sedimentary

Limonite (Ironstone) Concretion

FeO(OH)·nH2O (Hydrous ferric oxide)

AI-generated identification · may be incorrect

Hardness: 4-5.5 (Mohs scale); Color: Yellow-brown to dark brown/black; Luster: Dull to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3

Hardness: 4-5
Color: Yellow-brown to dark brown/black
Luster: Dull to sub-metallic

Identified July 1, 2026More sedimentary →

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Physical properties

Hardness: 4-5.5 (Mohs scale); Color: Yellow-brown to dark brown/black; Luster: Dull to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3

Formation & geological history

Formed through the oxidation and hydration of iron-rich minerals in sedimentary environments. Common in bog iron deposits or as cement in sandstones. Found globally, especially in oxidized zones of iron ore deposits.

Uses & applications

Historically used as a low-grade iron ore. Currently used as an ocher pigment in paints and as a geological specimen for education and mineral collecting.

Geological facts

Limonite is not a true mineral but a mineraloid mixture of various hydrous iron oxides. It gives many desert sands and soils their characteristic rusty yellow-brown color. The specimen displays a typical 'desert varnish' or weathered crust appearance.

Field identification & locations

Identify by its brownish-yellow streak, lack of magnetism (unlike magnetite), and density. Common in the coordinates provided (Visakhapatnam, India region), which features khondalite and charnockite belts known for iron-rich weathering products, laterites, and hematite/limonite deposits.