GMS Field Trip November 2024

GMS Field Trip
Copper Ore, Slag, and Sericite after Staurolite in Tennessee
Saturday, November 9, 2024

The history of this location goes back to the 1800s when copper was big business in the region. Unfortunately, early processing methods released sulfuric acid that wreaked havoc on the surrounding areas, killing almost all plant and animal life for miles around. It took many years of reengineering ore processing, and many more years to even begin repairing the ecosystem, work that continues to this day. The current owner of the property is dedicated to the environmental clean-up, so the area is recovering incredibly well and is safe for people like us to tromp around.

We often call this location a UGA – unique geological area – because it never disappoints in the diversity of the materials available. This trip marks the first time that we were treated to piles of copper ore that had been inaccessible before. The property owner moved piles of ore from one area to another to make it much easier for us to peruse. Members collected numerous samples from the piles including some stunning microminerals before we headed to our next destination – slag piles.

The slag there is the byproduct of copper ore processing from the 1800s to as late as the 1980s. Crushed ore was refined via chemical processes, then the concentrate was heated until it became molten. Flux, a substance that binds to impurities, was added to help make the impurities lighter than the copper. Impurities were skimmed off and cast aside as slag. More information about this process is available at www.tn.gov (click here or click here for the same pdf that we have downloaded). You may have seen slag that is very much like glass because of high silica content in the ore and flux. The ore at this location has a high iron content, so the slag is mostly iron.

At first thought, slag may not seem very interesting, but the slag here has some fun shapes, patterns, and often contains beautiful secondary microminerals. When the slag was poured off, it cooled into all sorts of shapes. At some point, cracking and resolidifying caused slag that has an other-worldly, reptile skin-like appearance with red and orange colors. We call it “slagon” skin, a portmanteau of slag and dragon. Members are always intrigued by it and it is one of my favorite kinds of slag to find. As water makes its way through the slag piles, secondary minerals form. Beautiful gypsum microcrystals and other micros we have yet to identify are but a loupe away from enchanting the diligent collector.

The trip continued with the opportunity to get nice and dirty digging and screening for sericite after staurolite. Staurolite is the state mineral of Georgia. Its crystals can twin to form a cross shape that is often called a “fairy cross”. Sericite is a type of mica that can form as staurolite alters during metamorphism or weathering. The alteration produces a sericite pseudomorph that retains the shape of the original staurolite, in this case, crystals or “blades” up to 6 or 7 inches long and 2 to 3 inches wide. What we have observed here is mostly incomplete alteration, so some of the staurolite remains inside the sericite pseudomorph. Most of the specimens are also peppered on the outside with tiny almandine garnets as well. Again, the owner of the property went to great lengths to dig large areas out for us and to provide large piles of material for us to search through. In one pile, members found a white sandy area that appears to be almost pure sericite, and they found sericite after staurolite specimens in it. In another area, a junior skillfully extracted specimen after specimen until he had a gallon baggie full! He donated one very large specimen to the club, and even offered specimens to other people. He had so much fun that he wanted to stay “just 20 more minutes”, but rain was beginning to tap us on the shoulder, so we had to call it a day.

As always, we cannot thank the property owner enough for everything he does to make each trip special and productive. He moves so much material around for us; stabilizes areas; makes parking easier; and he even scouts new places with new materials for us to collect! Members performed car ballets like pros as we caravanned around the property, and they were generous and kind with each other, making sure everyone got some goodies. It was a wonderful day thanks to Charles Carter and all of his hard work and time spent organizing the trip.

Lori Carter on behalf of Charles Carter
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