Kinds of Limestone
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.