sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation with Hematite and Jasper
Hardness: 5.5-7 (Mohs); Color: Alternating bands of silver-grey (hematite/magnetite), red (jasper), and yellow/orange (limonite/goethite); Luster: Metallic to dull; Structure: Fine-grained, persistent layering; Specific Gravity: 3.0-5.3
- Hardness
- 5
- Luster
- Metallic to dull
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Physical properties
Hardness: 5.5-7 (Mohs); Color: Alternating bands of silver-grey (hematite/magnetite), red (jasper), and yellow/orange (limonite/goethite); Luster: Metallic to dull; Structure: Fine-grained, persistent layering; Specific Gravity: 3.0-5.3
Formation & geological history
Formed in ancient oceans during the Precambrian (3.8 to 1.8 billion years ago). These are chemical sedimentary rocks formed when oxygen produced by cyanobacteria reacted with dissolved iron in seawater, causing it to precipitate as iron oxide layers on the seafloor.
Uses & applications
BIFs are the primary global source of iron ore used in the production of steel. Highly aesthetic specimens (like Tiger Iron or Jasper-rich BIF) are used for making jewelry, cabochons, and decorative ornamental items.
Geological facts
These rocks provide the most significant physical evidence for the 'Great Oxidation Event', a period when oxygen first accumulated in Earth's atmosphere. Most of the world's iron currently used in skyscrapers and cars was mined from BIFs.
Field identification & locations
Identify by characteristic distinct, parallel banding of iron oxides and silica. Often magnetic if magnetite is present. Found in ancient 'cratons' like the Hamersley Range in Australia, the Lake Superior region in North America, and the Transvaal Basin in South Africa.
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