igneous/metamorphic derivative
Gossan (Iron Hat)
Limonitic Gossan (predominantly FeO(OH)·nH2O)
Hardness: 4-5.5 (variable); Color: Rusty red, orange, yellow, and brown; Luster: Dull to earthy; Structure: Porous, cellular, or 'boxwork' texture; Specific gravity: 3.4-4.2.
- Hardness
- 4-5
- Color
- Rusty red, orange, yellow, and brown
- Luster
- Dull to earthy
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Physical properties
Hardness: 4-5.5 (variable); Color: Rusty red, orange, yellow, and brown; Luster: Dull to earthy; Structure: Porous, cellular, or 'boxwork' texture; Specific gravity: 3.4-4.2.
Formation & geological history
Formed by the intense oxidation and weathering of sulfide mineral deposits (like pyrite or chalcopyrite) near the Earth's surface. Groundwater leaches away sulfur and metals, leaving behind a residue of iron oxides.
Uses & applications
Historically used as an indicator for underlying ore deposits (gold, silver, copper). Sometimes used as a low-grade iron ore or mineral pigment (ochre).
Geological facts
Miners call this an 'iron hat' because it sits on top of valuable mineral veins. The 'boxwork' patterns inside can often reveal what type of original minerals were once there before they dissolved.
Field identification & locations
Identify in the field by its distinct rusty color, lightweight porous feel, and lack of sulfur smell compared to the unweathered rock below. Found in mining districts worldwide, particularly in arid or semi-arid regions.
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