Unique Proterozoic Banded Iron Stromatolite from Minnesota

Description: This banded iron is metasediment (sediment that has undergone metamorphism) and has a high content of magnetite (Fe3O4). Minute crystalline forms of other mineral-based compounds are also present; Whether the sediment was of bioorganic origin (stromatolite) is indeterminable, but likely. This one shows an unusual association of Silkstone (also known as Minnesota Tiger Eye) with the banded iron formation. Note the distortion of the bands near the wall of the intrusion. This material is the result of hydrothermal activity which is typically accompanied by various extremophilic organisms, making for a most unique duality of biogenic sources. In the 80 years of operation from 1904 over 100 million tons of iron ore was recovered from the Cuyuna Iron Range, much of which had measurable quantities of Manganese. See my other offerings for an example showing the pink Manganese-rich Rhodochrosite.

The specimen has been hand-polished to a glassine finish on one side with fine carbide and diamond abrasive, a very labor-intensive job. It comes from a 30-year-old deposit which is no longer available. The contrasting coloration seen here is most striking.

Stromatolites have persisted to the modern day in such places as Shark Bay, Australia where they continue their billions of years old lifestyle. While this piece would make a fine present for any natural history buff, anyone who appreciates art would also be glad to get thus wonderful specimen as a gift.