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Goethite

Kobushi Mine

Azusayama, Kawakami city, Minami-Saku county, Nagano pref., Japan

IMA approved name

Goethite pseudomorph after pyrite

Goethite

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)

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