Pyrite and Marcasite (Fool’s gold, “sulphur”) are brassy yellow,
metallic, heavy minerals which will scratch glass but which cannot
themselves be scratched by a knife, and which will leave a dark-greenish
to black mark or streak when rubbed across unglazed porcelain or chert
rock. Both are composed of iron sulphide, FeS₂—iron 46.6 per cent, and
sulphur 53.4 per cent. Although they have the same chemical composition,
they differ in internal atomic and crystalline structure, which is of
interest to scientific mineralogists. Pyrite may crystallize in cubes,
or in forms called pyritohedrons, named from pyrite, whereas marcasite
crystallizes in characteristic arrow-shaped or cockscomb forms.
Marcasite weathers a little more readily than does pyrite, but otherwise
they are much the same to the casual observer.
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.
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.