LIMESTONE is a sedimentary rock composed of particles of calcite
(calcium carbonate). The crystals may range from fine to coarse. Many
limestones contain other minerals, such as chert, clay, or sand, and in
some places they grade into dolomite (calcium magnesium carbonate).
Many limestones are white or gray. Yellow or brown shades are caused by
iron oxide impurities and dark gray to black colors by organic matter.
Limestones form in various ways. Some are deposited when calcium
carbonate precipitates from solution; others are formed when the shells
or skeletons of organisms such as brachiopods, clams, and corals
accumulate on a sea floor. If such fossils are very abundant, the rock
is called fossiliferous limestone. Limestone composed of tiny, rounded
concretions is called oolite or oolitic limestone.
Limestone effervesces freely in dilute hydrochloric acid, but dolomite
must be powdered before it will effervesce. In nature, limestones may be
dissolved by percolating water containing weak acid (such as carbonic
acid, composed of water and carbon dioxide). At many places in southern
and southwestern Illinois such solution of limestones has produced caves
and caverns.
Limestone outcrops are abundant in Illinois, especially along the bluffs
of the Mississippi, Ohio, and Illinois Rivers.
Limestone has many uses. It is used for building stone, road surfacing,
railroad ballast, in the manufacture of portland cement, and, if of high
purity, for making lime and chemicals and as a flux in smelting metals.
It also is used, as agricultural limestone, to add calcium to the soil.