|  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. 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 labor-intensive job.
The
polished side's reflected colors 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 encodes a defining event in geological history and evolution,
i.e., 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
1,500 million years, and about another 500 million years would
be required before the process exhausted the marine iron supply. |
