Millerite

specimen number:
2102373

location:
Hall's Gap, Lincoln Co., Kentucky, USA

description:
Classic specimen from contracting, an open geode half lined with crystallized millerite. In great condition.

The Halls Gap locality, situated along a prominent highway roadcut on U.S. Route 27 near Waynesburg in Lincoln County, Kentucky, is an internationally famous mineral occurrence renowned for its unique, self-contained pocket assemblages of acicular, hair-like millerite crystals. Geologically, the site does not represent a conventional ore body or massive vein system, but rather an exposure of the Mississippian-age Muldraugh Member of the Borden Formation, a sedimentary unit composed of argillaceous dolostone and silty limestone. Within this strata, numerous hard, unyielding quartz geodes ranging from one to five inches in diameter developed hollow interior cavities lined with chalcedony, quartz, and saddle-shaped dolomite crystals, which served as protective hosts for the crystallization of delicate, capillary nickel sulfide filaments of brass-yellow millerite alongside micro-crystals of pyrite, chalcopyrite, and secondary honessite. Historically, the locality was discovered not through corporate industrial prospecting, but as a direct result of massive blasting operations during the modernization and widening of the highway infrastructure in the mid-twentieth century. Systematic artisanal field collecting and academic research peaked during the 1960s, 1970s, and 1980s, during which time thousands of world-class micromount and cabinet-grade reference specimens were recovered from the vertical rock faces by independent collectors and regional geological societies. While formal specimen extraction and open roadside digging were subsequently halted by highway authorities due to safety hazards along the high-traffic thoroughfare, the roadcut remains a classic, globally recognized benchmark locality for geode-hosted sulfide mineralization through 2026.

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