Coal is so well known that little need be written about its
distinguishing characteristics. Most of the coal in Missouri is of about
bituminous rank, although some cannel coal, which is discussed below, is
also present.
Missouri bituminous coal occurs in the northern and western parts of the
state. It contains bands of dull coal, bands of glistening “glance”
coal, the sooty “mineral charcoal,” and common mineral impurities like
calcite, gypsum, pyrite and marcasite (“sulphur”), clay minerals, and
quartz. Bituminous coal breaks with essentially a cubical fracture.
It occurs in horizontal or nearly horizontal beds or “seams,” which may
be followed considerable distances laterally without necessarily
encountering much change. Usually, a fire clay or a fire clay-like
under-clay immediately underlies the coal, but the overlying rock (the
roof) may be shale (slate? see discussion of SHALE), sandstone, or less
commonly, limestone.
[Illustration: Bed of coal exposed by stream erosion, near
Columbia.]
Coal originates from pre-existing plants and may be thought of as Mother
Nature’s storage cellar of “preserved” plant life. The Missouri coal
began millions of years ago as mosses, tree-like ferns, conifers, and
various plants that reproduce by spores, which flourished in great
wide-spread swamps. Insects were abundant, as is indicated by their
remains. Rain was probably plentiful and climate favorable, so that such
vegetation thrived luxuriantly. Today, fallen forest timber of the
highland disappears by oxidizing and decaying in the air; but in swamp
land the leaves, stems, pollen and woody trunks fall into and under
water and under favorable conditions decompose through bacterial and
chemical action into layers and pools of slippery, oozy, blackish humic
gel (like brownish black gelatin, “jello”), which remains. Likewise, in
ages past, more and more plant material continued to live, fall, and
accumulate in the old coal swamps until very thick deposits of the woody
gel existed.
Eventually land-sea or climatic conditions changed, and plant life died
out as mud, sand, or other rock-forming material was swept in to cover,
as a lid, the stored-up plant remains. The weight of overlying beds
squeezed out excess water from the woody gel, and from the time of
covering through the present day, gases (like mine gases), and other
volatile constituents of the coal have been given off.
A bed of coal, which consists chiefly of black combustible carbon, with
volatile constituents and non-volatile ash substances, has resulted. Man
uses the coal by burning it directly, or it may be coked and the
volatile constituents recovered in coal tar and other compounds. The
mineral impurities like the calcite, gypsum, clay, sand, and brassy
pyrite or marcasite, are shaken through the grates as ash or melted as
clinkers.
In nature, the pyrite and marcasite minerals may oxidize in ground water
percolating over them to form dilute sulphuric acid, the acid mine
waters.
Cannel coal in Missouri has been found chiefly in old sinkhole deposits
through part of central Missouri. It is characterized by fracturing
conchoidally and having a more massive structure (instead of the layered
structure common to bituminous coal). Cannel coal burns to a very hot,
rather quick fire because of high volatile content, and is thought to
have developed from accumulations very rich in plant spores.
Coal mining is an important industry in Missouri, and a special bulletin
on coal has been published by the State Geological Survey at Rolla,
Missouri.