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March 2017 - Chert

Chert is a microcrystalline or cryptocrystalline sedimentary rock made mostly of silicon dioxide (SiO2). It can form as nodules, concretions, and as layered deposits. Like other silica rocks and minerals, chert breaks with a conchoidal fracture, often producing very sharp edges. Native Americans took advantage of this fracture pattern and intentionally knapped chert to make arrowheads and other cutting tools and weapons. Since chert forms in sedimentary rock, it often can contain fossils as well as banded layers.

There are two main ways chert forms. In some cases, chert develops when microcrystals of silica grow in deposits of limestone or chalk. This occurs when dissolved silica is transported through sedimentary layers by groundwater. Large numbers of silicon dioxide microcrystals grow from the dissolved silica into irregularly-shaped nodules or concretions. If there is a lot of silica causing large number of nodules to form, the nodules can merge together to develop a contiguous layer of chert within the sedimentary rock. When chert develops from dissolved silica it is classified as a chemical sedimentary rock.

The other way chert forms is from biologic remains. Certain marine organisms contain silica in their exoskeletons or spicules, such as sponges, radiolarian, and diatoms. When these organisms die, their remains fall to the bottom of the oceans or shallow seas. The silica dissolves, recrystallizes, and develops into chert nodules or entire layers of chert.

Most chert is tan, cream color, or gray. When iron impurities are included within the nodules or layers, chert can also be red, green, or black. In some cases red chert, or chert with other colors, is classified as jasper. The term "flint" is used to describe varieties of chert that form in chalk formations, whereas chert usually forms in limestone formations. Some people make a distinction between "flint" and "chert" as a matter of quality - chert being lower quality than flint. Sometimes jasper is also considered a higher quality of chert.



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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E21739 Brazel Street
Grand Marais, Michigan 49839


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