2–3 cm. The crystals often form stacked or parallel growth aggregates and rosettes. Alicewilsonite-(YCe) colour varies from pale yellow to yellow, lemon yellow, green yellow, orange-yellow, pale green to green, pale grey to grey, green grey, beige, and colourless. The streak is white; the lustre is vitreous. The cleavage is fair to indistinct, parallel to (001). The Mohs hardness is 3. Dcalc is 3.37 g cm−3. Alicewilsonite-(YCe) is optically biaxial (+), with α=1.554(3), β=1.558(3), γ=1.644(2), 2V (calc.) = 26∘, 2V (meas.) = 20(3)∘ (589 nm). The IR spectrum is reported. The composition (wt %, average of six analyses) is Na2O 7.42, CaO 0.72, SrO 21.49, BaO 1.41, Y2O3 8.52, La2O3 5.93, Ce2O3 9.52, Pr2O3 0.59, Nd2O3 1.75, Sm2O3 0.46, Gd2O3 0.83, Dy2O3 1.65, Ho2O3 0.34, Er2O3 1.21, Yb2O3 0.64, CO2 29.33, H2O 6.13, total 97.94. The empirical formula of the holotype calculated on the basis of six cations is Na2.11Ca0.11Sr1.83Ba0.08Y0.67(Ce0.51La0.32Pr0.03Nd0.09Sm0.02Gd0.04 Dy0.08Ho0.02Er0.06Yb0.03)Σ1.20(CO3)5.88 (H2O)3.00. The mineral is triclinic, P1, a=9.0036(6) Å, b=9.0175(6) Å, c=6.7712(5) Å, α=102.724(2)∘, β=116.398(2)∘, γ=60.003(2)∘, V=426.46(5) Å3, and Z=1. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are 6.07(31)(001), 4.372(100)(120, 2‾1‾1, 11‾0), 4.037(25)(1‾11, 1‾2‾1, 210), 3.201(25)(121, 2‾1‾2, 11‾1), 2.831(67)(1‾12, 1‾2‾2, 211, 1‾21, 21‾0), 2.601(39)(030, 3‾3‾1,3‾01), 2.236(24)(2‾4‾1, 2‾21, 4‾2‾1). 2.019(23)(003, 2‾22, 2‾4‾2‾, 420). 1.9742(24)(032, 3‾03, 3‾3‾3, 331, 03‾2, 301). The crystal structure, solved and refined from single-crystal X-ray diffraction data (R1=0.055), is of the weloganite type.