In the mines the silica is blasted loose from the natural deposit and loaded into trucks that take it to the processing mills at Elco and Tamms, where it is crushed and then transferred to huge grinding mills that pulverize it to a very fine powder. Air currents of different velocities separate the powder into various grades of fineness, and the finished silica goes into large paper bags for shipment.
_Origin of Silica._—Some of the history of the formation of silica deposits is not fully known, but it seems probable that their original character was quite different from what it is today. Investigations by Survey geologists suggest that the parent rock formation was limestone and chert. The limestone was composed of the mineral calcite, but scattered through it were myriads of small particles of quartz. Interlayered with it were bands and beds of chert that contained varying amounts of calcite.
As the rocks above them were worn away by the ceaseless erosion of streams and rivers, these original deposits were uncovered. Rain and snow-water then worked down into the limestone and chert deposits through cracks and crevices and dissolved the calcite in the rock. The quartz remained because it is much less soluble than the calcite. After many thousands of years all the calcite had been removed, leaving behind a “skeleton” of quartz—the silica deposits of today.
_Uses of Silica._-The silica mined and milled in southern Illinois has many uses. A superfine grade, known as white rouge, is used to polish optical lenses. Other grades are used in scouring compounds, metal polishes, paints, electrical resistors, high-temperature pipe coverings, fiberglass manufacture, plastics, silicone rubber, wood filler, caulking compounds, ceramic products, floor tile, billiard cue chalk, as foundry parting or facing, concrete admixture, in the manufacture of buffing compounds that are used to polish metal objects, and for other purposes. Industry uses many thousands of tons of silica each year.
Deposits of chert, chert gravel, and ganister also are among the variety of mineral materials found in extreme southern Illinois. Chert consists principally of minutely crystalline particles of quartz. Some chert is popularly called flint. In southern Illinois chert occurs in two principal kinds of deposits, those composed of solid ledges and those consisting of gravel. The term novaculite is used in southern Illinois for those solid deposits that are white, comparatively thick, and free of other interlayered materials. No novaculite is mined at present, but it is said to have been sold in past years for making sodium silicate and silica brick.