The Cave-In-Rock area of Hardin County in southern Illinois is known for producing some of the most spectacular display quality fluorite specimens on the planet. However, the fluorescent properties of this material can be hit or miss. Some specimens show show little to no fluorescence under UV, while others fluoresce typical blue due to activation by trace amounts of rare earth elements (REEs) in the fluorite crystal structure. In some cases, myriad hydrocarbon/petroleum inclusions will show a bright white response under UV, illuminating the fluorite from within. The specimen featured here has it all. This cluster of sky blue fluorite cubes is from the Minerva No. 1 Mine, and its beauty under visible light is spectacular. Under UV it fluoresces blue, brighter than average for Cave-In-Rock fluorite. In addition, some of the cubes contain brightly fluorescent hydrocarbon inclusions, visible as bright white wisps and blebs. The size of this specimen is 75 x 63 x 33 mm, and it weighs 138 grams.
Cluster of sky blue fluorite cubes from the Minerva No. 1 Mine, Cave-In-Rock, Illinois. Shown above under visible light.
Under short wave UV (254 nm), this fluorite shows a blue fluorescent response. Note the random, bright white fluorescent hydrocarbon inclusions, visible as wisps and small blebs, mainly on the right side of the specimen.
As shown above, the brightest fluorescent response is seen under long wave UV (365 nm), producing intense, deep, saturated blue fluorescence. Interestingly, the hydrocarbon inclusions do not fluoresce as brightly under long wave UV as they do under short wave.
Historically, the Minerva No. 1 mine exploited the largest bedding replacement orebody in the United States, and operated from the early 1940s until 1996. It changed ownership several times and was a producer of both fluorspar and zinc. It was also a famous producer of some of the most beautiful mineral specimens found in the Cave-In-Rock area. Today, little remains of the Minerva other than its tailings.
The site of the former Minerva No. 1 Mine as it appear today. All traces of the mining structures have been removed and the tailings are bulldozed flat. This photograph was taken in September, 2018.