Oiwa
Fukaura city, Nishi-Tsugaru county, Aomori pref., Japan
Synonym
Ohiwa
Occurrence
Intermediate acidic metasomatic rocks of Cristobalite zone
Spherulitic rhyolite of Azumagawa Rhyolite Member, a member of middle Miocene (20 - 18 Ma) Odose Formation.
Outcrop of spherulitic rhyolite of Azumagawa Rhyolite Member, Otose Formation. Opal spherules were formed by silicification alteration of rhyolite. Green nodules are opal spherules. The white host is rhyolite. Spherulitic rhyolite was observed in a zone of the rhyolite with flow structure.
Reported Minerals
- Opal
Mineral Assemblage
- Rhyolite(Host) - Opal
Localities
Related Occurrences
- Halloysite zone (Alunite zone, Argillized zone)
- Kaolinite zone (Alunite zone, Argillized zone)
- Minamiite-Alunite zone (Alunite zone)
- Pyrophyllite zone
- Smectite zone (Argillized zone)
- Interstratified mineral zone (Argillized zone)
- Illite-Chlorite zone (Propylite zone)
- Epidote-Actinolite zone
- Stilbite-Mordenite zone (Zeolite zone)
- Heulandite-Analcime zone (Zeolite zone)
- Laumontite-Albite zone (Zeolite z., Propylite z.)
- Wairakite-Albite zone (Zeolite zone)