BUSSYITE-(Y), A NEW BERYLLIUM SILICATE MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC

Bussyite-(Y), ideally (Y,REE,Ca)₃(Na,Ca)₆MnSi₉Be₅(O,OH,F)₃₄, a new mineral species, was found in the Mont Saint-Hilaire quarry, Quebec, Canada. The crystals are transparent to translucent, dark brown, with a white streak and vitreous luster. Crystals are blocky to prismatic, sometimes radiating, up to 3 mm in size. The mineral is brittle with a perfect {$1¯$01} cleavage. Associated minerals include aegirine, analcime, calcite, cappelenite-(Y), catapleiite, charmarite-2H and -3T, fluorite, helvine, kupletskite, microcline, perraultite, sérandite, and tainiolite. Bussyite-(Y) is monoclinic, space group C2, with unit-cell parameters refined from X-ray powder-diffraction data: a 11.545(2), b 13.840(2), c 16.504(4) Å, β 95.87(2)º, V 2623.1(6) ų, and Z = 4. Electron microprobe analysis gave the average composition and ranges, in wt.%: Na₂O 8.21 (8.43–8.07), K₂O 0.08 (0.10–0.05), BeO 9.75 (determined by structure refinement), CaO 5.25 (5.36–5.16), MnO 2.93 (3.20–2.57), BaO 0.03 (0.06–0.00), FeO 0.40 (0.60–0.25), Al₂O₃ 0.29 (0.34–0.21), Y₂O₃ 7.58 (7.79–7.37), La₂O₃ 0.48 (0.60–0.40), Ce₂O₃ 2.66 (3.09–2.37), Pr₂O₃ 0.55 (0.64–0.40), Nd₂O₃ 2.85 (2.93–2.81), Sm₂O₃ 1.45 (1.58–1.23), Eu₂O₃ 0.013 (0.17–0.12), Gd₂O₃ 1.97 (2.21–1.57), Tb₂O₃ 0.31 (0.40–0.25), Dy₂O₃ 2.20 (2.46–1.81), Ho₂O₃ 0.39 (0.48–0.33), Er₂O₃ 0.93 (1.07–0.86), Tm₂O₃ 0.16 (0.20–0.12), Yb₂O₃ 0.46 (0.60–0.36), Lu₂O₃ 0.01 (0.01–0.01), Nb₂O₅ 0.20 (0.25–0.15), SiO₂ 39.62 (40.03–38.96), ThO₂ 2.21 (2.43–1.90), F 3.49 (3.66–3.39), Cl 0.03 (0.03–0.02), H₂O 5.10 (determined by crystal-structure refinement), O = F + Cl –1.48, total 98.15. The empirical formula based on the crystal-structure analysis showing 34 anions is: 4{(Y_0.874Nd_0.221Ce_0.211Dy_0.154Gd_0.142Sm_0.108Er_0.063Pr_0.043La_0.038Yb_0.030Ho_0.027Tb_0.022Tm_0.011Eu_0.010Ca_0.789Th_0.105)_Σ1[Na_3.442Ca_0.800K_0.022]_Σ6.055(Mn_0.537Fe_0.072)_Σ0.899(Si_8.567Be_5.065 Al_0.222)_Σ13.724(O_24.107OH_5.893)_Ʃ30(F_2.386OH_1.60.3Cl_0.011)_Σ4}. The structure was refined to an R₁ index of 0.035 for 6531 unique, observed reflections. The structure has two chemically distinct layers parallel to (⁠$2¯$01): (1) a layer of [(Si,Be)O₄] tetrahedra and (2) a large cation Y-,Ca-, Mn-, Na-(O,F) polyhedral layer. Layers are cross-linked through shared O and F atoms. Significant amounts of OH are present, as indicated in the IR spectrum and crystal-structure analysis.