sedimentary
Gossan
Iron-rich regolith (mainly Limonite and Goethite, FeO(OH)·nH2O)
Hardness: 4-5.5 (variable); Color: rusty red, brown, yellow, and black; Luster: dull to earthy; Structure: porous, cellular, or massive; Specific Gravity: 3.3-4.3.
- Hardness
- 4-5
- Color
- rusty red, brown, yellow, and black
- Luster
- dull to earthy
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Physical properties
Hardness: 4-5.5 (variable); Color: rusty red, brown, yellow, and black; Luster: dull to earthy; Structure: porous, cellular, or massive; Specific Gravity: 3.3-4.3.
Formation & geological history
Formed by the intense oxidation and weathering of sulfide mineral deposits (like pyrite or chalcopyrite) near the Earth's surface. As surface water reacts with sulfides, it creates sulfuric acid, leaching metals and leaving behind a residue of iron oxides.
Uses & applications
Historically used as an indicator for underlying ore deposits (gold, silver, copper). Can be used as a low-grade iron ore or mineral pigments.
Geological facts
Minerals collectors and prospectors historically called gossans 'iron hats' because they sit on top of valuable ore veins. Gold is often concentrated in gossans because it is chemically stable while other minerals wash away.
Field identification & locations
Identified by its 'rotten' or honeycomb appearance and deep rust colors. Usually found in arid or semi-arid mining districts. Collectors look for them as 'pathfinder' rocks for better specimens below.
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