A mineral material unique in Illinois is the amorphous silica, or
tripoli, mined in the hills of southern Illinois about 20 miles north of
Cairo.
Most of the silica mines are found at the heads of valleys or in
tree-covered hill slopes along the less traveled roads. An arched
opening (fig. 20) with a road leading into it may be all that is visible
of the mine from the outside. Inside are many rooms, some 30 feet high,
separated by rounded pillars about 20 feet thick that have been left to
support the arched mine roof.
Most of the silica in the mines is white, and this is the part that is
mined. A silica deposit is made up of layers—some of white, powdery rock
material, others that look chalky but are firm or hard.
[Illustration: Figure 20—Entrance to a silica mine.]
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.