Iwashiroite-(Y), YTaO
4, was found from Suishoyama, Kawamata Town, Fukushima Prefecture, Japan. It is monoclinic,
P2/
a,
a = 5.262(5) Å,
b = 5.451(5) Å,
c = 5.110(5) Å, β = 95.12(10)°,
V = 146.0(2) Å
3, Z = 2. The five strongest lines in the powder XRD pattern [
d(Å),
I/
I0,
hkl] are (3.13, 100, 111), (2.95, 94, 111), (2.73, 26, 020), (1.890, 29, 220) and (1.862, 29, 022). Electron microprobe analysis gave; Y
2O
3 29.10, Ce
2O
3 0.10, Nd
2O
3 0.10, Sm
2O
3 0.36, Gd
2O
3 1.06, Tb
2O
3 0.25, Dy
2O
3 2.38, Ho
2O
3 0.56, Er
2O
3 2.09, Tm
2O
3 0.37, Yb
2O
3 3.33, Lu
2O
3 0.85, ThO
2 0.02, UO
2 0.15, CaO 0.17, Ta
2O
5 40.64, Nb
2O
5 16.66, TiO
2 0.41, total 98.60 wt%, and leads to the empirical formula, (Y
0.81Yb
0.05Dy
0.04Er
0.03Gd
0.02Lu
0.01Ho
0.01Sm
0.01Ca
0.01Tm
0.01)
Σ1.00(Ta
0.58Nb
0.39T
i0.02)
Σ0.99O
4 on the basis of the 4 oxygen atoms per formula unit. Iwashiroite-(Y) is translucent and amber brown to brown in color with a glassy to subadamantine luster. The streak is pale brown and cleavage is {010} good, {001} poor. The Mohs' hardness is 6. The calculated density is 7.1 g/cm
3. Iwashiroite-(Y) occurs as an aggregate of small euhedral platy crystals with a metamict substance in the main ore body of the granite pegmatite mine at the locality. Associated minerals are quartz, microcline and annite. The crystal structure was refined to
R = 0.034 with single crystal XRD data. Although the structure consists of TaO
6 octahedra and YO
8 polyhedra, iwashiroite-(Y) is not isostructural with fergusonite-beta-(Y).
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