Goethite
Kobushi Mine
Azusayama, Kawakami city, Minami-Saku county, Nagano pref., Japan
IMA approved name
Goethite pseudomorph after pyrite
Width of the cubic crystal: 10 mm. Goethite replacing pyrite. The earthy brown crust is dirt.
Recovered from a meteoric oxidation zone of skarn formed by contact metamorphism of S-type (Ilmenite-series) diorite.
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)