sedimentary
Banded Iron Formation (BIF)
Magnetite-hematite-chert formation
Hardness: 5-7 (Mohs), Color: Alternating bands of silver-black (iron) and reddish-yellow/brown (chert/jasper), Luster: Metallic to dull, Crystal Structure: Trigonal/Hexagonal, Cleavage: None/Conchoidal.
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Physical properties
Hardness: 5-7 (Mohs), Color: Alternating bands of silver-black (iron) and reddish-yellow/brown (chert/jasper), Luster: Metallic to dull, Crystal Structure: Trigonal/Hexagonal, Cleavage: None/Conchoidal.
Formation & geological history
Formed in marine environments as chemical precipitates when photosynthetic oxygen reacted with dissolved iron in Earth's early oceans. Most date to the Archean or Proterozoic Eons (3.8 to 1.8 billion years ago).
Uses & applications
Predominant global source of iron ore for steel production; lapidary use for decorative carvings and jewelry stones like Tiger Iron.
Geological facts
They represent the transition of Earth's atmosphere from oxygen-poor to oxygen-rich, known as the Great Oxidation Event.
Field identification & locations
Identify by heavy weight (high specific gravity), magnetism (if magnetite is present), and visible alternating layers of metallic iron oxides and silica. Common in Western Australia, Brazil, and the Lake Superior region.
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