Tokyoite, Ba_2Mn^<3+>(VO_4)_2(OH), a new mineral from the Shiromaru mine, Okutama, Tokyo, Japan

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Tokyoite, Ba2Mn3+(VO4)2(OH), the Mn3+-analogue of gamagarite, is found from the Shiromaru mine, Okutama, Tokyo, Japan. It is monoclinic, P21/m, a = 9.10(4), b = 6.13(2), c = 7.89(5) Å, β = 112.2(5)°, Z = 2. The strongest lines in the powder X-ray diffraction pattern are 3.73 (15) (111), 3.31 (100) (112), 3.08 (20) (020), 3.00 (16) (301, 212, 102), 2.90 (19) (120), 2.80 (62) (121, 300), 2.16 (18) (321), 1.963 (15) (204). Electron microprobe analysis gave V2O5 31.77, SiO2 0.15, Al2O3 0.07, Fe2O3 2.33, Mn2O3 11.27, CaO 0.07, BaO 51.91, SrO 0.22, Na2O 0.13, H2O (calc.) 1.59, total 99.51 wt. %, and lead to the empirical formula, (Ba1.92Na0.02Sr0.01Ca0.01)Σ1.96(Mn3+0.81Fe3+0.17Al0.01)Σ0.99[(V1.99Si0.01)O7.92](OH)1.00 on the basis of V + Si = 2 and (OH) = 1. It is translucent and black red in color with a vitreous luster. The streak is dark brownish red and cleavage is not observed. The Mohs hardness is about 4 to 4.5. The calculated density is 4.62 g/cm3. It has distinct pleochroism, from reddish orange to dark brownish red in thin section and the calculated mean refractive index is 2.03. It occurs as irregular grains under 15 μm, and the aggregates are up to 250 μm wide in association with braunite, hyalophane and tamaite. The mineral is formed under the later stage activity of low grade metamorphism.
2004-12-01