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.
LIMONITE (29)
LIMONITE is an iron oxide containing water and has a complex chemical composition. The limonite found in Illinois may be yellow, orange, red, brown, or black, but its streak is always yellowish brown. The mineral may have a glassy or an earthy luster. It may be too hard to be scratched by a knife. It is of medium weight.
Limonite is common and occurs as concretions and cavity fillings in sedimentary rocks, and as coatings on them, especially sandstone. It also occurs as iron rust, as scum on stagnant water, and it accumulates around rootlets in soils. Small amounts color limestone, dolomite, clay, shale, sandstone, and gravel. Some sands are firmly cemented by brown or black limonite and look much like iron ore. Clays containing a high percentage of limonite are called ocher.
In some states limonite is mined as an iron ore, and in Illinois it was so used in Hardin County in the middle 1800’s, but deposits are not large enough for profitable use now.
SPHALERITE (30)
SPHALERITE, zinc sulfide, is a major ore of zinc. It has a resinous luster and a white, yellow, or brown streak. Illinois sphalerite is generally yellow, yellowish brown, reddish brown, or brownish black. It is of medium weight, brittle, can be scratched by a piece of window glass but not by a penny. It is commonly opaque but may be translucent on thin edges.