banded iron formation (BIF)
A type of sedimentary rock consisting
of alternating layers of iron-rich and iron-poor
rocks. Most deposits of this type were laid down about 2.5 billion years
ago at a time when Earth's atmosphere still consisted mainly of nitrogen
and carbon dioxide. Oxygen
given off as a waste product by early photosynthetic bacteria immediately
bonded with the abundant dissolved iron to yield minerals like magnetite
and hematite. Banded iron formations (BIFs)
occur worldwide and give rich iron ore deposits in parts of Minnesota and
Michigan.
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• ORIGIN
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