There are two types of carbonate rocks in Illinois, limestone and
dolomite. In appearance they are almost indistinguishable, although they
are different chemically. Limestone is composed mainly of calcium
carbonate (the mineral calcite), whereas dolomite is calcium magnesium
carbonate (the mineral dolomite).
Limestone deposits are found at many places in the western,
southwestern, and southern marginal portions of the state. They are
largely of Mississippian and Ordovician age. Lesser deposits of
Pennsylvanian limestone are found in the central portion of the state.
In the northern fifth of the state, dolomites of Silurian and Ordovician
age are exposed at many places.
Limestone and dolomite are quarried for a variety of uses, including
aggregate for concrete, agricultural limestone, railroad ballast, chips
for bituminous roads, and for use in metallurgical and chemical
processes. They also are quarried for building stone at some places, and
marble is produced in southern Illinois.
[Illustration: Figure 10—Limestone and dolomite are quarried at many
places, but fluorspar, zinc, and lead are produced in restricted
mineralized areas.]
KEY
Quarry, Limestone or Dolomite
Cement or Lime
Fluorspar
Zinc and Lead
Limestone and shale or clay are burned together in kilns to produce
portland cement. Portland cement is manufactured from a Pennsylvanian
limestone in LaSalle County and from an Ordovician limestone in Lee
County.
Limestone or dolomite also is burned in kilns to make lime.
Mississippian limestone is used in Adams County and Silurian dolomite is
used in Cook County for lime making.