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MINERAL OF THE MONTH

November 2015 - Slag

Throughout the past 17 years since re-opening the Gitche Gumee Museum (after it was closed by its founder 21 years previous), each summer many people bring pieces of slag in for identification hoping that the specimens are agate. Slag can be found on the beaches west of Munising, MI, as well as at several other places in Michigan in other areas where blast furnaces were used to purify ore. I can understand why people think these specimens are agate since oftentimes like agates; they have conchoidal fractures, structure or other patterns, and translucency. I have learned over the years how to let people down easily and educate them at the same time.

Originally there were 29 blast furnaces in the Upper Peninsula of Michigan that were used to melt down and purify iron ore.  Only two sites remain including the Bay Furnace in Christmas, and Fayette on the Garden Peninsula east of Escanaba.  Iron ore was first discovered in the U.P. in the 1840s.  Although the iron ore rock was up to 72 percent pure, it was necessary to remove the impurities and extract the iron.

Iron is purified from iron ore in a huge container called a blast furnace. Beginning in the 1840s in Michigan's Upper Peninsula, iron ores such as hematite and magnetite were mined and then transported to blast furnaces.á To purify the ore, the rock was added to the blast furnace along with limestone and charcoal.á The mixture was heatedáto around 2282░F (1250░C) almost 300 degrees below iron's melting point of 2786░F (1538░C).

In this reduction reaction, the charcoal was used to heat the mixture and add carbon to the chemical reaction.  The limestone served as a flux that helped to catalyze the desired reaction and chemically bind to and remove impurities, such as silica.  In this reaction, the iron oxide was reduced to iron, the carbon was oxidized to carbon dioxide, and the impurities were formed into glass-like slag, which was separated and removed. A picture of the Bay Furnace in its reconstructed condition is shown below.

CITES: https://en.wikipedia.org/wiki/Slag

 


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

June 2012: Moqui Marbles

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

Winter 2011: Argentina Condor Agate

Spring 2011: Petrfied Wood

September 2011: Petoskey Stones

January 2012: Mary Ellen Jasper

March 2012: Mexican Crazy Lace Agate

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

August 2016: Puddingstone


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

 


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