mineral

Limonite

FeO(OH)·nH2O (Hydrated iron oxide)

Hardness: 4.0 - 5.5; Color: Yellow-brown to blackish-brown; Luster: Dull, earthy, or sub-metallic; Crystal structure: Amorphous (does not form distinct crystals); Specific Gravity: 2.7 - 4.3; Streak: Yellowish-brown.

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

Identified April 7, 2026More mineral →

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Physical properties

Formation & geological history

Formed as a secondary material through the oxidation and hydration of iron-rich sulfide and carbonate minerals. It is often found in the weathered 'gossan' zones above iron ore deposits and in sedimentary bog iron deposits.

Uses & applications

Historically used as an iron ore (though now largely replaced by hematite/magnetite) and widely used as a natural pigment (yellow ochre). It is also collected as a geological specimen.

Geological facts

Limonite was a primary source of iron for basic ironworking in antiquity. It is what gives many soils and sedimentary rocks (like sandstone) their characteristic yellow or rusty brown coloring.

Field identification & locations

Identify in the field by its characteristic yellowish-brown streak on a ceramic plate, its earthy smell when damp, and its yellow-brown color. It is common in bog environments and weathered outcrops.