Calcite with Sphalerite

specimen number:
2063811

location:
Shullsburg, Lafayette Co., Wisconsin, USA

description:
Non-lustrous slightly translucent rhombohedral crystals of calcite to 4.0 cm in size across the top of a crystallized black sphalerite matrix. In good condition with minimal micro-damage present. The specimen comes with a previous label from the collection of Ray DeMark.

Shullsburg, Wisconsin, located in Lafayette County within the Upper Mississippi Valley Zinc-Lead District, is renowned among mineral collectors for its distinctive calcite occurrences. Geologically, low-temperature hydrothermal fluids moving through fractured Ordovician limestone and dolomite deposited primary sulfide ores alongside associated secondary mineral pocket growth. Unlike localities known for twinned or deeply colored calcites, typical Shullsburg specimens are primarily untwinned, featuring clear to colorless or pale translucent scalenohedra. These central scalenohedral crystals are often recognized for unique composite growths, including fine stair-stepped rhombohedral overgrowths or sharp terminal caps. Found alongside dark sphalerite, galena, and marcasite matrix, classic calcite specimens recovered prior to the closure of local underground mines in 1979 remain highly valued historical representations of Midwestern regional mineralogy.

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