Illinois as a Mining State
Although Illinois is not usually thought of as a mining state, northwestern and southern Illinois together produced in 1963 nearly $5,000,000 worth of zinc, about $600,000 worth of lead, and $6,500,000 worth of fluorspar. The annual total value is about $12,000,000.
The southern Illinois fluorspar district has another distinction—for many years its mines have been the major domestic source of the nation’s fluorspar.
SILICA SAND
About 450,000,000 years ago, a shallow ocean covered Illinois. Its waves and currents carried clean white sand and deposited it as curving beaches, sand bars, and dunes. This sand differed from many sands in that it was composed almost exclusively of grains of the mineral quartz instead of being a mixture of quartz and other minerals.
Quartz is composed of silica (SiO₂), and sands such as the ancient Illinois sand that are composed of quartz are known as silica sands. Quartz is very hard and will scratch glass and some steel. Perfect quartz crystals, which are rare, are longer than they are thick and end in pyramids. Probably not many grains of the ancient sand came from perfect crystals; they more likely resulted from the decaying and breaking down of rocks such as granite, which are mixtures of quartz grains and other mineral particles.
The quartz grains probably did not come directly from their source to Illinois. Instead, it is likely they first were deposited elsewhere and formed into sandstone. That sandstone was subsequently broken down by weathering agents and the grains transported to the ancient Illinois sea by streams.
As a result of the erosive action of the agents that transported them, many of the originally angular grains, particularly the coarser ones, were rounded and their surfaces dulled like that of frosted glass ([fig. 13]). Consequently, they appear white, although they actually are colorless.
Since the ancient sea deposited its silica sand, other seas have covered Illinois at various times and each has left deposits of sand, mud, or limy materials. The silica sand thus was buried by hundreds of feet of other sediments and became sandstone. This sandstone is called the St. Peter Sandstone. It is named from the St. Peter River, now the Minnesota River, in Minnesota where the sandstone was first described and named by geologists. The overlying deposits also were consolidated into rock.
St. Peter Sandstone is exposed at the surface at many places in northern Illinois and in one small area in the western part of the state. The sandstone exposed in northern Illinois generally varies from 125 to 300 feet thick. The fact that it crops out at the surface indicates that the materials that formerly covered it have been removed.