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.
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.
Pyrite and marcasite have been used in the commercial manufacture of sulphuric acid, but elemental sulphur can now be utilized more economically, so that now no market exists for pyrite or marcasite in Missouri. In earlier times only large deposits containing thousands of tons of the mineral had any value. In some foreign countries pyrite is burned and the fumes utilized for the manufacture of sulphuric acid, while the cinder, an iron oxide and iron ore, is smelted to recover metallic iron.
The origin of pyrite and marcasite is as variable as its enclosing rock. No general statement can be made which will include the igneous rock pyrite, the Joplin marcasite, the “sulphur” of the coal, and occurrences in sink holes and various sedimentary rocks. A discussion of all these origins alone would fill a pamphlet as large as this one on Missouri rocks.
Conglomerate
Conglomerate is a rock composed of gravel, pebbles, and boulders cemented together, with more or less sand and clay between the larger fragments. It is truly a conglomeration of rock fragments as one would find loose today in a stream or ocean shore gravel bar, or in a hillside gravel bank. Probably the conglomerate most abundantly exposed in Missouri is that overlying the igneous rocks in the southeast part of the state.