Unique Proterozoic Stromatolites from Minnesota
Banded Iron Deposit with Hydrothermal Rhodochrosite Intrusion
Time: Early Proterozoic. ~ 1.8 Billion Years Old
fossil is 115 mm by 50 mm by 35 mm thick (maximum)
Tommald Formation, Merritt #2 Mine, Cayuga North Range, Crow Wing County
Description: This banded iron is metasediment (sediment that has
undergone metamorphism) and has a high content of magnetite (Fe3O4),
resulting in the midnight black and red coloration. Minute crystalline
forms of other mineral-based compounds are also present; Whether
the sediment was of bioorganic origin (stromatolite) is indeterminable,
but likely. The rich pink sections are the result of intrusion by
Rhodochrosite (Manganese Carbonate) and Rhodonite (Manganese Iron
Magnesium Calcium Silicate). 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.
Since the manganese minerals are intrusive, they are obviously younger,
although by how much is not known.
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 coloration seen here is most striking.
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.
click stromatolites pictures