mineral

Limonite (Bog Iron)

Limonite (Hydrated iron(III) oxide-hydroxide) - FeO(OH)·nH2O

Hardness: 4.0-5.5 on Mohs scale; Color: Yellow-brown to dark brown; Luster: Dull, earthy, or sub-metallic; Crystal Structure: Amorphous (pseudomorphous); Cleavage: None; Specific Gravity: 2.7–4.3.

Hardness: 4
Color: Yellow-brown to dark brown
Luster: Dull, earthy, or sub-metallic

Identified March 15, 2026More mineral →

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Physical properties

Hardness: 4.0-5.5 on Mohs scale; Color: Yellow-brown to dark brown; Luster: Dull, earthy, or sub-metallic; Crystal Structure: Amorphous (pseudomorphous); Cleavage: None; Specific Gravity: 2.7–4.3.

Formation & geological history

Formed as a secondary material through the oxidation or weathering of iron-rich mineral ores (like pyrite or magnetite). Often precipitates in swampy or marine environments as 'bog iron.' Age varies from recent to hundreds of millions of years based on source deposit.

Uses & applications

Historically used as a major iron ore. Today, it is primarily used as a pigment (yellow ochre) in paints, an additive in concrete, and for geological study. It has low value in jewelry but is significant for industrial mineral testing.

Geological facts

Limonite is not a true mineral but a mixture of hydrated iron oxides. It was used as far back as the Stone Age as a cave painting pigment. It is also found in meteorites and is responsible for the yellowish color of much of the Martian surface.

Field identification & locations

Identify by its distinctive yellowish-brown streak on a porcelain plate and its rusty, earthy appearance. Common in the iron ranges of North America (Minnesota, Michigan), Germany, and France. In the field, look for 'iron staining' and heavy, brittle rocks near wetlands or weathered outcrops.

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