Alteration States and Forming Minerals of Volcanic Products at Kuchinoerabu-jima Island, Kyushu

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  • 口永良部島における火山噴出物の変質状態と生成鉱物
  • クチノエラブジマ ニ オケル カザン フンシュツブツ ノ ヘンシツ ジョウタイ

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Abstract

Kuchinoerabu-jima Island, Tokara Islands, located in south of Kyushu, consists of plural volcanic bodies, and most of the rocks belong to hyperstheneaugite-andesite. The alteration phenomena of volcanic products in the island were investigated chemically and mineralogically.<BR>1) Most of the altered materials at the old thermal spring and solfatara fields are very siliceous and mainly consist of opal accompanied by alunite, kaolin, chalcedony-quartz TiO2 minerals. These extremely advanced stages of alteration were resulted from the hydrogen metasomatism of strong acid solution containing SO42-.<BR>2) Among the opaline minerals disordered a-cristobalite is predominant. Besides this, amorphous opal and β form like cristobalite in association with tridymite are found in small amounts. TiO2 minerals are mainly leucoxinic anatase and rutile. They are contained about 1-3 wt% in almost all altered materials except one of siliceous pyroclastic materials contained 8.3% of TiO2. One of the reason of such a high content of TiO2 was due to the change of distribution in particle size of altered materials during before and after eruption.<BR>3) Some argillized zones i.e. the assemblages of alunite, halloysite, kaolinite and micaceous mineral are formed at the altered areas in the west part of the island. These argillations were carried out at low temperature and with decreasing acidity of solution. Kaolinite in very fine particle size seems to be formed from mica mineral through the secondary alteration process in which Al2O3 is accumulated.

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