The mineral pyrite consists of iron and sulfur as the compound iron
sulfide (FeS₂) and, because of its shiny, brassy yellow color, is
sometimes called fool’s gold. Marcasite has the same composition as
pyrite, but its crystals have a different shape and it is often lighter
colored. Both minerals occur in various parts of the state; they are
particularly prevalent in some coal seams, and when so occurring are in
some cases called coal brasses or “sulfur.”
Pyrite and marcasite are used commercially as raw materials for making
sulfuric acid, although sulfur itself is more extensively used for that
purpose. At one time coal brasses recovered during coal-cleaning
operations at a northwestern Illinois coal mine were sold for acid
making. A large quantity of coal brasses probably could be recovered
from such operations at Illinois coal mines.