Can you explain " Banded iron formation " ?

Question

Thanks.

Answer 1

Banded iron formations are very large bodies of sedimentary rock laid down some 2.5 billion years ago. At that time, the Earth still had its original atmosphere of nitrogen and carbon dioxide. That would be deadly for us but it was hospitable to many different microorganisms in the sea, including the first photosynthesizers. These organisms gave off oxygen as a waste product, which immediately bonded with the abundant dissolved iron to yield minerals like magnetite and hematite.