sedimentary
Iron-rich Gossan with Quartz
Limonite and Goethite with Silicon Dioxide (SiO2)
Hardness: 5-7 (quartz is harder, oxides are softer); Color: Rust-red, yellow-brown, orange; Luster: Dull to earthy (oxides) or vitreous (quartz); Crystal Structure: Amorphous/masses; Cleavage: None.
- Hardness
- 5-7 (quartz is harder, oxides are softer)
- Color
- Rust-red, yellow-brown, orange
- Luster
- Dull to earthy (oxides) or vitreous (quartz)
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Physical properties
Hardness: 5-7 (quartz is harder, oxides are softer); Color: Rust-red, yellow-brown, orange; Luster: Dull to earthy (oxides) or vitreous (quartz); Crystal Structure: Amorphous/masses; Cleavage: None.
Formation & geological history
Formed by the intense oxidation and weathering of sulfide mineral deposits (such as pyrite or chalcopyrite) in the presence of oxygenated groundwater. This process leaves behind a 'cap' or gossan of iron oxides.
Uses & applications
Historically used as an indicator for mining prospectors to locate underlying ore bodies (gold, silver, copper). Today, it is primarily of geological interest or used as low-grade iron ore.
Geological facts
Gossans are often called 'iron hats' because they sit atop unweathered mineral veins. They are famous for being the surface signpost that led to many of the world's greatest gold and copper rushes.
Field identification & locations
Identify by the characteristic 'rusty' colors and porous or boxwork texture. Look for them in mining districts. For collectors, they represent the chemistry of ore alteration.
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