Mineral Formation: Fluorite
150 to 200 million years ago, hot water containing fluorine and other minerals was forced up from deep within the Earth. When this brine reached the calcium rich, limestone bedrock, fluorite crystals formed along the walls of fractures and voids in the rock. Flat layers of fluorite also formed parallel to the limestone beds, replacing the host rock.
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1000–540 million years ago, Illinois was part of Rodinia—a supercontinent that was breaking apart. Large faults formed from eastern Arkansas to southern Illinois.
525–250 million years ago, marine limestones and other sedimentary rocks were deposited in the Illinois Basin over the fault zone.
Late Mesozoic Activity
150–200 million years ago, as Pangea broke up, the faults became active again. Hydrothermal water flowed along fractures in the Mississippian limestone to form fluorite.