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 5 1/2 on Mohs scale. The empirical formula of takanawaite-(Y) is (Y
0.75Dy
0.08Yb
0.05Gd
0.02U
0.03Ti
0.02Fe
0.02)
Σ0.97(Ta
0.57Nb
0.45)
Σ1.02O
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) Å, β = 94.993(14)°, and
V = 293.74(7) Å
3, and
Z = 4, with a calculated density 6.97 g/cm
3. The eight strongest lines in the powder XRD pattern [
d (Å), (
I/I0),
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).
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