sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (Fe2O3 + SiO2)
Hardness: 5.5-7 (Mohs scale); Color: Metallic grey, black, and reddish-brown alternating bands; Luster: Sub-metallic to dull; Crystal structure: Trigonal/Hexagonal; Cleavage: None/Indistinct; Specific Gravity: 3.5-5.3.
- Hardness
- 5
- Color
- Metallic grey, black, and reddish-brown alternating bands
- Luster
- Sub-metallic to dull
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Physical properties
Hardness: 5.5-7 (Mohs scale); Color: Metallic grey, black, and reddish-brown alternating bands; Luster: Sub-metallic to dull; Crystal structure: Trigonal/Hexagonal; Cleavage: None/Indistinct; Specific Gravity: 3.5-5.3.
Formation & geological history
Formed in marine environments billions of years ago (mostly Archean and Proterozoic eons, 2.4 to 1.8 billion years ago) when oxygen produced by photosynthetic cyanobacteria reacted with dissolved iron in the oceans to precipitate alternating layers of iron oxide and silica.
Uses & applications
Primary source of iron ore globally; used in the manufacture of steel for construction, machinery, and vehicles. High-quality lapidary specimens (like Tiger Iron) are used in jewelry and ornaments.
Geological facts
BIFs are the primary evidence for the 'Great Oxidation Event'. They are responsible for almost all the iron we use today. Some of the most famous deposits are in the Hamersley Range in Western Australia and the Mesabi Range in Minnesota, USA.
Field identification & locations
Identify by the distinct, rhythmic alternating layers of dark iron minerals (magnetite/hematite) and lighter silica (chert/quartz). It is often heavy for its size and can be weakly magnetic. Commonly found in ancient cratonic regions.
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