Magnesiorowlandite–(Y), Y₄(Mg,Fe)(Si₂O₇)₂F₂, a new mineral in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan

Abstract

Magnesiorowlandite, Y₄(Mg,Fe)(Si₂O₇)₂F₂, a Mg–analogue of rowlandite–(Y), was found in a pegmatite at Souri Valley, Komono, Mie Prefecture, central Japan. The mineral occurs as aggregates composed of gray massive and white powdery parts. The aggregates are up to 1 cm in diameter scattered in the pegmatite. The mineral is associated with quartz, albite, K–feldspar, muscovite, allanite–(Ce), gadolinite–(Y), and ‘yftisite–(Y)’. It is transparent and gray to white in color with a vitreous to oily luster. The streak is white and cleavage is not observed. The Mohs hardness is 5 to 5½. The calculated density is 4.82 g/cm³. It is biaxial negative and refractive indices are α = 1.755 (5) and γ = 1.760 (5) with non–pleochroism. Electron microprobe (WDS) analysis gave SiO₂ 28.61, FeO 2.94, MnO 0.35, MgO 2.77, CaO 0.03, Y₂O₃ 36.02, La₂O₃ 0.29, Ce₂O₃ 2.64, Pr₂O₃ 0.64, Nd₂O₃ 4.72, Sm₂O₃ 2.82, Gd₂O₃ 4.45, Tb₂O₃ 0.69, Dy₂O₃ 4.87, Ho₂O₃ 0.50, Er₂O₃ 1.64, Tm₂O₃ 0.34, Yb₂O₃ 2.02, Lu₂O₃ 0.69, ThO₂ 0.24, F 4.56, –F₂=O 1.92, total 99.91 wt% (average of 16 analyses), and led to the empirical formula, (Y_2.71Nd_0.24Dy_0.22Gd_0.21Ce_0.14Sm_0.14Yb_0.09Er_0.07Pr_0.03Tb_0.03Lu_0.03Ho_0.02Tm_0.02La_0.01Ca_0.01Th_0.01)_∑3.98(Mg_0.58Fe_0.35Mn_0.04)_∑0.97Si_4.00O_13.97F_2.03 on the basis of O + F = 16. The mineral is triclinic, P1, a = 6.527(6), b = 8.656(9), c = 5.519(5) Å, α = 99.09(8), β = 104.17(7), γ = 91.48(8)°, V = 297.9(5) ų, Z = 1. It is non–metamict, and the strongest lines in the powder XRD pattern [d(Å) (I/I₀) hkl] are 4.95 (33) 110; 3.64 (37) 021; 3.54 (38) 111; 3.08 (100) 201, 021; 2.92 (26) 211, 210; 2.68 (32) 112; 2.65 (26) 130, 012, 002; 2.63 (28) 220. The crystal structure was determined and refined to R₁ = 0.0736 for 1645 reflections with I > 2σ(I) of single crystal XRD data. In the crystal structure, the Si₂O₇ group, together with (Mg,Fe)O₄F₂ octahedron, connects the 7–coordinated and 8–coordinated REEs polyhedra to form the 3–dimensional structure.