Watatsumiite, KNa₂LiMn₂V₂Si₈O₂₄, a new mineral from the Tanohata mine, Iwate Prefecture, Japan

Abstract

Watatsumiite, KNa₂LiMn₂V₂Si₈O₂₄, 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) ų 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 SiO₂ 52.64, TiO₂ 3.13, VO₂ 15.10, FeO 0.35, MnO 12.28, MgO 1.61, CaO 0.03, BaO 0.88, Na₂O 7.10, K₂O 4.89, Li₂O 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_7.97O₂₄ 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 VHN₁₀₀ 707-946 kg/mm² (Mohs 51/2-6). The calculated density is 3.24 g/cm³. Watatsumiite has moderate pleochroism from yellowish tint to pale yellowish green, and is optically biaxial positive with 2V = 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.