metamorphic
Banded iron formation (iron-rich metamorphic rock)
Banded iron formation, chiefly hematite/magnetite with chert or quartz (Fe₂O₃/Fe₃O₄ + SiO₂)
AI-generated identification · may be incorrectHardness about 5.5–6.5; gray, brown, reddish, and yellowish bands with earthy to submetallic luster. Fine-grained, layered structure; individual minerals are not clearly crystalline in the photo, and cleavage is absent or poor.
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Physical properties
Hardness about 5.5–6.5; gray, brown, reddish, and yellowish bands with earthy to submetallic luster. Fine-grained, layered structure; individual minerals are not clearly crystalline in the photo, and cleavage is absent or poor.
Formation & geological history
Chemical sediment deposited in ancient oceans and later lithified and metamorphosed, commonly 2.5–1.8 billion years old. Alternating silica-rich and iron-rich layers formed as ocean chemistry and oxygen levels changed.
Uses & applications
Major industrial source of iron ore where sufficiently enriched; attractive pieces are used as educational specimens, lapidary material, and décor. It is generally not used as a gemstone unless polished.
Geological facts
The reddish-brown colors usually reflect hematite and oxidation, while dark gray portions may contain magnetite. A magnet may attract magnetite-rich areas, and a streak test can help distinguish hematite (red-brown streak) from magnetite (black streak).
Field identification & locations
Common in Precambrian terrains, especially the Lake Superior region, Western Australia, Brazil, India, and South Africa. Look for thin, repetitive silica-and-iron bands, high density, and possible magnetism; the image could also represent an iron-rich metamorphic fragment rather than classic BIF.
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