Takanawaite-(Y), a new mineral of the M-type polymorph with Y(Ta,Nb)O4 from takanawa mountain, ehime prefecture, Japan

Abstract

Takanawaite-(Y), a new mineral of the M-type polymorph with an generalized formula of Y(Ta,Nb)O4, 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 5 1/2 on Mohs scale. The empirical formula of takanawaite-(Y) is (Y0.75Dy0.08Yb0.05Gd0.02U0.03Ti0.02Fe0.02)S0.97(Ta0.57Nb0.45)σ1.02 O4, leading to a simplified formula of Y(Ta,Nb)O4. 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) Å, β = 94.993(14)°, and V = 293.74(7) Å3, and Z = 4, with a calculated density 6.97 g/cm3. The eight strongest lines in the powder XRD pattern [d (Å), (I/I0), hkl] are 3.1334 (100) 121̄ , 2.9525 (85) 121, 2.7391 (29) 040, 2.6494 (21) 200, 1.9115 (24) 202̄ , 1.9050 (39) 240, 1.8546 (26) 042, and 1.5681 (19) 242̄ . Takanawaite-(Y) is a distinct mineral compared to iwashiroite-(Y), yttrotantalite-(Y), and formanite-(Y).