sedimentary

Limonite with Trace Fossils

Hydrous Iron Oxide Mixture (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4-5.5 (Mohs); Color: yellowish-brown to dark brown; Luster: dull, earthy; Crystal structure: amorphous; Cleavage: none; Specific gravity: 2.7-4.3. This specimen contains small tubular trace fossils (possibly worm burrows or root casts).

Hardness: 4-5
Color: yellowish-brown to dark brown
Luster: dull, earthy

Identified April 6, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as a secondary material through the oxidation and hydration of iron-rich mineral deposits or by chemical precipitation in bogs. It often replaces other minerals or preserves fossils in sedimentary environments.

Uses & applications

Historically used as an iron ore (yellow ochre) and as a pigment for paints and cosmetics. Today, it is mostly of interest to geologists and collectors of sedimentary structures.

Geological facts

Limonite is not a true mineral but a mixture of hydrous iron oxides, primarily goethite. It is the substance that gives many soils their yellow or rusty color and is often found in 'ironstone' formations.

Field identification & locations

Identify in the field by its distinct mustard-yellow streak and earthy appearance. Look for it in areas with weathered iron-bearing rocks or wetlands. The presence of small tubes or burrows suggests it formed in a lagoonal or shallow marine environment.