Rock Identifier
Limonite (Iron Ore) (Limonite (an amorphous mixture of hydrated iron oxides, primarily FeO(OH)·nH2O)) — mineral
mineral

Limonite (Iron Ore)

Limonite (an amorphous mixture of hydrated iron oxides, primarily FeO(OH)·nH2O)

Hardness: 4.0 - 5.5 on the Mohs scale; Color: Yellow-brown to dark brown; Luster: Dull, earthy, or submetallic; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7 - 4.3.

Hardness
4
Color
Yellow-brown to dark brown
Luster
Dull, earthy, or submetallic
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Physical properties

Hardness: 4.0 - 5.5 on the Mohs scale; Color: Yellow-brown to dark brown; Luster: Dull, earthy, or submetallic; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7 - 4.3.

Formation & geological history

Formed as a secondary material through the oxidation and weathering of primary iron-bearing minerals (like pyrite or magnetite) in sedimentary or metamorphic layers. It is often found in gossans (weathered rock caps) above ore deposits.

Uses & applications

Historically used as a low-grade iron ore and as a source of yellow ochre pigment. Today, it is primarily of interest to collectors or as an indicator of underlying metallic ore deposits.

Geological facts

Limonite is not a true mineral but a 'mineraloid' because it lacks a consistent crystal structure. It was one of the earliest pigments used by humans, appearing in Neolithic cave paintings.

Field identification & locations

Identify in the field by its characteristic yellowish-brown 'mustard' streak when rubbed against a tile. Commonly found in boggy areas (Bog Iron), mining waste piles, or as coatings on other stones. Collectors look for botryoidal (grape-like) habits or pseudomorphs.

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