sedimentary
Iron Gossan (Limonite/Goethite)
Hydrated Iron Oxides (FeO(OH)·nH2O) from oxidation zones
Hardness: 4-5.5 (Mohs scale); Color: Yellowish-brown to rusty dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous to microcrystalline; Cleavage: None; Specific Gravity: 2.7–4.3
- Hardness
- 4-5
- Color
- Yellowish-brown to rusty dark brown
- Luster
- Dull to earthy
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Yellowish-brown to rusty dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous to microcrystalline; Cleavage: None; Specific Gravity: 2.7–4.3
Formation & geological history
Formed by the chemical weathering and oxidation of primary sulfide minerals (like pyrite or chalcopyrite) in an ore deposit's 'cap.' This process occurs over geological time as oxygenated groundwater percolates through metallic veins near the Earth's surface.
Uses & applications
Historically used as a low-grade iron ore. In exploration geology, it serves as a 'signpost' for miners to find deeper deposits of copper, gold, or silver. Small amounts are used as mineral pigments (ochre).
Geological facts
Gossans are known as 'iron hats' and are famous for marking several of the world's largest gold and copper discoveries. Because they represent the 'rust' of a metal deposit, they often contain rare secondary minerals in the vugs (cavities) seen in the specimen.
Field identification & locations
Identify in the field by its distinct 'rusty' appearance, porous/cellular 'boxwork' texture, and yellow-brown streak. Found globally in mining districts. Collectors should look for micro-crystals inside the small holes (vugs) using a magnifying glass.
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