Fluorite with Calcite

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A thin plate of deep blue crystallized fluorite, the crystals measuring to 1.3 cm on edge. Needlelike crystals of calcite are scattered throughout. Beautiful when backlit, in good condition. From the 1970s.pThe Minerva Mine No. 1, situated near Cave-In-Rock within Hardin County, Illinois, is a historic underground operation renowned as a premier source of fluorite, sphalerite, galena, and barite specimens. Geologically, the deposit belongs to the famous Illinois-Kentucky Fluorspar District and consists of stratiform, Mississippi Valley-Type (MVT) bedding-replacement deposits hosted within Mississippian-age limestone formations. Hydrothermal fluids migrating along regional faults dissolved portions of the limestone, precipitating minerals in open breccias and expansive solution cavities where multi-colored, chemically zoned fluorite cubes grew alongside highly lustrous sphalerite complexes. Large-scale commercial underground operations at the mine commenced in 1942 under the Minerva Oil Company and continued through subsequent acquisitions by companies including Allied Chemical and Ozark-Mahoning. Industrial production ceased when the mine was permanently closed and flooded in 1996 due to foreign competition and declining economic reserves, concluding over five decades of continuous major operations in the district.