sedimentary
Gossan (with Limonite and Hematite)
Goethite (FeO(OH)), Limonite (FeO(OH)·nH2O), and Hematite (Fe2O3)
Hardness: 4.0 - 5.5 (Mohs scale); Color: Rusty orange, reddish-brown, yellowish-ochre to dark grey/black; Luster: Dull, earthy, or sub-metallic; Crystal structure: Amorphous or microcrystalline; Streak: Yellowish-brown (Limonite) to reddish-brown (Hematite).
- Hardness
- 4
- Luster
- Dull, earthy, or sub-metallic
- Streak
- Yellowish-brown (Limonite) to reddish-brown (Hematite)
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Physical properties
Hardness: 4.0 - 5.5 (Mohs scale); Color: Rusty orange, reddish-brown, yellowish-ochre to dark grey/black; Luster: Dull, earthy, or sub-metallic; Crystal structure: Amorphous or microcrystalline; Streak: Yellowish-brown (Limonite) to reddish-brown (Hematite).
Formation & geological history
Formed through the oxidation and weathering of iron-rich sulfide minerals (like pyrite or chalcopyrite) near the surface. These are often found in the 'iron hat' located above major ore bodies, typically from the Proterozoic to recent times.
Uses & applications
Historically used as an indicator for underlying valuable ore deposits such as copper, gold, or silver. Limonite is also a minor iron ore and a source of natural earth pigments (ochre).
Geological facts
Gossans are nicknamed 'iron hats'. Prospectors throughout history have used the presence of these rusty rocks to find hidden sulfide deposits underground, as the distinctive coloring signals chemical leaching has occurred.
Field identification & locations
Identify in the field by its intensely rusty, porous, or 'vuggy' texture and high weight relative to other surface rocks. Often found in mining districts or along fault lines where mineralization occurred.
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