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September 2009 - Banded Iron Formation

Jasperite Boulder
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.

Mineral of the Month Archives

May 2007: Rainbow Fluorite

June 2007: Lake Superior Michipicoten Agate

July 2007: Labadorite

August 2007: Rain Flower Agate

Fall 2007: Malachite

December 2007: Nepheline Syenite

January 2008: Native Copper

February 2008: Amazonite

March 2008: Lake Superior Agate

April 2008: Shadow Agate

May 2008: Apohpylite

June 2008: Ocean Jasper

Summer 2008: Marra Mamba Tiger's Eye

September 2008: Mohawkite

October 2008: Mexican opal

November 2008: Prehnite

December 2008: Picture Jasper

January 2009: Sea Shell Jasper

February 2009: Polychrome Jasper

March 2009: Selenite Desert Rose

Spring 2009: Coyamito Agate

July 2009: Obsidian Needles

August 2009: Goethite

September 2009: Banded Iron Formation

Fall 2009: Fairburn Agate

March 2010: Fossilized Dinosaur Bone

April/May: 2010 Kentucky Agate

June 2010: Nantan Meteorite

July 2010: Mookaite Jasper

Aug/Sept 2010: Polyhedroid Agate

Fall 2010: Ammonite Fossil

September 2011: Petoskey Stones

Spring 2011: Petrfied Wood

Winter 2011: Argentina Condor Agate

January 2012: Mary Ellen Jasper

March 2012: Mexican Crazy Lace Agate

June 2012: Moqui Marbles

September 2012: Chlorastrolite Greenstone

March 2013: Jacobsville Sandstone

August 2013: Unakite

November 2013: Skip-an-Atom Agate

April 2014: Tiger's Eye

September 2014: Black Corundum

February 2015: Condor Agate

June 2015: Petoskey Stone

November 2015: Slag

June 2016: Lake Superior Copper Replacement Agates

March 2017: Chert

July 2017: Kona Dolomite

December 2017: Septarian Nodule

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Gitche Gumee Museum.
E21739 Brazel Street
Grand Marais, Michigan 49839


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