Part 2

Lime.—When limestone or dolomite is heated to a high temperature, it undergoes a change and carbon dioxide is liberated. The weight of the gas set free is equal to somewhat less than half the weight of the rock if the rock is pure. The solid product remaining after the gas has been driven off is known as lime. The heating process is called burning.

Besides being used in making mortar and plaster, lime is valuable in many other ways, especially in various chemical processes of modern industry. Plants at Chicago and Quincy make lime from Illinois limestone and dolomite.

Cement.—Portland cement, sometimes called simply cement, is made at LaSalle, Oglesby, and Dixon in northern Illinois from limestone and a smaller, carefully measured amount of clay or shale. Cement also is made at Joppa in southern Illinois. The blend of materials is thoroughly ground and mixed, then heated at a high temperature in huge rotating horizontal furnaces, called kilns, until it forms a clinker. After it cools, the clinker is ground to a flourlike powder. This is basically the cement that binds together the mixture of sand and limestone (or dolomite), or of sand and gravel, to make the concrete from which roads, bridges, buildings, and other structures are made.

Aggregate.—The crushed stone used in making concrete is known as concrete aggregate. This is a major use for Illinois limestones and dolomites. Large amounts of stone aggregate also are used in bituminous roads, popularly called “blacktop” roads.

Agricultural Uses.—Agricultural limestone (agstone) is applied to farm land to neutralize soil acids and otherwise benefit the soil. Both limestone and dolomite are so used. Chickens are fed small limestone chips to provide calcium for the formation of egg shells. Pigs and cows get calcium from mineral supplements containing powdered pure limestone.

Other Uses.—Illinois limestone or dolomite is used in steel making, as building stone and marble, as road stone, as ballast for the road beds of railroad tracks, for making refractory dolomite used in the steel industry, and for a variety of less common uses.

Quarries

There are about 200 stone quarries in Illinois. Most of the larger quarries (fig. 3) are in the Chicago, Joliet, Kankakee, and East St. Louis areas, but one or more limestone or dolomite quarry occurs in many counties.

If all the stone taken from Illinois quarries in 1963 were removed from a hole 100 feet square, the hole would penetrate into the earth about 8 miles. It would take more than 350,000 railroad cars holding 100 tons each to haul away the stone. Limestone, dolomite, and their products added over 80 million dollars to the economy of the state in 1963, approximately 8 dollars for each person in Illinois.

Most Illinois limestone and dolomite is quarried from open pits, but in some places, as in the rocky bluffs along the Mississippi River, the stone is taken from underground mines (fig. 4). There is also a dolomite mine in Chicago. At the quarries the first step is the removal of the earth overlying the stone. Next, in both pit and mine, the stone is blasted to free it from the parent deposit and break it into pieces. Mechanical shovels (fig. 3) load the stone into trucks that take it to the crushing plant where powerful crushers further break the stone into pieces. The pieces are sorted into various sizes by large screens. At some of the plants, the stone is ground into powder.

Location of Limestone Deposits

The geologic map of Illinois prepared by the Illinois State Geological survey shows, with reasonable exactness, what bedrock formations would crop out at the surface if the overlying clay, sand, gravel, and earth were removed. Thick dolomite formations would be exposed in much of the northern fifth of the state, but would be rare elsewhere. Thick limestone formations would occur in an almost continuous zone, varying in width from 3 to 25 miles, along the Mississippi River from Rock Island to southern Illinois and then eastward across the extreme southern tip of the state. Limestone also would be seen along the Illinois River from Havana southward.

In the central area of the state, limestones are present, but they are rarely over 25 and often less than 15 feet thick. Consequently, most of the larger quarries are in the northern, western, and southern parts of Illinois. The thinner limestones, nonetheless, are of much importance and are quarried at many places, chiefly to provide agricultural limestone, road stone, and limestone for making cement.

The Geological Survey locates and maps limestone and dolomite deposits and analyzes and tests samples to determine the best possible uses for the stone. Many reports have been published about the character and general use of the deposits in various parts of the state. Other reports deal with the use of limestone and dolomite for specific purposes such as cement making, building and decorative stone, rock wool, terrazzo chips, and lime.

METALLIC ORES AND FLUORSPAR

Lead and Zinc

Lead mining was one of the earliest industries of Illinois. The early settlers’ need for bullets for procuring food and for defense of their lives and property made lead an important commodity, and the deposits of lead ore in the northwestern corner of Illinois were quickly exploited. The ore was the mineral galena (fig. 5), for which the city of Galena in Jo Daviess County is believed to have been named.

Galena is a dark, shiny mineral that breaks readily into cubes or combinations of cubes. It is composed of lead and sulfur (PbS). Galena itself is not suitable for use as a metal; the lead must first be separated from the sulfur.

The earliest method of recovering lead from galena was crude. A pile of logs, smaller pieces of wood, and ore was built on sloping ground. Just below it a pit was dug. When the wood was set on fire, the heat caused the lead and sulfur to separate, and the molten lead trickled down into the pit. The smelting process was later improved, and stone “furnaces” were built to house the operations.

Crevice Deposits and Residual Deposits.—Most of the lead ore mined in the early days of the northwestern Illinois mining district came from crevice deposits in the dolomite bedrock and from residual deposits at or near the surface of the ground. The crevices were vertical narrow joints or fissures. Ore was not continuously present along them but occurred from place to place in “pods” (fig. 6 and fig. 7). Dimensions of the pods varied, but typical ones were about 3 feet wide, 5 feet high, and a few to a few hundred feet long. The galena, for the most part, occurred in a mixture of clay and weathered dolomite that filled, or partly filled, the crevices.

The residual deposits were found where the action of the weather for many thousands of years had dissolved the dolomite from the outcropping parts of a crevice deposit and left behind a residue of brown or red clay containing galena.

Some of the crevice and residual deposits worked by the early miners cropped out at the surface, but most of them were covered by earth. Other crevices were exposed in the bluffs of the Mississippi River and extended back into them for 1,000 feet or more.

SOIL A GALENA CLAY AND ROTTED DOLOMITE B GALENA CLAY AND ROTTED DOLOMITE DOLOMITE

When the richer deposits of ore in the crevices were worked out, some mines were deepened into the dolomite bedrock, but usually less rather than more galena was found.

As the amount of galena decreased, however, another mineral, which had been present before in only small amounts, was found in increasing quantities. This was sphalerite—a yellow, brown, or black mineral of resinous appearance that is composed of zinc and sulfur (ZnS). It does not look like a metallic ore.

At first the sphalerite was not used because there were no smelters in the area that could separate the zinc from the sulfur with which it was combined in the ore. Between 1850 and 1870, however, smelters were built in northwestern Illinois and southern Wisconsin and the ore was shipped there. Sphalerite is now the principal ore mineral produced in northwestern Illinois.

The sphalerite and galena of southern Illinois, described later, are similar to that found in northwestern Illinois.

Mining and Milling.—One large mine is producing zinc and lead near Galena at present. Smaller mines operate irregularly. The ore may occur as pockets, irregularly shaped rather flat masses, vertical or inclined veins (fig. 8), or small particles scattered through the dolomite. The principal ore bodies that have been worked in recent years have been of irregular shape, both horizontally and vertically, and usually have been between 50 and 200 feet wide and from a few to as much as 100 feet high. They lie at a depth of roughly 300 feet.