metamorphic
Banded iron formation (iron-rich metamorphic rock)
Banded iron formation, typically hematite–magnetite–quartz (Fe₂O₃/Fe₃O₄/SiO₂)
AI-generated identification · may be incorrectDark gray to black, fine- to medium-grained, with rusty orange weathering from iron oxides and lighter quartz-rich bands. Hardness is about 5.5–7; metallic to earthy luster, no consistent cleavage, and commonly strongly magnetic if magnetite-rich.
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Physical properties
Dark gray to black, fine- to medium-grained, with rusty orange weathering from iron oxides and lighter quartz-rich bands. Hardness is about 5.5–7; metallic to earthy luster, no consistent cleavage, and commonly strongly magnetic if magnetite-rich.
Formation & geological history
Chemical sediment deposited in ancient Precambrian oceans, then compacted and metamorphosed; most examples formed roughly 3.8–1.8 billion years ago during periods of widespread oceanic oxidation.
Uses & applications
Major iron ore when hematite or magnetite is abundant; attractive pieces are used as lapidary specimens, bookends, and geological teaching samples.
Geological facts
Alternating silica and iron-oxide layers record the rise of oxygen in Earth’s atmosphere and helped create many of the planet’s largest iron deposits.
Field identification & locations
Identify by thin, contrasting iron-rich and quartz-rich bands, heavy weight, rusty staining, and a magnetite response to a strong magnet. Common in the Lake Superior region, Western Australia, Brazil, South Africa, India, and Ukraine.
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