Tanohataite, limn₂si₃o₈(OH): A new mineral from the Tanohata mine, Iwate Prefecture, Japan

Tanohataite, LiMn₂Si₃O₈(OH), the Li analogue of serandite, has been found in a metamorphosed manganese ore deposit of the Tanohata mine, Iwate Prefecture, Japan. The mineral has the triclinic space group P1 with a = 7.612(7), b = 7.038(4), c = 6.700(4) Å, α = 90.23(6)°, β = 94.70(7)°, γ = 105.26 (8)°, V =345.0(3) ų, and Z = 2. The seven strongest lines in the X-ray powder diffraction pattern are [d(Å), (I), (hkl)]: 6.64(35)(001), 3.67(26) (200), 3.13(89)(1̄02), 3.11(69)(21̄1), 2.95(100)(102), 2.81(33)(120), and 2.18(40)(103). Electron microprobe analysis and laser ablation microprobe-inductively coupled plasma-mass spectrometry gave an SiO₂ content of 51.97; MnO, 37.99; MgO, 1.06; CaO, 0.41; Na₂O, 1.97; Li2O, 3.34; total, 96.74 wt%, corresponding to an empirical formula of (Li_0.78Na_0.22)Σ_1.00(Mn_1.86 Ca_0.03Mg_0.09)Σ_1.98Si_3.01 O₈(OH) on the basis of O = 9. Tanohataite is transparent and pinkish white with a vitreous and silky luster. The streak is white. The cleavage is perfect on {001} and {100}. On the Mohs' scale, the hardness is 5-5^1/2. The calculated density is 3.33 g/cm³. Optically, tanohataite is biaxial positive with 2V_calc = 82(2)°, α = 1.593(3), β = 1.618(3), and γ = 1.653(3). Tanohataite occurs as an aggregation of fibrous crystals in veinlets composed mainly of quartz, aegirine, Mn-arfvedsonite, nambulite, natronambulite, and barite.