sedimentary
Gossan (Iron Hat)
Gossan (secondary iron oxide cap)
Hardness: 5-6 (variable), Color: Rusty red, yellow, brown, and weathered green/black, Luster: Dull to earthy, Structure: Vuggy or cellular (porous), Cleavage: None
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Physical properties
Hardness: 5-6 (variable), Color: Rusty red, yellow, brown, and weathered green/black, Luster: Dull to earthy, Structure: Vuggy or cellular (porous), Cleavage: None
Formation & geological history
Formed by the intense oxidation of sulfide-bearing ore deposits. Acidic groundwater leaches minerals, leaving behind a residue of iron hydroxides like limonite and hematite.
Uses & applications
Historically used as a visual indicator for prospectors to find underlying ore deposits (gold, silver, copper). Sometimes processed for its iron content.
Geological facts
Gossan is often called 'iron hat' because it sits atop an unoxidized ore body. It was a primary tool for early miners to strike it rich without modern equipment.
Field identification & locations
Identify by its 'rusty' appearance and much lighter weight than the minerals it replaced. Commonly found in mining districts of Arizona, Australia, and Cornwall.
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