Goethite
| FeO(OH)
These samples of goethite are displayed in the Smithsonian Museum of Natural History. Goethite is an oxide mineral of iron with the composition FeO(OH). This sample is about 8x7 cm and it is from Park County, Colorado .
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This sample is goethite with quartz and is about 15x15 cm. It is from Morocco.
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This goethite sample is about 18 x 20 cm and it is from Chestnut Hill, Pennsylvania .
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This goethite sample was formed after pyrite when chemical conditions changed. It retains the pyrite's basic cubic shape, so it is a pseudomorph. Pyrite is an iron sulfide, and goethite is an iron oxide hydroxide. 
This compares the goethite sample with a cubic pyrite sample from Ambassaguas mine, Amejun, La Rioja, Spain.
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These goethite samples appear to have formed as stalactites. The sample above left is about 13x17 cm and is from Germany. The sample above right is about 10x15 cm and is from La Disha mine, Guerrero, Mexico.
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This sample of goethite is about 10 x 16 cm and is from Ironwood, Michigan.
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Mindat: Goethite
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