Goethite
Mount Takeshi
Shimo-Takeshi, Ueda city, Nagano pref., Japan
Closed to collecting as it is a natural monument of Ueda city
IMA approved name
Goethite pseudomorph after pyrite
Width of the cubic crystal: 10 mm. Goethite replacing pyrite. The brown host is metasomatised rhyolite - dacite tuff of Reisenji Volcanics Member, Uchimura Formation.
Recovered from a meteoric oxidation zone of metasomatised tuff.
The first Goethite pseudomorph after pyrite from Japan was reported from this location in 1893.
The formation process of this specimen is considered by reactions relating iron bacteria in oxygen-containing meteoric water. For example:
[Inorganic reaction]
2FeS2 (Pyrite) + 7O2 + 2H2O = 2Fe2+ + 4SO42- + 4H+
[Iron bacteria]
4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O
[Inorganic reaction]
4Fe3+ + 2H2O + 3O2 = 4FeOOH (Goethite)
Other localities
- Kobushi Mine (Pneumatolytlic Skarn, replacing Pyrite)
- Chichibu Mine (Hydrothermal Skarn, Reniform)
- Nakatatsu Mine (Hydrothermal Skarn, Reniform)
- Mikawa Mine (Hydrothermal, Acicular)
- Gunma Mine (Bog Iron, replacing Plants)
- Sukayu hot spring (Bog Iron)
- Mount Takeshi (Tuff, replacing Pyrite)