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December 2017 - Septarian Nodule

Septarian nodules are round concretions found in sedimentary rocks. Concretions are hard solid masses formed by the accumulation of matter within sediment. Although scientists do not agree on the details and specifics of their formation, there are several theories. One proposal suggests they formed when there was dehydration and shrinkage of clay, gel, or organic cores within sedimentary pockets. Others believe there was an expansion of gases produced by the decay of organic matter that fractured material within sedimentary pockets. One more theory is that an earthquake, compaction, or other geologic forces fractured material within sedimentary pockets. However the sedimentary material fractured, mineral-rich fluids filled in the spaces between the fractures allowing calcite, siderite, pyrite, and other minerals to crystalize and fill in the open areas within the cavity.

The most well-known septarian nodules are found in Utah. The Utah septarians developed during the Cretaceous period between 50 and 70 million years ago. At this time the Gulf of Mexico had expanded northwest into the area that is now southern Utah. Scientists believe that volcanic eruptions killed marine organisms, causing them to sink to the bottom of the shallow sea. The decomposing material chemically attracted sediments causing the mixture to form into mud balls. When the ocean receded, the mud balls dried out causing the interior sections to crack and shrink. Over time, mineral-rich solutions infiltrated the cracks causing crystals to form and fill in the cracks. Septarian nodules, sometimes called lightning stones, can also found on the Lake Michigan shoreline as well as in New Zealand, England, Morocco, and Madagascar.

Septarians are composed of calcite (the yellow centers), aragonite (the brown lines), and limestone (the outer grey surface).  Occasionally, fossils can be seen within or on the surface of the nodules.


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


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