sedimentary
Banded Iron Formation (BIF)
Banded Iron Formation (Magnetite-Hematite-Chert)
Hardness: 5-7 (Mohs scale), Color: Alternating bands of silver-grey (magnetite/hematite) and blue-black or red (chert), Luster: Metallic to dull, Crystal structure: Microcrystalline, Specific gravity: 3.5-5.0
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Physical properties
Hardness: 5-7 (Mohs scale), Color: Alternating bands of silver-grey (magnetite/hematite) and blue-black or red (chert), Luster: Metallic to dull, Crystal structure: Microcrystalline, Specific gravity: 3.5-5.0
Formation & geological history
Formed in ancient shallow oceans during the Precambrian eon (3.8 to 1.8 billion years ago) through the precipitation of iron and silica as photosynthesis increased oxygen levels in the oceans.
Uses & applications
Primary global source of iron ore for steel production; occasionally used for jewelry (cabochons) and ornamental carvings due to attractive banding.
Geological facts
These formations are evidence of the 'Great Oxidation Event,' recording the transition when Earth's atmosphere and oceans first began to accumulate significant oxygen.
Field identification & locations
Identified in the field by heavy weight (high density), distinct dark/light parallel layering, and magnetism if magnetite is present. Commonly found in major Precambrian shields like Australia, Brazil, and the Lake Superior region.
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