CLAY is an unconsolidated rock made up of a group of hydrous aluminum
silicate minerals, of which chlorite, montmorillonite, kaolinite, and
illite are the most abundant. These minerals are formed by the
weathering or alteration of other rocks and minerals.
Clays are very fine grained and their minerals have tiny, flat crystals
that can be distinguished from each other only by laboratory methods.
Although clays may appear to be similar, their compositions vary
greatly.
Some clays are white, but most are colored by iron compounds and organic
matter. Wet clays have an earthy odor and generally are slick and
plastic, but dry clays are relatively hard and are greasy to the touch.
Clays are abundant in Illinois, especially in soils, in shales, and as
clay deposits. In Illinois the underclays that occur beneath coal beds
are particularly well suited to the manufacture of bricks, pottery,
stoneware, and drain tile.