Watatsumiite, KNa
2LiMn
2V
2Si
8O
24, the V- and Mn-analogue of neptunite, is found in metamorphosed manganese ore deposit of the Tanohata mine, Iwate Prefecture, Japan. The mineral is monoclinic,
Cc, a = 16.450(16) Å,
b = 12.492(7) Å,
c = 9.995(8) Å, β = 115.32(6)°,
V = 1857(2) Å
3 and
Z = 4. The seven strongest lines in the X-ray powder diffraction pattern are [
d (Å), (
I), (hkl)]: 9.58(84)(110), 4.52(85)(002), 3.52(63)(131), 3.19(100)(330, 132), 2.94(90) (223), 2.90(66)(222, 510) and 2.49(93)(441, 621). Electron microprobe analysis and laser ablation microprobe-inductively coupled plasma-mass spectrometry gave SiO
2 52.64, TiO
2 3.13, VO
2 15.10, FeO 0.35, MnO 12.28, MgO 1.61, CaO 0.03, BaO 0.88, Na
2O 7.10, K
2O 4.89, Li
2O 1.6, total 99.61 wt.%, corresponding to (K
0.94Ba
0.05)
Σ0.99Na
2.08Li
0.97(Mn
1.57Mg
0.36Fe
0.04)
Σ1.97(V
1.66Ti
0.36)
Σ2.02Si
7.97O
24 on the basis of O = 24. Watatsumiite is transparent and yellowish green with vitreous luster. The streak is white with greenish tint. The cleavage is poor. The fracture is conchoidal and brittle. The hardness is VHN
100 707-946 kg/mm
2 (Mohs 51/2-6). The calculated density is 3.24 g/cm
3. Watatsumiite has moderate pleochroism from yellowish tint to pale yellowish green, and is optically biaxial positive with 2
V = 60(5)°, α = 1.686 (2), β (calc) = 1.694, γ = 1.720 (5), v > r perceptible. Watatsumiite occurs as short prismatic or massive aggregates in veinlets composed mainly of quartz, K-feldspar and potassicleakeite.
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