sedimentary

Limonite (Ironstone Concretion)

Limonite (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4 to 5.5 on Mohs scale; Color: yellowish-brown to dark brown or blackish; Luster: earthy to sub-metallic; Crystal structure: Amorphous/Microcrystalline; Cleavage: none; Specific Gravity: 2.7 to 4.3.

Hardness: 4 to 5
Color: yellowish-brown to dark brown or blackish
Luster: earthy to sub-metallic

Identified July 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms as a secondary mineral through the oxidation and hydration of iron-rich minerals (like pyrite or magnetite) or by precipitation from iron-rich groundwater. Found in sedimentary deposits and weathered rock zones (gossans).

Uses & applications

Historically used as an iron ore and as a pigment for paints (brown ochre). It is mostly collected as a mineral specimen today, especially when forming unique botryoidal or concretionary shapes.

Geological facts

Limonite is not a singular mineral but a mixture of hydrated iron oxides, primarily goethite. It was one of the earliest pigments used by humans, appearing in Neolithic cave paintings.

Field identification & locations

Identified by its yellow-brown streak (when rubbed on unglazed porcelain) and its relative weight compared to non-metallic rocks. Common in boggy environments and weathered soil profiles. Collectors look for unique 'moqui marbles' or iridescent surfaces.