sedimentary
Gossan
Ferruginous gossan (Iron-rich capping)
Hardness: 5-6 (variable), Color: Rusty brown, orange, red, and yellow, Luster: Dull to earthy, Crystal Structure: Amorphous/massive or after sulfide minerals, Specific Gravity: 3.5-4.5
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Physical properties
Hardness: 5-6 (variable), Color: Rusty brown, orange, red, and yellow, Luster: Dull to earthy, Crystal Structure: Amorphous/massive or after sulfide minerals, Specific Gravity: 3.5-4.5
Formation & geological history
Formed by the oxidation and leaching of sulfide-rich ore bodies at the surface. Rainwater and oxygen react with metal sulfides like pyrite to create iron oxides and hydroxides like limonite and hematite.
Uses & applications
Used historically as an indicator ('iron hat') by prospectors to find underlying valuable metal deposits of copper, gold, or silver.
Geological facts
The term comes from the Cornish language word 'gos' meaning blood, referring to the red color of the iron oxides. Gossans have led to the discovery of major mining districts worldwide.
Field identification & locations
Identify in the field by its sponge-like, porous (cellular) texture and intense rust coloring compared to surrounding rocks. Commonly found in arid or semi-arid mineralized terrains.
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