sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (predominantly Hematite, Magnetite, and Chert)
Hardness: 5.5-7 (individual layers vary), Color: Alternate bands of reddish-brown, black, and gray, Luster: Sub-metallic to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture in chert layers), Specific Gravity: 3.0-5.3.
- Hardness
- 5
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Physical properties
Hardness: 5.5-7 (individual layers vary), Color: Alternate bands of reddish-brown, black, and gray, Luster: Sub-metallic to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture in chert layers), Specific Gravity: 3.0-5.3.
Formation & geological history
Formed in marine environments during the Precambrian Eon (mostly 3.8 to 1.8 billion years ago) through the precipitation of iron oxides and silica as photosynthesis increased atmospheric oxygen level.
Uses & applications
Primary global source of iron ore used in steel production. High-quality specimens with 'Tiger Iron' chatoyancy are used in lapidary work and jewelry.
Geological facts
These formations are our primary evidence for the 'Great Oxidation Event.' Without the cyanobacteria that caused these rocks to form, Earth's atmosphere might never have become oxygen-rich.
Field identification & locations
Identify by distinct rhythmic layering and high density (feels heavy for its size). Test for magnetism with a strong magnet. Commonly found in ancient cratons like the Superior Province (USA/Canada), or Western Australia.
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