2022 年 117 巻 1 号 論文ID: 220728
Oxyyttrobetafite–(Y) is the first member in the betafite group of the pyrochlore supergroup found in albite–rich pegmatite from Souri Valley, Komono, Mie Prefecture, Japan. This new mineral occurs as small anhedral grains with sizes of 20 to 200 µm in cylinder–shaped aggregates with a substrate of thalénite–(Y) and synchysite–(Y). Small amounts of aeschynite–(Y), thorianite, and thorite are also associated in the same occurrence with oxyyttrobetafite–(Y), and gadolinite–(Y) is also included at the boundary between the aggregate and albite. The physical properties are: brown in color, brittle, transparent, non–fluorescent, vitreous luster, white streak with a Mohs hardness of 5, and a calculated density of 5.54 g·cm−3. Oxyyttrobetafite–(Y) is an optically isometric material with brown color under the microscope with a refractive index of n = 2.3 calculated using the Gladstone–Dale relationship. The empirical formula of oxyyttrobetafite–(Y) calculated on the basis of B = 2 with A2B2X6Y composition is (Y1.58Dy0.13Yb0.07Er0.06Tm0.05Gd0.04Ho0.03Sm0.02Tb0.02Eu0.01Lu0.01)Σ2.02
(Ti1.85Ta0.09Fe0.05Sn0.02Nb<0.01)Σ2O7.05 and leads to the ideal formula of Y2Ti2O6O, which requires TiO2 41.44 wt% and Y2O3 58.56 wt%, total 100 wt%. The structure is isometric cubic with the space group Fd3m and unit cell parameters of a = 10.11090(10) Å, V = 1033.64(3) Å3, and Z = 8 by single crystal X–ray diffraction measurements. The seven strongest peaks in the powder X–ray diffraction pattern [d in Å (I/I0) hkl ] were 2.918(100) 222, 2.527(18) 400, 2.321(13) 331, 1.788(53) 440, 1.525(46) 622, 1.162(13) 662, and 1.033(9) 844 with unit cell parameters of a = 10.121(3) Å, V = 1036.6(9) Å3, and Z = 8. The crystal structure was refined to R1 = 0.018 for 159 observed reflections with the criteria of I > 2σ (I ). Oxyyttrobetafite–(Y) is characterized by Y dominance at the A sites, Ti dominance at the B sites, and O dominance at the X and Y sites in the A2B2X6Y pyrochlore–type formula.
