Secondary tungsten minerals in quartz veins in the Ishidera area, Wazuka, Kyoto Prefecture, Japan: anthoinite, mpororoite, and Fe-free hydrokenoelsmoreite

摘要

Secondary minerals of tungsten that are the products of alteration of scheelite present in quartz veins in the Ishidera area, Wazuka, Kyoto Prefecture, have been examined using XRD, SEM-EDS, EPMA, and XRF. From the results, three tungsten minerals were identified: anthoinite, mpororoite, and hydrokenoelsmoreite. The two former minerals have not been reported to be found in Japan. This is, therefore, the first discovery of anthoinite and mpororoite in Japan. The two minerals form a white powdery mixture with pseudomorphing scheelite. Chemical analysis of the mixture shows that the Al/W ratio is approximate to 1 and that the Fe_(2)O_(3) content is very low, suggesting that the ideal formulae of anthoinite and mpororoite are WAlO_(3)(OH)_(3) and WAlO_(3)(OH)_(3)·2H_(2)O, respectively, even though the original mpororoite had a high content of Fe_(2)O_(3) substituting for Al_(2)O_(3). In addition to these two minerals, another tungsten mineral was also found within the scheelite-pseudomorphs. It occurs as aggregates of regular octahedral crystals up to 50 μm in length. The XRD data are in good agreement with those for hydrokenoelsmoreite, but chemical analysis shows that the major components are WO_(3), Al_(2)O_(3), and H_(2)O with no Fe_(2)O_(3). Up to this time, only Fe-containing hydrokenoelsmoreite, once termed ferritungstite according to the old nomenclature, has been widely reported to be found in Japan. This paper is the first to report the occurrence of such an Fe-free hydrokenoelsmoreite in Japan. It is likely that these three secondary minerals of tungsten at Wazuka were formed in an environment where the supply of H_(2)O and Al_(2)O_(3) and the leaching of calcium ions from scheelite took place simultaneously. The source of Al is ascribed to the decomposition of muscovite in the quartz veins.