Illinois has two principal varieties of limestone, referred to
technically as limestone and dolomite. “Limestone” may be used as a
general name for both varieties.
Limestone consists principally of crystalline particles of the mineral
calcite (fig. 1). This mineral is glassy in appearance and is composed
of calcium, carbon, and oxygen combined to form calcium carbonate—CaCO₃.
Dolomite is largely made of crystalline particles of the mineral
dolomite, which also has a glassy appearance and consists of calcium,
magnesium, carbon, and oxygen—CaMg(CO₃)₂. The crystalline particles of
limestone and dolomite vary in size. Some are coarse enough to be seen
easily, others are so small that they can be distinguished only with a
microscope.