Limestone is a bedded or layered rock found abundantly in Missouri in
bluffs, creek beds, hill sides, and is known to underlie the soil in
most of the south half of the state. It occurs in thin slabs, thick
layers, and in massive beds which may make a small cliff in themselves.
Limestone is soft enough to be scratched with steel. It is commonly
white to grayish, but may be stained tan, yellowish, or reddish by iron
oxide, or darkened through shades of gray to black by the presence of
very finely-divided, black carbonaceous matter. It may be
microscopically fine-grained (and then it can be used in lithographic
printing in the reproduction of very fine images), or its grains may
vary in size up to one-half inch in cross section.
[Illustration: Limestone (dolomite) bluff near Jefferson City.]
It is determined as limestone with certainty by wetting with dilute cold
acid; then it “bubbles” or effervesces, and eventually dissolves
entirely. Ordinary or regular limestone contains the mineral calcite,
but the magnesian variety of limestone, dolomite, contains the mineral
dolomite, which does not effervesce freely in lump size in dilute acid,
but which does effervesce when _powdered_ or when treated with hot acid
or concentrated acid. The preferred acid to use is muriatic
(hydrochloric, the “not-cut” soldering acid) diluted one part of acid to
one part of water. Caution! This acid mixture should be stored in a
glass or porcelain container away from children or animals! Acid strong
enough to dissolve rock will ruin clothes, destroy flesh, and is
poisonous! Dilute sulphuric (storage battery) acid will also give the
effervescence test, and the acid of very strong vinegar will react with
limestone slowly. In making the test it should be recognized that the
limestone which acts as a cement in sandstone, or limestone impurities
in shale will also effervesce, but those minor parts of the rock will
dissolve and leave the residues of sandstone or shale, which are
insoluble.
[Illustration: A coarse-grained limestone effervescing in dilute
muriatic acid. (This photograph and other close-up views taken by J.
F. Barham and Allen Barnes, University photographers.)]
[Illustration: Solid dolomite does not effervesce in dilute acid.
Note the white rock powder scrapings adjacent.]
[Illustration: Dolomite powder does effervesce in dilute muriatic
acid. Not all dolomite is this fine in grain.]
Some limestones are chemical deposits but many are consolidated
accumulations of fossil shells and shell fragments—organic limestone.
For example, a widespread limestone, the so-called Burlington limestone,
extending across central Missouri, contains many crinoid stem fragments
and plates, attesting to the abundance of crinoids living in the sea at
the time this limestone was laid down. Crinoids are sea animals which,
because of their branching structure and superficial resemblance to
plants, have been nicknamed “sea lilies.” Except for calcareous cave and
spring deposits, almost all limestone formations in Missouri contain a
few fossils of animals which lived in the ocean, and therefore Missouri
limestones are considered marine in origin. They offer evidence for the
very interesting land-sea changes which this state has undergone in the
geologic past.
[Illustration: Limestone composed almost entirely of crinoid (marine
animal fossil) stem plates, from near Columbia.]
Pure limestone is composed of 100% calcium carbonate (calcite mineral),
whereas pure dolomite contains 54.35% calcium carbonate and 45.65%
magnesium carbonate (dolomite mineral). Magnesium carbonate has slightly
higher acid-neutralizing properties than calcium carbonate, weight for
weight, and because analyses of limestone to be used for soil sweetening
and agricultural fertilizer purposes are commonly reported in calcium
carbonate equivalents, a dolomite or dolomitic limestone may be reported
over 100% calcium carbonate equivalent. Unless one understands the full
meaning of the report he may be bewildered by a statement of the value
over 100%.
The calcium and magnesium which form limestone (or dolomitic limestone)
in the ocean are carried there in solution by the streams which drain
the land. Rain water percolating through the ground and rocks becomes
slightly acidified with carbon dioxide (like the carbonated water in
beverages) and dissolves the calcium and magnesium from primary igneous
rocks like gabbro and basalt which are weathering, or from preexisting
limestones which primitively were derived from igneous rocks. This
calcium and magnesium in solution are responsible for the hardness of
the water. In fact, the hard water in Missouri springs, wells, and
streams is hard because it contains either or both calcium (“lime”) and
magnesium in solution.
This soluble calcium and magnesium flows on in the stream to the ocean
because of its combination with the dissolved carbon dioxide. In the
shallow parts of the ocean, as on the continental shelves where the
water is less than 600 feet deep, the limestone is deposited in layers
just like the white lime layer deposits on the bottom of the teakettle
in which hard water has been boiled. Chemical processes, temperature
changes, evaporation of the ocean water, and organisms are responsible
for most of the limestone deposition. Extensive limestone deposition is
taking place today off the coast of Florida and around the tropical
islands of the southern Pacific.
The uses of limestone are numerous. It is an excellent building stone in
either the rough, sawn, or dressed state. It is used for rubble stone,
rip-rap, railroad ballast, crushed gravel, and aggregate in concrete. It
is one of the raw materials of Portland cement. Quicklime and hydrated
lime are prepared from limestone which has been heated to drive off the
chemically combined carbon dioxide.
Limestone is added as a fluxing material in metallurgical processes. It
is the lowest priced source of alkali in chemical industry. Pulverized
limestone may be used as a filler in paints, putty, paper, or rubber;
and rock wool is made by melting and blowing a limestone having a
suitable chemical composition. Two formations develop a “spongy”
appearance (“sponge rock” or “sponge limestone”) upon weathering and are
utilized abundantly in the eastern part of the state for rock gardens
and for ornamental and decorative stone.
Many tons of limestone are used each year in Missouri as a soil
fertilizer because it neutralizes acidity, coagulates the clay,
furnishes calcium to the plants by way of the colloidal clay, and frees
other chemical elements so that they become available to the plants. No
doubt rocks other than limestone will be crushed and added to the soil
in the future, but today our attention is focussed chiefly on limestone
and dolomite.
The value of a limestone quarry for agricultural purposes depends upon
availability, amount of overburden, purity of the stone, ease of
crushing, and size of deposit. For instance, a stone of 90% purity,
which is close at hand, will probably be more valuable than one of 98%
purity which must be hauled fifteen miles. Bare hillsides or creek banks
where a crusher can be set up to handle the stone without extra lifting
are preferable for quarry sites. Usually the overburden is less in such
an exposed face. Impurities in limestone deposits may be large chert
(flint) nodules which can be hand-sorted out, sand grains, clay which
settled into and onto the stone during its accumulation, and pyrite
(fool’s gold) or other minerals of lesser importance. Clay impurities
simply act as useless extra weight which must be handled. Sand grains,
however, are hard, and will abrade and wear out crushing equipment.
Chert and fine-grained silica likewise are harder than steel and will
wear a crusher excessively. Pure limestone (calcite or dolomite mineral)
has a hardness less than that of steel and will only polish or wear the
metal slightly.
[Illustration: Typical, intermittently-operated, farm limestone
quarry near North Kansas City.]
It will probably pay to give some thought to this matter of crushing
when selecting a quarry site for agricultural limestone. The several
beds of stone available should be tested not only for amount, but kinds
of impurities. Samples sent in for testing must be _representative_ of
the rocks under consideration or the analytical results are meaningless.
The writer does not believe this point can be over-emphasized. Time
after time he has seen samples taken of geological deposits for testing
which no more represented the deposits than a bantam rooster picked out
of a chicken pen would represent the egg-laying or weight-production
possibilities of the flock of Plymouth Rock hens.
If five layers or beds of stone are to be properly tested, then five
samples must be taken, _one broken from each layer of solid rock in
place_. The five layers may have the same color, or look much the same,
but fine grains of sand, hardly visible without magnification, may be
abundant in some layers and not in others. If circumstances do not
permit having five different tests made, but allow only one sample to be
run, then specimens should be taken from all five beds, their sizes
being in proportion to the relative amounts expected to be quarried from
each bed, and all five specimens sent to the analyst, who can crush and
mix them.
A single grab sample taken from loose rock on a hillside, in expectation
that it will represent the rocks inside, depends as much on luck as
betting on the weather next 4th of July, a year hence. The chemist who
analyzes the limestone for calcium can usually report on the kind of
impurity if he will take the time to do it.
“Cotton Rock” Limestone
“Cotton rock” refers to a white to slightly gray or buff variety of
limestone which has a “soft”, somewhat chalky and porous appearance that
is suggestive of cotton. Missouri “cotton rock” is usually dolomitic.
Although the term “cotton rock” has no standing in a technical sense,
its fairly wide use indicates that the name has descriptive value.