Lower Proterozoic Banded Iron from Michigan
"The Rusting of the Earth"

This banded iron testament to the rusting of the earth in the Lower Proterozoic is classic and dense, with high content of magnetite (Fe3O4), a natural magnet. The pictures speak adequately to its inherent beauty. The color changes dramatically with angle of incident light from black to metallic silver (compare the pictures). Note the various size streaks of red rusted sediment.

This rock resulted from accumulations of ferrous Iron (Fe+2) in aquatic and marine environments. The iron readily combined with the microorganism-produced and increasingly abundant oxygen to produce Fe2O3 iron precipitates, thus removing it from the atmosphere. Extensive banded iron formations (BIF) were laid down during the Proterozoic, while sedimentary rock formations such as shale, sandstone, and carbonates (i.e., limestone) were far less prevalent. By the early proterozoic some 2.3 billion years ago, oxygen levels has risen sufficiently to enable the formation of ferric oxides (Hematite) that made prominent so-called red beds.

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