MINERAL OF THE MONTH: September 2009 – Banded Iron Formation

In the iron-rich areas around Lake Superior, there are deposits of banded iron formation. In addition to being called banded iron formation, many rockhounds label specimens as Jasperite, Jaspite, or Jaspelite. This is a distinctive type of rock found in Precambrian sedimentary rocks. The structures consist of repeated layers of iron oxides, either magnetite or hematite, alternating with bands of iron-poor chert or jasper. Some of the banded iron formations date back to 3 billion years ago. However, most formed around 1.8 billion years ago: others are much younger.

The total amount of oxygen locked up in the chemical bonds of the banded iron beds is believed to be perhaps twenty times the volume of oxygen present in the modern atmosphere. Banded iron beds are an important commercial source of iron ore. They exist in several areas of the world including Western Australia, Minnesota, and the Upper Peninsula of Michigan.

Scientists believe that the banded iron layers formed in sea water as the result of oxygen released by photosynthetic bacteria. The oxygen combined with dissolved iron in the oceans to form insoluble iron oxides, which precipitated out forming a thin layer at the bottom of the ocean. It seems that the amount of oxygen varied, perhaps by season, so that at other times chert or jasper formed intermediate layers when oxygen levels were lower.

The picture of the Jasperite boulder included above is in a boulder garden at my friends house (Kat and Ken), who live near Aitkin, MN. The other photo I took of a specimen for sale at the Ishpeming Gem and Mineral Show earlier this month.

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