Takanawaite-(Y), a new mineral of the M-type polymorph with Y(Ta,Nb)O_4 from Takanawa Mountain, Ehime Prefecture, Japan

摘要

Takanawaite-(Y), a new mineral of the M-type polymorph with an generalized formula of Y(Ta,Nb)O_4, is identified from Takanawa Mountain, Ehime Prefecture, Japan. It is dark brown in color and occurs as a single crystal with a tabular habit (up to 5 mm) or a radial aggregate of fine crystals. The hardness is 51/2 on Mohs scale. The empirical formula of takanawaite-(Y) is (Y_(0.75)Dy_(0.08)Yb_(0.05)Gd_(0.02)U_(0.03)Ti_(0.02)Fe_(0.02))_(Σ0.97)(Ta_(0.57)Nb_(0.45))_(Σ1.02)O_4, leading to a simplified formula of Y(Ta,Nb)O_4. The Ta/(Ta + Nb) ratio varies between 0.55 and 0.57. Takanawaite-(Y) naturally occurs in a submetamict state, and a heating treatment is required to determine the crystal structure. The heating experiment revealed the consistency of takanawaite-(Y) and its M-type polymorph. The mineral crystallizes in a monoclinic M-type structure with a space group I2/a, unit cell parameters a = 5.3182(8), b = 10.9583 (13), c = 5.0595 (7) A, β = 94.993(14)°, and V= 293.74(7) A~3, and Z = 4, with a calculated density 6.97 g/cm~3. The eight strongest lines in the powder XRD pattern [d(A) (I/I_0), hkl] are 3.1334 (100) 121, 2.9525 (85) 121, 2.7391 (29) 040, 2.6494 (21) 200, 1.9115 (24) 202, 1.9050 (39) 240, 1.8546 (26) 042, and 1.5681 (19) 242. Takanawaite-(Y) is a distinct mineral compared to iwashiroite-(Y), yttrotantalite-(Y), and formanite-(Y).