Magnesiorowlandite-(Y), Y_4(Mg,Fe)(Si_2O_7)_2F_2, a new mineral in a pegmatite at Souri Valley, Komono,Mie Prefecture, central Japan

摘要

Magnesiorowlandite, Y_4(Mg,Fe)(Si_2O_7)_F_2, 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 51/2. The calculated density is 4.82 g/cm~3. 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 SiO_2 28.61, FeO 2.94, MnO 0.35, MgO 2.77, CaO 0.03, Y_2O_3 36.02, La_2O_3 0.29, Ce_2O_3 2.64, Pr_2O_3 0.64, Nd_2O_3 4.72, Sm_2O_3 2.82, Gd_2O_3 4.45, Tb_2O_3 0.69, Dy_2O_3 4.87, Ho_2O_3 0.50, Er_2O_3 1.64, Tm_2O_3 0.34, Yb_2O_3 2.02, Lu_2O_3 0.69, ThO_2 0.24, F 4.56, -F_2=O 1.92, total 99.91 wt% (average of 16 analyses), and led to the empirical formula,(Y_(2.71)Nd_(0.24)Dy_(0.22)Gd_(0.21)Ce_(0.14)Sm_(0.14)Yb_(0.09)Er_(0.07)Pr_(0.03)Tb_(0.03)Lu_(0.03)Ho_(0.02)Tm_(0.02)La_(0.01)Ca_(0.01)Th_(0.01))Σ3.98(Mg_(0.58)Fe_(0.35)Mn_(0.04))Σ0.97Si_(4.00)O_(13.97)F_(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) Å, Z = 1. It is non-metamict, and the strongest lines in the powder XRD pattern [d(Å) (I/I_0) 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 R_1 = 0.0736 for 1645 reflections with I＞ 2σ(I) of single crystal XRD data. In the crystal structure, the Si_2O_7 group, together with (Mg,Fe)O_4F_2 octahedron, connects the 7-coordinated and 8-coordinated REEs polyhedra to form the 3-dimensional structure.