sedimentary
Banded Iron Formation (BIF) / Tiger Iron variety
Banded Iron Formation (composed primarily of Magnetite Fe3O4, Hematite Fe2O3, and Chert/Quartz SiO2)
Hardness: 6.5–7 on Mohs scale; Color: Alternating bands of silver-grey, black, and rusty brown/red; Luster: Metallic to sub-metallic on iron bands, dull to vitreous on chert; Structure: Microcrystalline and layered; Specific Gravity: 3.0–5.0 (high density due to iron content).
- Hardness
- 6
- Color
- Alternating bands of silver-grey, black, and rusty brown/red
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Physical properties
Hardness: 6.5–7 on Mohs scale; Color: Alternating bands of silver-grey, black, and rusty brown/red; Luster: Metallic to sub-metallic on iron bands, dull to vitreous on chert; Structure: Microcrystalline and layered; Specific Gravity: 3.0–5.0 (high density due to iron content).
Formation & geological history
Formed in Precambrian marine environments (mostly 2.4 to 1.8 billion years ago) through the rhythmic precipitation of iron oxides and silica. This occurred when photosynthetic cyanobacteria began releasing oxygen into the oceans (The Great Oxygenation Event), reacting with dissolved ferrous iron.
Uses & applications
Major industrial source of iron ore for steel production. High-quality colorful specimens (Tiger Iron) are used in lapidary work for cabochons, carvings, and bookends.
Geological facts
These formations record the transition of Earth's atmosphere from anaerobic to oxygen-rich. Many of the world’s largest iron mines, such as those in the Hamersley Range of Australia or the Mesabi Range in the USA, are located in these deposits.
Field identification & locations
Identify in the field by its distinct heavy weight for its size and its rhythmic, zebra-like layering. Many specimens are weakly magnetic. Commonly found in Western Australia, Brazil, South Africa, and the Lake Superior region of North America.
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