Part 4

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.

The uncovering was not a single, simple event but rather a series of events that took place at various times during the many years since the St. Peter sand was deposited. Among these was the up-bowing of the rocks of central northern Illinois into a broad arch. Streams then began to cut across the arched rock, slowly but persistently stripping away the top layers until the core of the arch was laid bare. Among the rocks thus exposed was the St. Peter Sandstone, which may be seen in northern Illinois in the valleys and tributaries of the Rock River near Dixon and Oregon and along the Illinois and Fox Rivers and some of their tributaries near Ottawa, Wedron, Millington, and Troy Grove. The St. Peter Sandstone at Starved Rock and Matthiessen state parks near LaSalle and along the highway between Dixon and Oregon is eroded into scenic bluffs and canyons.

Silica Sand Industry

The Illinois silica sand industry is based on the St. Peter Sandstone. It centers around Ottawa, Wedron, Troy Grove, and Utica in LaSalle County and in Oregon in Ogle County. Two principal grades of silica sand are produced—washed and crude. The value of the silica sand produced in Illinois in 1963 was about $9,000,000.

Washed Sand.—Although the St. Peter Sandstone is composed almost entirely of quartz grains, a small amount of clay is present. For some uses it is not necessary to remove the clay, for others its elimination is important and is achieved by washing the sand.

In the mining of silica sand that is to be washed, the sandstone is first blasted loose from the parent deposit to break it into sand or pieces of various sizes. Some of the larger pieces may require a second blasting to disintegrate them.

At some pits the material is loaded mechanically and transported to the washing plant. At others a powerful stream of water is directed against the broken sandstone (fig. 14) and the resulting mixture of sand and water flows to a collecting basin from which it is pumped through large pipes to the processing mill.

In both types of operation the sand is thoroughly washed at the plants. After it is washed, the sand is further processed to suit the needs of its users. Much of it is screened into different size grades.

Uses of Washed Silica Sand.—The washed silica sand produced in Illinois has many uses, some of which are briefly mentioned below. The suitability of the sand for some purposes depends in part on its having been screened to specified sizes.

The high purity of Illinois washed silica sand makes it suitable for making glass, which is more than half silica sand. Each year over a million tons is used for this purpose. The purity of the sand also is of importance for chemical and metallurgical uses such as the manufacture of sodium silicate and silicon carbide and in alloying.

The hardness of the sand makes it useful for grinding large sheets of plate glass to prepare them for polishing and also makes it an effective abrasive agent for sandblasting. Metal castings in foundries and the exteriors of buildings are cleaned by this process. Illinois produces thousands of tons of sand yearly for such abrasive purposes.

Because the coarser grains of the washed silica sand are rounded, strong, and available in uniform sizes, oil operators use thousands of tons of it annually in the hydraulic fracturing of oil-bearing strata. The sand is mixed with oil, other petroleum products, or water and is forced by powerful pumps into sandstone or limestone formations that contain oil. The great force thus exerted opens fractures in the rock strata and pushes the liquid and sand into them. When the pressure is relieved, the sand grains serve as props to hold the fractures open. The oil can then flow more easily into the wells and oil production is thus increased.

The washed sand, because it is clean and does not dissolve in water, is used to filter impurities from drinking water. Its whiteness makes it a desirable constituent in plaster, mortar, and precast building panels.

Because it is round grained and withstands high temperatures without melting, large tonnages of the washed silica sand are used to make molds into which molten metal is poured to make various kinds of castings.

A special type of coarse silica sand from Illinois that is carefully prepared so that it is always of the same grain size is used throughout the world as a standard in laboratories that test cement and other commercial products.

Some silica sand is ground to a fine, white powder. The powder, called ground quartz, ground silica, silica flour, or potter’s flint, has many uses. It is an ingredient in paints, potters use it in making pottery and china, it goes into scouring powders, into molds used for precision types of metal castings, and into enamels.

Crude Silica Sand.—The crude silica sand produced from the St. Peter Sandstone generally is yellow or yellowish white and is not washed before it is used. It probably originally was white, but iron oxide, similar to the rust that forms on iron, now coats many of the sand grains and colors the small amount of clay in the sand. Thousands of tons of crude silica sand are mined annually (fig. 15). Because it is highly heat resistant, foundries buy much of it to make the molds used for castings, especially steel castings, and for automobile engine blocks, train wheels, and a variety of other metal products. Crude silica sand also is used around industrial furnaces to seal cracks and openings to prevent the loss of heat, in certain ceramic products, and for adjusting the silica content of the raw materials used for making portland cement.

Studies of the St. Peter Sandstone

The Illinois Geological Survey has made field studies and prepared maps showing where the St. Peter Sandstone is exposed in northern and western Illinois. Many samples have been screened and examined under a microscope to determine how the sand of different deposits, or different parts of the same deposit, varies in grain size and mineral composition. The possibility of using Illinois silica sand for making silica brick also has been investigated.

GRAVEL AND SAND

Some 225,000 years ago, most of what is now Illinois was buried under the ice of the Illinoian glacier. Two earlier glaciers had covered large parts of Illinois, and another, known as the Wisconsinan glacier, came into the state later, about 50,000 to 70,000 years ago (fig. 16).

The relatively small glaciers in the United States today, such as those in the northern Rocky Mountains, are concentrated in valleys and are called valley glaciers. The glaciers that covered Illinois were parts of huge ice sheets that extended over much of the North American continent and are called continental glaciers. They spread over most of Canada, then pushed southward to bury New England and a great area in the north-central part of the United States north of the Ohio and Missouri Rivers.