The uses of clay and shale are determined to a large degree by the
properties of their clay minerals and to a lesser degree by the
impurities present. A clay or shale containing the clay mineral illite,
and other similar but less important clay minerals, commonly becomes red
when fired and gets hard at a relatively low temperature. It therefore
is used to make red bricks, drain tile, building tile, and other
structural clay products.
Another clay mineral, kaolinite, generally burns to a light color and is
difficult to fuse. Therefore, clays composed wholly or mainly of
kaolinite can be used for making buff or light-colored bricks and for
the manufacture of highly heat-resistant (refractory) bricks.
The clay mineral in the southern Illinois clay that was used to
decolorize oil is montmorillonite. This clay is now used in sweeping
compounds, as an oil absorbent, as animal litter, and for other
purposes.