Author: ChrisN

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.

CITES

• https://en.wikipedia.org/wiki/Concretion
• Tony Hisgett, https://commons.wikimedia.org/wiki/File:Calcite_septarian_nodule_ROM.jpg
• Sven Teschke, https://commons.wikimedia.org/wiki/File:Septarie_by_Steschke.JPG
• Keith Pomakis, https://commons.wikimedia.org/wiki/File:Septarian_Nodule.jpg
• http://www.therockshed.com/information.html

This rock is named after the Kona Hills, located in Marquette County, Michigan. This is an ancient formation of fossil stromatolite that is between 2.1 and 2.8 billion years old. Most dolomite found throughout the world is gray or white. Kona dolomite is quite colorful and is found nowhere else. Dolomite is a calcium-magnesium carbonate and fizzes in warm acid.  Dolomite is limestone with magnesium added. The Kona formation is dominantly dolomite with interstratified layers of shale, graywacke and quartzite. The dolomite beds are from a few inches to many feet thick, but even the purest beds contain thin cherty layers and other sedimentary clastic material.

The original organisms that created this fossil rock were some of the first life on earth. There was nothing to eat so these early cyanobacteria survived by using the energy of the sun via photosynthesis. Cyanobacteria were responsible for producing the first oxygen on earth as a byproduct of this photosynthetic process. Thus, if it were not for stramatolitic organisms, according to scientists, we would probably not be here.

The cyanobacteria grew in shallow water as flat mats to maximize exposure to the sun. The fossil rock formed as the cyanobacteria mats trapped, bound and, and cemented sedimentary grains. Over time, as the trapped sediments blocked the energy of the sun, the cyanobacteria would develop another mat on top of the mound. The mounds grow in various shapes including conical, stratiform layers, branching, and domal.

The earliest stromatolite mounds are believed to be 3.7 billion years old. They peaked around 1.25 billion years ago. Originally scientists believed these species of cyanobacteria were extinct, until live stromatolite mounds were discovered in Sharks Bay, Australia in 1958. Even though 99 percent of all species that have ever existed on earth are extinct – it is amazing that stromatolitic organisms were the first living things on earth – and they are still here. Living stromatolites mounds can be found in Australia, Chili, Brazil, Mexico, Belize, British Columbia and the Yukon (Canada), Minnesota (US), South Africa, and the Bahamas. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone.

Fossilized stromatolite rock can be found throughout the world. Kona dolomite is one example. Because of its tremendously old age, and due to trace minerals, Kona dolomite can be found in a variety of colors including pink, brown, yellow, tan, cream, red, and orange. The colors are often complex with mottling, banding and lacing. The black wavy lines in Kona are the remains of the microbial mats. The unique pink and red pigments are caused by the iron in the soil.

In the rough before polishing the rock does not look nearly as nice – but still cool.

Lapidary enthusiasts enjoy working with this rock because they make beautiful slabs used in crafting clocks, wind chimes, spheres, bookends, cabochons, and other items. Kona is also used for building stones and for making refractory bricks for furnace linings. Due to its vivid coloration it is also used for decorative purposes as crushed stone for borders and walkways.

CITES

• http://www.michigan.gov/documents/deq/GIMDL-PU21B_216237_7.pdf
• Mathew Parker, https://commons.wikimedia.org/wiki/File:Tolypothrix_(Cyanobacteria).JPG
• Vincent Pointer, https://commons.wikimedia.org/wiki/File:20130118-HighbormeCay-Stromatolite-03.JPG

NOTE

• Thanks to Bob and Peg Clark for their donations of Kona dolomite specimens and slabs to the Gitche Gumee Museum.

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.

This web page update features the copper replacement agate. They can only be found in the Keweenaw Peninsula, located in Michigan’s Upper Peninsula. They can be found in the mine dumps near abandoned copper mines from the Kearsarge Lode. Most of these agates are small – less than an inch in diameter. They are extremely rare and difficult to find, usually requiring labor intensive work to free them from the basalt matrix rock. These agates are well sought after because of their rarity, their interesting patterns, and the vast array of other mineral inclusions. In addition to copper these agates often have other mineral inclusions including silver, calcite, malachite, tenorite, epidote, and pumpellyite, In most cases these small agates are fully husked, requiring them to be cut to expose their inner beauty.

The copper replacement agate shown below includes malachite, pumpellyite, and epidote inclusions.

The agate below has white chalcedony bands on the left and clear calcite on the right. Copper is in the outer shell, surrounding the specimen. The green is epidote and the red is a copper oxide.

The two photos below show copper replacement agate nodules in basalt matrix rock. In addition to copper there is also prehnite, epidote, and pumpellyite (dark green inclusions). These photos were taken by Dave Schuder.

The specimen shown below has basaltic matrix on the left and copper replacement on the right. The copper bands alternate with the chalcedony bands. This photo is from my agate book, Agates Inside Out.

CITATION: The first three photos shown above (used with permission) are from the Wayne W. Sukow collection http://www.sukowssuperiorminerals.com.

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.  To purify the ore, the rock was added to the blast furnace along with limestone and charcoal.  The mixture was heated to 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