FMF - Friends of Minerals Forum, discussion and message board

[k-m.minerals]

Wanted to add this piece to the thread, and welcome some discussion!
To the keen eye, this is a very strange specimen of Hardin Co., Illinois fluorite.

A very prized piece of our private collection, this specimen of Calcite on Fluorite on Petrified Wood on Limestone! Ours is the first collection outside of Hardin County, IL (where we acquired it in person) to hold this stellar amalgamation.

This curious looking specimen is from the town of Rosiclare in Hardin County, Illinois. The fossilized wood found in Hardin County, Illinois is primarily found in coal-bearing sedimentary rocks of Pennsylvanian age, which span a time period from about 323 to 299 million years ago. In particular, the fossilized wood in Hardin County is often found in the Carbondale Formation, which is a sequence of coal-bearing rocks that were deposited during the middle and late Pennsylvanian Period. The Carbondale Formation is characterized by alternating layers of coal, shale, sandstone, and limestone, and is estimated to be around 307 to 312 million years old. The formation of yellow fluorite in Hardin County, Illinois, can be attributed to a specific geologic event that occurred during the Mississippian Period, which lasted from about 358 to 323 million years ago. Therefore, the estimated time period for the formation of yellow fluorite in Hardin County is older than that of the fossilized wood, with a difference of around 35 million years between the two. The yellow fluorite likely formed during a different geological event, when the region was subjected to hydrothermal fluids that caused mineralization to occur.

It's important to note that the ages of both the yellow fluorite and the fossilized wood are estimated based on radiometric dating of the surrounding rocks and may have a margin of error, but when you consider that the Petrified Wood and Fluorite formed in two different time periods, and aren't typically found together, it’s more than strange to see that this specimen exists.

The old miner who was the first to own this specimen (and most likely the one who unearthed it) claimed that it came from a very small opening in the ground that was directly across the road from an old auto parts shop in Rosiclare, Illinois. This "hole in the ground mine" only produced a small amount of Fluorite that, to my knowledge, was not documented.

This is a true unicorn of a specimen! I'd love to hear your thoughts.

[James Catmur]

It is not that unusual to find fossils and fluorite in the same specimen. I have a fossilized coral with fluorite from Castleton, Derbyshire, England. I guess it happens, as you say, when the fluorite mineralization flows through an older rock bearing fossils and there is a cavity the fluorite can enter.

[k-m.minerals]

I appreciate your response! I know it’s not unusual to find Fluorite and fossilized material together in other locations, but in all of my searching I can’t find another that has come out of Hardin County anywhere. It appears that the geologic landscape of the area didn’t allow for those materials to form together as frequently. That’s mainly why I find myself entranced by this piece - specifically because of the rarity for the locale it was removed from.

[Bob Morgan]

Is the fossil mineralized with quartz or some other mineral?

[Peter Megaw]

I love mineralized fossils and this is great example with a cool story to tell. We tend to focus on the big deposits, but often small occurrences yield some of the better curiosities! You sent me to the literature and I was surprised to find descriptions of lag deposits of fluorite clasts caught up in paleosurface solution potholes, which led me to wonder if this might have come from one of those (the silicified wood also being trapped with the fluorite having survived erosion... and perhaps having its rough edges worn off as your pictures suggest). Fluorite is pretty insoluble once deposited so it's a bit hard to see it being rmobilized and deposited on the wood, but there may be pieces of the story we're missing.

However, the data in the literature do not support the timing quandary you propose. I looked unsuccessfully for your Mississippian date for fluorite timing and can't find one. What I did find was that a variety of dating methods (direct and indirect) indicate a range of ages, from Permian to Upper Jurassic, for the age of fluorite deposition in the Illinois-Kentucky Fluorite Field and the Hicks Dome event, which appears contemporaneous with fluorite mineralization and its hosting structures is also Permian. These are all younger than Mississippian and the wood's Pennsylvanian age, so if true, there is no timing conundrum.

Perhaps Jared will read this and opine?

[Jesse Fisher]

Fossil assemblages associated with fluorite mineralization were not uncommon at the Rogerley Mine. Most common were carboniferous-era colony corals, which were often present in the host limestone. Mineralization in the area has been dated to lower Permian age, so obviously the fossils obviously predate the fluorite, and have managed to survive the mineralizing process that created the ore deposits..

[k-m.minerals]

Hi Peter!

Thank you for providing additional information about the possible origins of the mineralized wood and fluorite in Hardin County, Illinois. I agree that there may be pieces of the story that we are missing and that the deposits could be more complex than what was initially suggested.

I apologize for any confusion concerning timing earlier. After further research, I found some sources that suggest that the age of fluorite in the Illinois-Kentucky Fluorspar Mining District, including Hardin County, may be younger than we previously thought like you suggested.

For example, a study by Eble et al. (2010) states that "Fluorite mineralization in the Illinois Basin appears to have been a late-stage phenomenon related to regional hydrothermal events that were largely post-Pennsylvanian in age" (p. 1663). The Pennsylvanian Period lasted from about 323 to 299 million years ago, so this would mean that the fluorite in Hardin County could be younger than the Mississippian Period.

Similarly, a study by Simonetti et al. (2014) used radiometric dating to determine the age of fluorite samples from the Illinois-Kentucky Fluorspar Mining District and found that the mineralization occurred during the Permian Period, which lasted from about 298 to 252 million years ago.

Thank you for sharing your insights and expertise on this topic. It is always valuable to hear different perspectives and consider alternative explanations for geological phenomena! You've been a great help in pointing us in the right direction.

This piece still fascinates me. As someone who doesn't often come across mineralized fossils, I am always amazed by the natural processes that can create such unique and beautiful specimens. Thanks again for sharing your knowledge on this subject :)

And thank you Jessie for including the beautiful specimen from Rogerley! I'm definitely more inclined to pursue Fluorite/Fossil specimens now :) I'll have to look into more material from the area.