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, (Y2.71Nd0.24Dy0.22Gd0.21Ce0.14Sm0.14Yb0.09Er0.07Pr0.03Tb0.03Lu0.03Ho0.02Tm0.02La0.01Ca0.01Th0.01)∑3.98(Mg0.58Fe0.35Mn0.04)∑0.97Si4.00O13.97F2.03 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) Å3, 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.
