Pyrite and marcasite have been called “fool’s gold” because so many persons have been fooled, sometimes with serious financial consequences, by their slight resemblance to true gold. True gold is soft, usually slightly orange-yellow in color, malleable, and unaffected by ordinary acids; and it leaves a gold-colored streak when rubbed on unglazed porcelain or a hard white rock. Pyrite, in contrast, is quite hard (harder than steel), is brassy yellow with perhaps a slight greenish tinge except where tarnished, is brittle, is corroded by acids or oxidizing ground waters, and leaves a greenish black to black streak on a white rock. One readily notices the difference in color between pyrite and gold (such as is in a piece of good quality jewelry), when the two are viewed close together. Yellowish, partially weathered mica has also been mistaken for gold.
[Illustration: Brassy, granular pyrite in hand specimen.]
[Illustration: Pyrite in crystal cubes, and replacement of fossils.]
Pyrite and marcasite occur abundantly in most of the metal-mining districts of Missouri, as the “brass,” or “sulphur balls,” etc., in coal, as small nodules or pellets in some limestone, shale and sandstone, as replacements of fossils, and as minute crystals in granite, porphyry, and the other igneous rocks. Several marcasite mines have been developed in old sinkhole deposits in south central Missouri, but these are not in production at the time of this writing. The sinkhole iron mines of south central Missouri contained pyrite-marcasite before oxidation to the iron oxide ore, and some of them still contain the sulphides in their lower levels.
[Illustration: Marcasite crystal cluster from Joplin region. The arrowhead or cockscomb crystal form is characteristic of marcasite.]
Marcasite weathers (oxidizes) very readily under most conditions, with the formation of (1) yellowish brown iron oxide, the mineral limonite, which may stain rocks, soil, stream bank, etc., and (2) weak sulphuric acid water. The sulphuric acid solution may react with more marcasite or pyrite and evolve a gas, hydrogen sulphide, H₂S, which has a rotten-egg odor. This explains the foul odor often noticed around old coal mine dumps. Heat is evolved in these reactions, and coal waste on the dump may be ignited by the heat of the chemical reactions. The burning pyrite, or elemental sulphur, gives off sulphur dioxide, “burning sulphur fumes,” which add to the odor and heat around a coal mine dump. The burning coal waste and the chemical reactions may raise the temperature of the coal waste pile high enough to fire or “burn” the shale rock to a red, partially vitrified, natural brick-like material, which is sold or distributed as “coal dump shale” or “burned shale,” or “red shale” for all-weather surfacing of drives or walks.