mineral

Limonite (Iron Stone)

Limonite (Hydrated Iron Oxide) - FeO(OH)·nH2O

Hardness: 4-5.5 (Mohs); Color: Yellowish-brown to dark brown/black; Luster: Dull to submetallic; Crystal Structure: Amorphous (does not form distinct crystals); Cleavage: None; Specific Gravity: 2.7-4.3.

Hardness: 4-5
Color: Yellowish-brown to dark brown/black
Luster: Dull to submetallic

Identified March 14, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the oxidation and hydration of iron-rich mineral deposits (like pyrite or magnetite). It often forms in the oxidation zones of ore deposits or as bog iron in swampy environments. Age varies based on parent rock exposure.

Uses & applications

Historically used as a low-grade iron ore and as a pigment for ochre paints. Today, it is primarily a collector's specimen or used as a pigment in specialized paints and coatings.

Geological facts

Limonite is one of the three main ores of iron. It is often a 'pseudomorph,' meaning it replaces the internal molecules of other minerals while keeping their shape. Famous prehistoric cave paintings often used limonite for yellow and brown hues.

Field identification & locations

Identify in the field by its yellow-brown streak (scratched on porcelain) and its lack of magnetism compared to magnetite. Commonly found in gossans (weathered rock caps) and sedimentary iron beds throughout the Appalachian Mountains and the Great Lakes region.

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