In the southeastern tip of Illinois lie deposits of a mineral that
contains the chemical element fluorine. This element is used in making
the propellant that activates aerosol sprays, a plastic that resists
chemicals and oil and is strong enough to be used for bearings,
compounds that are said to help to prevent tooth decay, and many other
useful chemicals.
The mineral is fluorite (fig. 9), commonly called fluorspar. It is
composed of calcium fluoride (CaF₂), a compound of calcium and fluorine,
and is a glassy mineral that is generally white or gray but may be
purple, rose, yellow, blue, or green. In rare instances it is colorless.
Fluorspar mining in Hardin and Pope Counties began with lead mining.
Galena was first discovered there in 1839 in a well being dug at the
town of Rosiclare. Mining of galena began in the early 1840’s, and
somewhat later ore was being smelted by three furnaces, all of which
have long since disappeared.
The veins that were worked for galena also contained fluorspar, but as
there was little or no use for fluorspar in the 1840’s it was considered
waste. In time, uses developed, however, and about 1870 it was mined and
shipped in commercial quantities. Since then the tonnage and value of
the fluorspar produced from the Rosiclare area have increased until
fluorspar is the major product.
[Illustration: Figure 9—Group of cubic fluorite crystals.]
The fluorspar mining district north of the town of Cave in Rock in
eastern Hardin County also was an early producer of galena. In that area
the fluorspar-galena deposits are elongate and approximately flat. The
first miners followed the ore bodies from outcrops by tunneling into the
hillsides. In the late 1930’s and early 1940’s, many holes were drilled
into the bedrock in search of new deposits. Ore was found that contained
not only galena and fluorspar but also important amounts of sphalerite.
_Vein Deposits._—In the Rosiclare district, the fluorspar and its
accompanying minerals occur as steeply inclined veins a few inches to 25
feet or more wide (fig. 10), usually in limestone strata. The veins are
not uniformly thick but widen or narrow from place to place both
vertically and horizontally. They occur along faults—planes along which
the rocks of the earth’s crust have broken and slipped. A fault may be a
single plane of slippage but more often is a zone of broken and
displaced rocks. In most of the faults that contain fluorspar, the
slippage is vertical, or nearly so. Along one of the faults in the
Rosiclare district, the rocks on one side of the fault have moved
downward as much as 650 feet in relation to the rocks on the other side.
Some faults are more than 10 miles long, and the depth to which they
extend into the earth is unknown. Fluorspar has been mined from one of
them at depths of 800 feet. Not all faults, nor all parts of any one
fault, contain fluorspar.
[Illustration: Figure 10—Diagrammatic cross section of fluorspar
vein along a fault. The strata on the left side of the fault have
moved downward with reference to those on the right side.]
SOIL
FAULT
FLUORSPAR VEIN ALONG FAULT
down
SANDSTONE A
LIMESTONE B
SHALE C
SANDSTONE D
LIMESTONE E
up
LIMESTONE B
SHALE C
SANDSTONE D
LIMESTONE E
_Bedding Deposits._—In contrast to the vein deposits of the Rosiclare
district, the bedding deposits of the Cave in Rock area are flat, or
nearly flat, commonly 5 to 15 feet thick, and from a few to 200 feet
wide (fig. 11). They may be as much as 2000 feet long, widening or
narrowing and thickening or thinning throughout their extent. They are
called bedding deposits because they lie along the beds or layers of the
limestone in which they occur. Most of the ore bodies are associated
with a fracture or a small fault.
[Illustration: Figure 11—Diagrammatic cross section of bedding
deposit of fluorspar, lead, and zinc ore.]
SOIL
SANDSTONE AND SHALE
ORE
LIMESTONE
FRACTURE OR SMALL FAULT
_Grades and Uses of Fluorspar._—There are three principal grades of
fluorspar—metallurgical, acid, and ceramic. The metallurgical spar is
used as a flux in making steel and in metal foundries. Acid spar is used
to make hydrofluoric acid, which plays a part in the preparation of
uranium isotopes and in the production of a synthetic mineral essential
in refining aluminum. The acid is also used in the production of
high-octane gasoline and is the basis for a variety of important
chemical compounds, among them refrigerants and insecticides. Ceramic
grade fluorspar is used in making enamels, glazes, and certain kinds of
glass.
_Mining and Milling._—Fluorspar and its associated ores are mined in
different ways in the Rosiclare and Cave in Rock districts because the
types of deposits differ. However, in both areas most of the larger
mines are entered by vertical shafts. In the mines, explosives are
placed in holes drilled by machines and are then detonated to shoot down
the ore (fig. 12). Mine cars, trucks, or conveyor belts carry the ore to
the bottom of the mine shaft where hoists raise it to the mills at the
surface. In the mills a variety of ore-classifying machines separate the
galena, sphalerite, and fluorspar from the waste mineral materials
(chiefly limestone and calcite) with which they occur. Some fluorspar
after the separation process is almost flour-fine. To increase its use,
much of this spar is mixed with a binder and made into pellets or
briquets one-half to one inch in size.
[Illustration: Figure 12—Machine loading fluorspar ore in mine near
Cave in Rock.]
_Geological and Chemical Studies._—Because much of the fluorspar
produced in southern Illinois has come from veins along faults,
geologists have mapped the faults of the area by investigating the
distribution and nature of the various bedrock outcrops. The work was
complicated by the mantle of earth and vegetation that covers the
bedrock at many places. However, a geologic map was made that shows
where the various rock formations—sandstone, limestone, and shale—lie
beneath the surface and where the numerous faults crisscross the
district.
The first geologic map of the fluorspar district was made in 1920 by the
Illinois and U. S. Geological Surveys. New maps on a much larger scale
have been made recently by the Illinois Survey to meet the needs of the
modern fluorspar industry.
The Illinois Survey also has studied the ores and ore deposits of
southern Illinois to determine how they were formed. The records of many
borings and pits sunk to find ore have been collected and filed at the
Survey to guide future prospecting.
Survey chemists are finding new uses for Illinois fluorspar. Their
research has produced new organic fluorine compounds that are being
tested for use in agriculture, medicine, and industry. They also have
worked out easier and cheaper methods of making certain fluorine
compounds. Survey chemical engineers have helped to obtain needed
information about the physical properties of the pellets made from
fluorspar powder.