sedimentary

Gossan (Limonite/Ironstone)

Hydrated Iron Oxide (FeO(OH)·nH2O)

Hardness: 4-5.5 (Mohs scale); Color: Rusty brown, yellowish-orange to reddish-brown; Luster: Dull to earthy, greasy when wet; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 2.7–4.3

Hardness: 4-5
Color: Rusty brown, yellowish-orange to reddish-brown
Luster: Dull to earthy, greasy when wet

Identified July 3, 2026More sedimentary →

Explore Gossan (Limonite/Ironstone) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed by the oxidation of sulfide minerals (like pyrite or chalcopyrite) in an ore deposit. As water and oxygen react with the minerals, sulfuric acid is produced, leaving behind these iron-rich oxides as a cap or 'hat' over the deposit.

Uses & applications

Historically used as an indicator for miners to find valuable underground metal deposits (gold, silver, copper). Sometimes used as a low-grade iron ore or mineral pigment (ochre).

Geological facts

Miners call these 'iron hats' because they sit on top of ore veins. They are often porous or 'vuggy' because the original minerals have been leached away.

Field identification & locations

Identify by its rusty color, heavy weight compared to non-metallic rocks, and staining. In the field, its presence suggests chemical weathering of minerals nearby. Commonly found in mining districts and arid environments.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary