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Calcite

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Sharp colorless gemmy scalenohedral crystals of lustrous calcite measuring to 1.5 cm in size across the top of an iridescent brown rhombohedral calcite crystal covered matrix. This is in terrific condition. Previously from the collection of Louis Lafayette, this would have been collected in the 1960s. This is one of the better examples from this area that I have seen in quite a while. The Mich Mine, located near Oskaloosa in Mahaska County, Iowa, was a small-scale coal mining operation that became famous among mineral collectors for its remarkable calcite specimens. Geologically, the mine accessed carbonaceous shales and coal seams within the Pennsylvanian-aged Cherokee Group, where hydrothermal fluids circulating through the sedimentary strata deposited minerals in localized limestone concretions and septarian nodules. The locality is specifically known for its complex calcite growth sequences, typically featuring two distinct generations: a primary layer of rhombohedral crystals with a dark brown, petroleum-rich color and a vivid iridescent "oil-slick" luster, followed by a second generation of sharp, snowy white scalenohedral crystals that provide a striking visual contrast. Historically, the mine was active during the early to mid-20th century as part of the regional coal mining industry that flourished in southern Iowa. While the mine is no longer operational and the site is closed to the public, specimens from the Mich Mine are highly coveted in the mineralogical community as some of the most aesthetic examples of Midwestern sedimentary calcite ever recovered.