sedimentary

Limonite Concretion (Ironstone)

Limonite / Goethite (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4-5.5 (Mohs); Color: Dark brown to yellow-brown or reddish; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7–4.3.

Hardness: 4-5
Color: Dark brown to yellow-brown or reddish
Luster: Dull to earthy

Identified April 3, 2026More sedimentary →

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Physical properties

Hardness: 4-5.5 (Mohs); Color: Dark brown to yellow-brown or reddish; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7–4.3.

Formation & geological history

Formed by the precipitation of iron oxides/hydroxides from groundwater within sedimentary rock layers, often as nodules or concretions in sandstone or shale. They can form over thousands to millions of years.

Uses & applications

Historically used as a low-grade iron ore. Today, used primarily for educational specimens, aquascaping, and as earth-tone pigments (ochre).

Geological facts

Limonite is not a true mineral but a mixture of hydrated iron oxide minerals. These rounded nodules are sometimes called 'shaman stones' or 'Moqui marbles' in specific regions like the Southwestern USA.

Field identification & locations

Identify in the field by its high density (heavy for its size), rusty streak when scratched, and characteristic pitted or rounded 'nodular' surface. Commonly found in eroded sedimentary basins.