Banded Iron Formation
(c) 2000 Andrew Alden, licensed to About.com, Inc. (fair use policy)
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.
The black parts of this polished slab are thin layers of dark, semi-metallic hematite, and the red layers are jasper, an iron-rich chert. The gray and bright golden areas are part of the same layers, made of tiger-eye quartz. It forms when quartz replaces the fibrous mineral crocidolite (also known as blue asbestos). The dark mineral flashes in golden highlights as you turn it in the light, an effect called chatoyancy.
Learn more about the ancient origin of iron.
See more sedimentary rocks in the Sedimentary Rocks Gallery.
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