sedimentary
Limonite Concretion
Limonite (Hydrated Iron Oxide) FeO(OH)·nH2O
Hardness: 4-5.5 (Mohs), Color: Yellowish-brown to dark brown/black, Luster: Dull or earthy, Crystal structure: Amorphous or microcrystalline, Cleavage: None, Specific gravity: 2.7-4.3
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs), Color: Yellowish-brown to dark brown/black, Luster: Dull or earthy, Crystal structure: Amorphous or microcrystalline, Cleavage: None, Specific gravity: 2.7-4.3
Formation & geological history
Formed by the precipitation of secondary iron minerals within sedimentary rock layers, often as minerals grow around a nucleus like a leaf, shell, or pebble in oxygenated water environments.
Uses & applications
Primarily of interest to collectors and geologists; historically used as a low-grade iron ore and as a source of yellow pigment (ochre).
Geological facts
Concretions like this are often mistaken for meteorites or fossils due to their unusual shapes. They can range in size from tiny beads to giant boulders over several feet wide.
Field identification & locations
Identify by its heavy weight relative to its size and its distinct yellowish-brown streak when rubbed on unglazed porcelain. Commonly found in sandstone or shale beds.
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