Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 99, Issue 6
December
Displaying 1-2 of 2 articles from this issue
  • Satoshi MATSUBARA, Ritsuro MIYAWAKI, Kazumi YOKOYAMA, Masaaki SHIMIZU, ...
    2004 Volume 99 Issue 6 Pages 363-367
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    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.
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  • Hiroyuki MIURA, Keiji WADA, Yoshio KATSUI
    2004 Volume 99 Issue 6 Pages 368-374
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    An outcrop of manganese oxide about one meter thick, inter-bedded with several volcanic ash layers, is found at the Yunotaki hotspring, Hokkaido, Japan. The manganese wad is precipitated from hydrothermal water. On the basis of the previously known age of the inter-bedded volcanic ash layers, linked to well-known eruptions, it has been possible to determine the chronology of the manganese oxide precipitation. The ash layers were identified by the chemical composition of the separated volcanic glasses. Since the lowermost manganese oxide layer of this outcrop occurs immediately above the mud flow of Mt. Ponmachineshiri (one volcano of Mt. Meakan) without the presence of any intermediate soil layer between them, it's age of formation could be near the age of the mud flow, i.e. 4500 - 5000 years BP. The manganese oxide layers in the top 50 cm of the outcrop are comparatively softer than those occurring below 50 cm and commonly have the texture of the aggregate of a filament structure. Manganese oxide occurring below 50 cm is hard and has a diagenetically altered texture reflecting change in the filament structure to a grain aggregate structure. These oxides have lost their structural water and the mineralogy has begun to change from todorokite (10Å phase) to birnessite (7Å phase) during the diagenetic process.
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