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May-June 2010

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Illinois Mineral Locality Index

The bedrock under Illinois is predominantly composed of Paleozoic sedimentary rocks. In the southern portion of the state, these rocks were deposited in a northwest-to-southeast spoon-shaped basin. They are hosts to economic deposits of coal, hydrocarbons, fluorspar, lead and zinc ore, glass-sand, and dolostone and limestone that produce lime and rock aggregate. To complicate things for the mineral collector, these rocks are mostly covered by glacial till and glacial outwash deposits.

The Illinois-Kentucky fluorspar district in the extreme southeast corner of the state has been a major fluorite producer, with some zinc and lead production. For more than 150 years, until 1995, it was one of the largest mineral specimen-producing districts in the United States. Unfortunately, few specimens collected prior to the mid-1970s were documented with the exact mine or in many cases even the correct subdistrict. In the opposite northwest corner of the state, is the Illinois portion of the Upper Mississippi Valley zinc-lead district. This area has been an active mining district from about the middle of the nineteenth century until the 1970s. Although the Wisconsin portion of the district has eclipsed the Illinois portion in the number of mineral specimens produced, both have yielded some nice specimens.

Arthur E. Smith, now deceased (see March/April 2010 issue In Memoriam column), was a longtime consulting editor of Rocks & Minerals.

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