OPAL FROM HOSAKA, FUKUSHIMA PREFECTURE, JAPAN

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. T...

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Published inGanseki kōbutsu kōshō gakkaishi Vol. 68; no. 10; pp. 295 - 302
Main Authors Tsutsumi, Sadao, Sakamoto, Takabumi
Format Journal Article
LanguageEnglish
Published Japan Association of Mineralogical Sciences 1973
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Summary: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.
ISSN:0021-4825
1883-0765
DOI:10.2465/ganko1941.68.295