The geology of Hardin County, and the adjoining part of Pope County . In thin section, fluorite is seen to penetratecalcite irregularly as veinlets which sometimes terminate with idiomorphicoutlines. Slight projections of the fluorite along calcite crystallographicdirections may be observed, as though the fluorite was controlled by calcitetwinning to a small degree, though more commonly the fluorite does not showthis directional replacement but rather enlarges irregular calcite displays the finest replacement pattern observed, where it is developedin a lattice-like pattern fo
The geology of Hardin County, and the adjoining part of Pope County . In thin section, fluorite is seen to penetratecalcite irregularly as veinlets which sometimes terminate with idiomorphicoutlines. Slight projections of the fluorite along calcite crystallographicdirections may be observed, as though the fluorite was controlled by calcitetwinning to a small degree, though more commonly the fluorite does not showthis directional replacement but rather enlarges irregular calcite displays the finest replacement pattern observed, where it is developedin a lattice-like pattern following very definitely the twinning directions ofcalcite. A photomicrograph of this is shown. Elsewhere galena shows idio-morphic outlines where it is in contact with sphalerite on one side and withcalcite on the other. Fluorite appears entirely enclosed by galena, especiallyalong a fracture between fluorspar and calcite grains. In some places galenaalso completely surrounds small patches of sphalerite. Illinois State Geological Sukvey Bulletin No. 41, Plate II-A. A. Quartz (Q), chalcopyrite (Ch), and galena (G), replacing calcite (Ca).No definite order among the sulphides is suggested. The galena is seen to be idio-morphic. The same specimen in other portions shows replacement of calcite byfluorite and of fluorite by sulphides. The transition from fluorite to calcite, whilesometimes sharp, generally is indistinct or gradual. Elsewhere, also, the specimensuggests that some quartz deposition accompanied the fluorite; much was closelyfollowing and preceded the sulphides, and some may have followed the black spots in the key—corresponding to numerous very small white areas inthe photomicrograph—represent blende.
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectgeology, bookyear1920
