mineral
Gossan (Iron Hat)
Iron oxide-rich weathered rock (primarily Limonite and Goethite with secondary Quartz)
Hardness: 4-5.5 (variable), Color: Rusty brown, yellow-ochre, or dark red, Luster: Dull to earthy, Crystal Structure: Amorphous to microcrystalline, Specific Gravity: 3.3-4.3. Often porous or cellular.
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (variable), Color: Rusty brown, yellow-ochre, or dark red, Luster: Dull to earthy, Crystal Structure: Amorphous to microcrystalline, Specific Gravity: 3.3-4.3. Often porous or cellular.
Formation & geological history
Formed by the intense oxidation and weathering of sulfide-rich ore bodies (like pyrite or chalcopyrite). Surface waters leach metals, leaving behind a residue of iron hydroxides.
Uses & applications
Historically used as an indicator for underlying precious or base metal deposits (gold, silver, copper). Sometimes used as local iron ore or pigments.
Geological facts
Gossans were once called 'iron hats' by miners because they sit directly on top of valuable ore deposits, providing a 'cap' that signals wealth beneath.
Field identification & locations
Identify by its heavy iron staining, cellular 'boxwork' texture, and association with altered bedrock. Commonly found in mining districts globally, such as Arizona, USA or Cornwall, UK.
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