sedimentary
Gossan (Iron Cap)
Limonitic Gossan (Fe2O3·nH2O mixed)
Hardness: 4.0-5.5 (variable), Color: Rusty brown, yellow, or ocher, Luster: Dull or earthy, Structure: Highly porous or cellular (boxwork), Cleavage: None
- Hardness
- 4
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Physical properties
Hardness: 4.0-5.5 (variable), Color: Rusty brown, yellow, or ocher, Luster: Dull or earthy, Structure: Highly porous or cellular (boxwork), Cleavage: None
Formation & geological history
Formed by the intense oxidation and weathering of primary sulfide mineral deposits (like pyrite or chalcopyrite) near the earth's surface. Groundwater leaches away soluble metals and leaves behind iron oxides.
Uses & applications
Historically used as a surface indicator for underlying metal ore deposits (gold, silver, copper). Sometimes used as low-grade iron ore or pigment.
Geological facts
Miners call this 'iron hat' because it sits atop a mineral vein. It is a sign of secondary enrichment where valuable metals may have concentrated just below the water table.
Field identification & locations
Identify by its 'burnt' or rusty appearance and lightweight, porous structure. Commonly found in arid or temperate regions over mineralized zones. Collectors value them as indicators of locality.
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