metamorphic
Banded iron formation (likely iron-rich metamorphic rock)
Banded iron formation; hematite–magnetite with chert/jasper (Fe₂O₃/Fe₃O₄ and SiO₂)
AI-generated identification · may be incorrectDark metallic to earthy bands alternate with rusty brown iron-oxide zones; typically fine-grained, dense, and hard (about Mohs 5.5–6.5). The specimen shows planar banding and weathered hematite-rich surfaces; magnetism may occur if magnetite is present.
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Physical properties
Dark metallic to earthy bands alternate with rusty brown iron-oxide zones; typically fine-grained, dense, and hard (about Mohs 5.5–6.5). The specimen shows planar banding and weathered hematite-rich surfaces; magnetism may occur if magnetite is present.
Formation & geological history
Chemical sediment deposited in ancient oceans, commonly 2.5–1.8 billion years ago, then compacted, lithified, and often metamorphosed. Oxidation during weathering produces the reddish-brown limonite/hematite coloration.
Uses & applications
Used as iron ore where grade is sufficient; attractive pieces are sold as lapidary rough, polished slabs, or display specimens. This rough sample has limited commercial value unless unusually magnetic or from a notable locality.
Geological facts
Banded iron formations record the oxygenation of Earth’s early atmosphere, when dissolved iron precipitated from seawater. Major examples occur in the Lake Superior region, Western Australia, Brazil, India, and South Africa.
Field identification & locations
Identify by alternating iron-rich dark bands and silica-rich lighter bands, high density, rusty streak, and possible attraction to a magnet. A streak test, magnet test, and fresh fracture are more diagnostic than surface color.
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