mineral

Limonite (Iron Ore)

Hydrous ferric oxides (FeO(OH)·nH2O)

Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3.

Hardness: 4
Color: Yellowish-brown to dark brown
Luster: Dull to earthy

Identified January 5, 2026More mineral →

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Physical properties

Hardness: 4.0-5.5; Color: Yellowish-brown to dark brown; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7-4.3.

Formation & geological history

Formed as a secondary mineral through the oxidation and hydration of iron-rich minerals such as pyrite, magnetite, or hematite. It often forms in near-surface environments like swamps (bog iron) or gossans (weathered tops of ore veins).

Uses & applications

Historically used as a major iron ore; currently used as a natural earth pigment (ochre) in paints, dyes, and coatings, and sometimes as a low-value gemstone when polished.

Geological facts

Limonite is not a true mineral but a mineraloid mixture of various iron hydroxides. It was the source of 'Bog Iron' used during the Industrial Revolution and has been used as a pigment since prehistoric times (cave paintings).

Field identification & locations

Identify in the field by its characteristic yellowish-brown streak on a porcelain plate and its earthy, rusty appearance. It is commonly found in weathered zones of sulfide deposits and in sedimentary bog environments. Collectors value it as a pseudomorph (where it replaces the shape of another crystal).

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