mineral

Limonite (Ironstone)

Limonite (mixture of hydrated iron(III) oxide-hydroxides)

Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown; Luster: Submetallic, dull, or earthy; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Hardness: 4
Color: Yellowish-brown to dark brown
Luster: Submetallic, dull, or earthy

Identified June 15, 2026More mineral →

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Physical properties

Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown; Luster: Submetallic, dull, or earthy; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Formation & geological history

Formed by the oxidation and hydration of iron-rich mineral deposits (like pyrite or magnetite) near the surface; typical in sedimentary bog environments or as a weathering crust. Likely from the Triassic basins common near Durham, NC.

Uses & applications

Historically used as a low-grade iron ore and as a source of yellow ochre pigment. Currently used for specimen collecting and occasionally in cement production.

Geological facts

Limonite is not a true mineral but a mineraloid mixture. It often forms pseudomorphs, meaning it replaces the internal structure of other minerals while keeping their outward shape.

Field identification & locations

Identified by its characteristic yellowish-brown streak, earthy appearance, and lack of magnetism compared to magnetite. Common in the Piedmont region of North Carolina as 'ironstone' or 'concretionary' material.

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