Archaean Banded Iron from Wisconsin

This banded iron is metasediment (sediment that has undergone metamorphism) and has a high content of magnetite (Fe3O4), a natural magnet, and yielding the midnight black coloration; minute crystalline forms of other mineral-based compounds are also present. The color changes with angle of incident light from black to metallic silver.

This Archaean banded iron was laid down prior to Earth’s Great Oxygenation Event that began some 2.45 billion years ago. It was putatively formed in sea water because of oxygen produced by abundant photosynthetic cyanobacteria combining with dissolved iron in Earth's oceans to form insoluble iron oxides that precipitated to form thin layer upon thin layer of sediment. The banding likely resulted from cycling of available oxygen levels.

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 lighter areas are small calcite crystals.