sedimentary
Limonite and Goethite (Ironstone)
Hydrous Iron Oxide Mixture (FeO(OH)·nH2O)
Hardness: 4 to 5.5 on Mohs scale; Color: Yellowish-brown to dark brown or black; Luster: Dull, earthy to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7 to 4.3; Fractures: Uneven to conchoidal.
- Hardness
- 4 to 5
- Color
- Yellowish-brown to dark brown or black
- Luster
- Dull, earthy to sub-metallic
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Physical properties
Hardness: 4 to 5.5 on Mohs scale; Color: Yellowish-brown to dark brown or black; Luster: Dull, earthy to sub-metallic; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7 to 4.3; Fractures: Uneven to conchoidal.
Formation & geological history
Formed through the chemical weathering of iron-rich minerals such as pyrite, magnetite, and hematite in oxidizing environments. Often found as a secondary mineral in the gossan (rusted cap) of ore deposits or within sedimentary bog iron deposits. Some specimens date back to the Precambrian, but many are relatively modern in geological terms.
Uses & applications
Historically used as a major ore of iron; used as a natural pigment (yellow ochre) in paints, dyes, and cosmetics; occasionally used by collectors for its unique botryoidal or slag-like formations.
Geological facts
Limonite was once the primary source of 'Yellow Ochre' used by early humans for cave paintings. Because it is a mixture of several hydrous iron oxide minerals rather than a single chemical compound, it is technically classified as a mineraloid rather than a true mineral.
Field identification & locations
Identify in the field by its distinct yellow-brown streak (produced by rubbing the specimen on an unglazed porcelain plate), its high weight relative to size (high specific gravity), and its common proximity to weathered metal veins. It often exhibits a 'rusty' appearance and can be found in stream beds or as 'iron hat' surface caps.
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