The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists
Online ISSN : 1883-0765
Print ISSN : 0021-4825
ISSN-L : 0021-4825
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Volume 68, Issue 10
Displaying 1-3 of 3 articles from this issue
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  • OPAL FROM HOSAKA, FUKUSHIMA PREFECTURE, JAPAN
    Sadao Tsutsumi, Takabumi Sakamoto
    1973 Volume 68 Issue 10 Pages 295-302
    Published: October 05, 1973
    Released on J-STAGE: August 07, 2008
    DOIhttps://doi.org/10.2465/ganko1941.68.295
    JOURNAL FREE ACCESS
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    It is well known that opal occurs from Hosaka, Fukushima Pref., northeastern Japan. The opal is found as crusts of cauliflower-like nodular masses and patchy areas in a plagioliparite or pearlite. Commonly it is milky white or bluish white in colour and rarely shows a colour play of precious opal. The section of nodular massive opal shows a distinct banded structure consisting of successive layers with different translucency. Of these layers, transparent or translucent ones are usually composed of spherulitic chalcedony, while milky white layers are occupied by opal. In addition, it was found that montmorillonite and clinoptilolite are associated with opal.
    The X-ray powder diffraction patterns of opal from Hosaka are divided into three as follows; (1) a more or less disordered type of low cristobalite with a characteristic reflection at about 4.3Å. (2) a diffuse pattern resembled high cristobalite with 4.3Å reflection. (3) a diffuse pattern resembled high cristobalite. Chemical analyses of several kinds of opal with different degree of crystallinity indicated that SiO2 decreased with the disordering, whereas Al2O3, Fe2O3, MgO, CaO, alkalies and H2O increased.
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  • PARGASITES IN LHERZOLITE AND WEBSTERITE INCLUSIONS FROM ITINOME-GATA, JAPAN
    KEN-ICHIRO AOKI, IKUKO SHIBA
    1973 Volume 68 Issue 10 Pages 303-310
    Published: October 05, 1973
    Released on J-STAGE: August 07, 2008
    DOIhttps://doi.org/10.2465/ganko1941.68.303
    JOURNAL FREE ACCESS
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    Pargasite occupies up to 4 volume per cent of lherzolite inclusions and up to 8 per cent of websterite inclusions. It occurs as discrete primary crystals and as interstitial grains among other minerals averaging less than 2 volume per cent. New analyses are presented for six representative pargasites. They are characterized by high Cr2O3 and wide variation of K2O and Mg/(Mg+Fe) ratio.
    The following origin of the pargasites is hypothsized from the mineralogy and chemistry of the host rocks. Original garnet lherzolite and websterite were located at a depth of up to 50 to 75km. During upward transport, to 25km deep in the uppermost part of the mantle, the garnet was finally transformed to olivine+plagioclase+spinel and both orthopyroxene and clinopyroxene were recrystallized toward a relatively rich Tschermak's molecule. Jadeite and Tschermak's components separated from pyroxenes and silica liberated from the breakdown of garnet produced plagioclase, pargasite and orthopyroxene. However, minor newly-formed minerals were not always in equilibrium with major constituents.
    It is demonstrated that the lherzolite mantle just below the M-discontinuity contains only up to 0.04 per cent water.
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  • CLAY MINERALS IN THE MIOCENE LOW-GRADE METAMORPHIC ROCKS, TANZAWA MOUNTAINS, KANAGAWA PREFECTURE, CENTRAL JAPAN
    KEIJI KIMBARA
    1973 Volume 68 Issue 10 Pages 311-328
    Published: October 05, 1973
    Released on J-STAGE: August 07, 2008
    DOIhttps://doi.org/10.2465/ganko1941.68.311
    JOURNAL FREE ACCESS
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    [in Japanese]
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