sedimentary
Banded Iron Formation (Chert/Jasper)
Banded Iron Formation (BIF)
Hardness: 6.5-7 (Mohs); Color: Grey, brownish-yellow, and dark metallic; Luster: Dull to sub-metallic; Structure: Microcrystalline (Cryptocrystalline); Cleavage: None (Conchoidal fracture).
- Hardness
- 6
- Color
- Grey, brownish-yellow, and dark metallic
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Grey, brownish-yellow, and dark metallic; Luster: Dull to sub-metallic; Structure: Microcrystalline (Cryptocrystalline); Cleavage: None (Conchoidal fracture).
Formation & geological history
Formed in ancient seawater billions of years ago (Precambrian) as a result of oxygen production by cyanobacteria, which caused dissolved iron to precipitate out of the oceans as minerals like hematite or magnetite interleaved with silica.
Uses & applications
Primarily used as iron ore for steel production; polished specimens are used in lapidary art, jewelry, and for geological education.
Geological facts
These rocks represent a 'heartbeat' of the early Earth, recording the Great Oxygenation Event that occurred roughly 2.4 to 2.0 billion years ago.
Field identification & locations
Identify by the distinct fine layers or bands of varying colors. Common in the Lake Superior region (USA/Canada), Australia, and Brazil. Collectors look for high-contrast banding.
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