sedimentary
Banded Iron Formation (BIF) / Ferruginous Quartzite
Banded Iron Formation containing Hematite/Goethite and Chert/Quartz
Hardness: 5-7 Mohs; Color: Rusty red, dark brown, and metallic grey; Luster: Dull to sub-metallic; Crystal structure: Microcrystalline; Specific Gravity: 3.5-5.0.
- Hardness
- 5-7 Mohs
- Color
- Rusty red, dark brown, and metallic grey
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-7 Mohs; Color: Rusty red, dark brown, and metallic grey; Luster: Dull to sub-metallic; Crystal structure: Microcrystalline; Specific Gravity: 3.5-5.0.
Formation & geological history
Chemical precipitation from seawater during the Precambrian eon, where dissolved iron combined with oxygen produced by early life forms. Common in cratonic regions of West Africa like the Birimian Supergroup.
Uses & applications
Primary source of iron ore globally; smaller colorful specimens are used as garden stones or lapidary material for hobbyists.
Geological facts
These rocks represent the 'Great Oxygenation Event' of Earth's history, where cyanobacteria began pumping oxygen into the oceans, causing dissolved iron to rust out of solution and settle on the seafloor.
Field identification & locations
Identify by its weight (heavy for its size) and distinct alternating bands of iron oxides and silica. Common in northern Ghana and Burkina Faso. It will typically leave a reddish-brown streak on a porcelain plate.
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