Fluorite (cleavage octahedron)

specimen number:
2292316

location:
Southern Illinois Fluorspar District, Illinois, USA

description:

An attractive polished cleavage fluorite octahedron. The size provided measures from point to point.

The Southern Illinois Fluorspar District, centered in Hardin and Pope counties, forms the northern portion of the famous Illinois-Kentucky Fluorspar District. Geologically, the region is characterized by complex faulting and the intrusion of igneous rocks, such as those at Hicks Dome, which occurred during tectonic activity in the Permian period. Mineralization took place when warm, mineral-bearing hydrothermal fluids rose through these fault systems, depositing fluorite (calcium fluoride) alongside sphalerite, galena, and barite within Mississippi Valley-type ore bodies. These mineral deposits occurred as both vertical vein fillings and horizontal bedding-replacement deposits in Mississippian-age limestones. Historically, early 19th-century miners in the district initially targeted lead, treating the fluorite as waste. However, the late 19th-century development of basic open-hearth steel manufacturing, which utilized fluorite as a flux, created a massive industrial market. The district subsequently boomed, supplying critical raw materials during both World Wars and eventually leading to the designation of fluorite as the Illinois state mineral in 1965, before cheap foreign imports forced the last local mining operations to close in 1995.

During the height of the Illinois-Kentucky Fluorspar District mining era in southern Illinois, underground miners developed a specialized pocket-knife crafting tradition during their breaks. Taking raw, blocky cubic masses of fluorite (locally called fluorspar), they carefully tapped the stone along its four perfect cleavage planes to split the mineral cleanly into eight-sided, double-pyramid geometries. This process bypassed the natural cubic crystal habit to reveal a internal octahedral form. Miners referred to these hand-cleaved octahedrons as "diamonds" or "devils dice". Because the last operating underground mines in Hardin County closed in 1995, these artifacts, showcasing vibrant purple, yellow, and blue zoning, have become historic, hand-crafted representatives of regional mining heritage.

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