PYRITE and MARCASITE are iron disulfide compounds. They look much alike
but have different crystal forms. Both are brittle, hard, brassy yellow
with metallic luster, and opaque. The best distinguishing feature is
crystal shape. The pyrite crystals are cubes, but the marcasite crystals
are blade- or needle-shaped.
Pyrite and marcasite have been mistaken for gold because they are yellow
and metallic and hence are sometimes referred to as “fool’s gold”. They,
however, are harder than gold, tarnish, and leave a dark streak, whereas
gold is soft, very heavy, does not tarnish, and leaves a yellow streak.
Gold is malleable, but pyrite or marcasite are reduced to powder if they
are pounded and give off a noticeable odor of sulfur dioxide gas if they
are heated.
Pyrite and marcasite are found in many deposits in Illinois. They occur
as grains or larger masses in some clays, shales, and limestones. They
also occur with the lead and zinc ores of northwestern Illinois and, in
small amounts, with the fluorite and associated minerals in the extreme
southern part of the state.
Both pyrite and marcasite are common as surface coatings, veins, and
concretionary structures in coal and in dark shales associated with
coal. They are referred to as “coal brasses” or “sulfur” when found as
impurities in coal.
A potential use for pyrite and marcasite is in the manufacture of
sulfuric acid for industrial use. Coal brasses recovered from Illinois
coal have been so used.