Proterozoic Banded Iron Stromatolites Section From Michigan

Description: This banded iron is metasediment (sediment that has undergone metamorphism) and has a high content of magnetite (Fe3O4), making it a natural magnet, also resulting in the midnight black coloration. Mminute crystalline forms of other mineral-based compounds are also present; Whether the sediment was of bioorganic origin (stromatolite) is indeterminable, but likely.

The specimen has been hand-polished to a glassine finish on one side with fine carbide and diamond abrasive, a very labor-intensive job.

The reflected colors of the polished side highly depends on the angle of reflected light. Depending on angle, colors vary from black to gray and red to silver. This is a very pretty example of banded iron that portrays a defining event in geological history and evolution: the rusting of the earth due to bacterial-produced oxygen that also led to the creation of atmospheric oxygen. At the time this banded iron was formed, the earth had been rusting for about 1.5 billion years, and about another 500 million years would be required before the process exhausted the marine iron supply.