Chapter II: Introduction (1)

Missourians are interested in the rocks and minerals which they find on their farms, in excavations, and while on their vacation trips. Some of the specimens are unusual in shape or appearance, some are crystalline and beautiful, some may be ores of economic importance, but many simply arouse the curiosity of the finder.

Many of these specimens are received each year at the University at Columbia, and each is usually accompanied by a request for information on the correct name for the specimen, its composition, its commercial value, and the manner of its formation.

Frequently the requests include questions of a broader geological nature, or seek the recommendation of a general, easily-read book written on rocks and minerals which may be purchased at a book store or consulted at a library. Moreover, many persons ask how they may determine for themselves the geological specimens which they have collected.

This little booklet has been prepared with the intention of answering the questions most commonly asked by citizens of the state about Missouri rocks and minerals.

Descriptions and photographs of Missouri rock and mineral occurrences are provided, and essential facts about the geological conditions of their formation are simply told. A determinative key is supplied in order that the reader may identify and name most of the common specimens which he collects within the state (and elsewhere, also). No special determinative equipment will be suggested, and only non-technical language will be employed because the chief objective here is to furnish a useful, understandable geological account of the _common_ Missouri rocks and minerals to the average person without geological training. In fact, for the purposes of identification no differentiation is made between mineral and rock, although the professional geologist does separate them in definition. For our purpose, a rock is an aggregate of mineral particles, but a mineral is a substance (without life) having more definite and constant properties than a rock. For those interested further, more technical and more nearly correct definitions, with explanations, are given at the back, on page 74.

The rarer minerals and those requiring special equipment for determination may be sent to the Department of Geology of the University of Missouri at Columbia for identification free of charge.[1]

Names used locally, and sometimes incorrectly from a strictly technical sense, for rocks and minerals will follow the generally accepted names, and both will be duplicated in the index at the back of the pamphlet to facilitate finding either one.

DETERMINATIVE KEY

A rock or mineral specimen which is unfamiliar to the collector may be identified by using the information in this booklet in either of two ways: (1) the reader may turn through the pages and compare his specimen with the photographs of others named there and read their descriptions until he finds a match for his specimen; or (2), the better way, he may classify his specimen first by the use of the determinative key which follows and be directed thereby to the pages in the book for confirmation of the name by the photographs, description, and discussion of the substance. The writer recommends the second method and has prepared this booklet on the assumption that the determinative key will be used.

The simplest and probably the best means of separating specimens of different rocks and minerals is on the basis of hardness, which means _resistance to scratching_. Crushing strength is different from hardness; therefore, in testing for hardness, do not attempt to pulverize. Merely determine if the specimen can be scratched with the substance indicated.

Determination of the mark or “streak” of a mineral when rubbed on a hard white rock or unglazed porcelain is demonstrated in the photograph on page 56.

A. Specimens that can be scratched readily with the THUMB NAIL.
1. Become muddy when rubbed with a wet finger.
Page
Shale 20
Fire Clay 24
Flint Fire Clay 26
Diaspore Clay 27
2. Crumble easily into hard sand grains.
Sandstone 30
3. Chalky, white, porous.
Tripoli 37
Weathered Chert 36
“Cotton Rock” Dolomite 15
4. Clear and glassy, or glistening white; may split and show glassy,
flat faces.
Gypsum 63
B. Specimens scratched readily with a POCKET KNIFE, or IRON NAIL but
not with the thumb nail.
1. Loose sand grains scratched off.
Sandstone 30
2. Granular, but grains are tightly interlocked; also “bubbles” or
effervesces in dilute muriatic (hydrochloric) acid.
Limestone 10
Dolomite 10
Marble 15
Formed in a cave.
Cave Onyx 16
Travertine 16
3. Chalky white, porous.
Weathered Chert 36
4. Black.
Coal 49
Black Shale 20
5. Pebbles or gravel cemented together.
Conglomerate 54
6. Powder becomes muddy when wetted.
Hard Shale 20
Flint Fire Clay 26
Diaspore Clay 27
7. Intense red; leaves a red mark or streak when rubbed on a hard
white rock or on unglazed porcelain.
Hematite 56
Iron Band Diaspore 58
Paint Ore 58
8. Yellow, brown, or black and leaves a yellow-to-brown mark or
streak when rubbed on a hard white rock or on unglazed
porcelain.
Limonite 57
Ochre 57
9. Heavy, black, leaves a black or brownish black mark or streak
when rubbed on a hard white rock or on unglazed porcelain.
Manganese Ore 58
10. Heavy, with bright metallic luster, and lead-colored on a
freshly broken surface.
Galena 59
11. Looks like rosin, or may be ruby-colored or black, but has a
high resinous luster on freshly broken surface.
Sphalerite 60
12. Glassy luster; water-white, milky, honey-colored, pink, gray;
may occur in six-sided crystals, sometimes pyramid-shaped;
always breaks with flat glistening faces; always reacts in the
lump with cold dilute muriatic (hydrochloric) acid.
Calcite 16
13. Like calcite above but may have a pink, pearly luster and curved
crystal faces; reacts with cold dilute acid when powdered but
not readily in lump form.
Dolomite 18
14. Opaque white, glassy or bluish, very heavy, lustrous on freshly
broken surface; does not react with acid.
Barite 61
15. Flaky, micaceous like “isinglass”.
Mica 44
C. Specimens TOO HARD to be scratched readily on a fresh surface with
a pocket knife or iron nail; weathered specimens may be slightly
scratched.
1. Very fine-grained throughout, compact; occurs in nodules,
pebbles; breaks with a slick, curved, oyster-shell-like
(conchoidal) fracture.
Chert, if white, gray or stained yellow or red 34
Flint, if black 34
Agate, if banded 37
Petrified Wood, if it shows the grain or bark of wood 37
2. Granular like sandstone but extremely hard and breaks through the
grains as readily as around them.
Quartzite 41
Quartzitic Sandstone 30
3. Fine-grained, dark green to dark gray to greenish black; occurs
in boulders north of Missouri River and in the granite and
porphyry country or southeastern Missouri.
Basalt 46
4. Very fine-grained, compact, pink, red, brown, gray; usually
“freckled” or sprinkled with grains about 1/16 inch in diameter.
Porphyry 45
Rhyolite 45
Rhyolite Porphyry 45
5. Coarse-grained (BB-shot size to considerably larger), glassy
luster where freshly broken; pink, red, grey.
Granite 38
Gneiss, like granite but _banded_; occurs in boulders north of the
Missouri River 38
6. Coarse-grained, dark green, dark gray, greenish-black.
Gabbro 54
Diabase 48
7. Brassy, metallic, heavy; leaves a black to greenish black mark or
streak when rubbed on a hard white rock or on unglazed
porcelain.
Pyrite 51
Marcasite 51
8. Glassy fragments breaking with rough fracture, or may occur in
six-sided crystals; clear, water-white, milk-white, gray or
pink; in sand grains; in granite.
Quartz 41
9. Intense red; leaves a red mark or streak when rubbed on a hard
white rock or on unglazed porcelain.
Hematite 56
Iron Band Diaspore 58
Paint Ore 58
10. Yellow, brown or black, but leaves a yellow to brown mark or
streak when rubbed on a hard white rock or on unglazed
porcelain.
Limonite 57

ROCK AND MINERAL DESCRIPTIONS

Limestone and Dolomite

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.

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.

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.

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.

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.

Marble

Marble, in a scientific sense, is a metamorphic rock and does not occur as such in Missouri. However, marble has been used as a name in commercial trade to refer to a crystalline, fairly pure limestone, which possesses most of the useful qualities of true marble. In that sense the “marbles” quarried near Ozora and Carthage, Missouri, are very excellent stone. No doubt some recrystallization has occurred in connection with the faulting in the Ozora region, and this may be interpreted as mild metamorphism. The Carthage “marble” is quarried from beds of limestone well developed for structural purposes. These “marbles” effervesce in acid, of course, just as described for limestone.

In this connection it is interesting to note that the polish on limestone or marble is not durable where exposed to the weather in the same sense as is the polish on granite. Because limestone and marble are softer than granite they may be cut and polished at lower cost, but because of their ease of attack by acid, water, and abrasion they soon become dull when used as an exterior stone. For interior decoration they are excellent, of course. Granite contains hard minerals which happen not to be attacked appreciably by dilute acids, and therefore it retains a polish for a long time even where exposed to the weather.

Cave Onyx and Deposits

The stalactites (rock icicles) hanging from cave ceilings, stalagmites built up from the floors, and other drip stone deposits of caves are largely calcite, the mineral of limestone. Again, this can be recognized by the limestone acid test (effervescence, see limestone). Cave onyx may be banded like agate. It is then commonly called Mexican onyx. The name travertine has also been applied to such deposits from water.

Travertine

Travertine is a general name for calcium carbonate deposits of varying size, shape, color, texture, and purity which originate largely through evaporation of spring or surface water. Its composition of calcium carbonate, calcite mineral, is easily confirmed by effervescence in acid, like limestone.

Calcite

Calcite (sometimes called “tiff” locally in south-_western_ Missouri), the essential mineral in limestone, can be recognized by several definite characteristics:

1. It bubbles, “fizzes,” or _effervesces_ in dilute acid. See page 11.
2. It is easily scratched with a knife.
3. It breaks or cleaves into rhombohedral shapes, of which at least
one flat, glistening side is visible on every individual grain
in the broken surface of limestone.
4. It has a glassy luster on crystal and cleavage faces.
5. It crystallizes into six-sided crystal forms, which can be
differentiated from quartz (also six-sided) by tests (1) and
(2) above.

The one single test of calcite which is most diagnostic, and which appeals to most persons, is number one above, effervescence of the solid lump in dilute acid. The bubbles are filled by carbon dioxide gas which comes from, and is freed from, the calcite by the reaction of it with the acid. Calcite is calcium carbonate, CaCO₃.

Many Missourians have not realized that the ordinary, everyday limestone (fine to coarse granular), which is so abundant here, is composed of a mineral—calcite which makes up the grains. The strikingly beautiful calcite crystals (displayed in museums) derived from the calcite crystal caves found in some mines in the Joplin district are accepted without question as _mineral_ specimens of calcite, but the idea that all of the commonplace glistening grains in the local limestone are also mineral grains is a new thought to most persons. A pure limestone is composed entirely of calcite. Even impure limestones which contain subordinate amounts of quartz sand, chert, clay, or iron oxide are in the main also calcite. Dolomite and dolomitic limestones contain the mineral dolomite.

The mineral of ordinary marble is calcite; dolomite marble contains dolomite. The cementing material in sandstone and a common accessory mineral in shale are calcite. It is truly a wide-spread and abundant mineral. Even the lime deposit in the bottom of the tea-kettle, the water heater, boiler, or automobile cooling system is calcite, or aragonite, a twin brother to calcite.

The use of calcite in the form of limestone is treated under limestone. As for the use of large calcite crystals, they are sold as ornaments and curiosities. Visitors to the Missouri State Fair may recall the exhibit of a beautiful, reconstructed crystal cave which was lined with large calcite crystals. Calcite crystals have been shipped in car-load lots to beautify grottos, notably some in Iowa and Illinois, and are displayed in almost all prominent museums.

Water-white (clear), optical-quality calcite crystals, which command a high price, are relatively rare and have not been found in Missouri.

The optical property of calcite which accounts for its high value is its ability to separate, or refract, every single ray of light passing through it into two widely separated, easily distinguishable rays, hence doubling their number. This is called double refraction, and is shown by the double image of an object viewed through the calcite. Instruments which polarize light may contain calcite crystals. The artificial product, “Polaroid”, is used for a similar purpose.

Dolomite

Dolomite mineral occurs in Missouri as a constituent of dolomitic limestone or as a vein and cavity filling in the rocks of the Joplin mining district and as a lining in cavities in the dolomitic limestones of the southern and eastern parts of the state.

Dolomite when _powdered_ (by scraping the surface of the specimen, for dolomite is softer than steel or glass) effervesces freely in cold dilute hydrochloric (muriatic) acid, but the lump dolomite effervesces _very slowly, if at all_. Calcite effervesces freely in the lump with cold dilute acid. This acid test is the one certain test for dolomite, and works with the thick-bedded formations as well as with the showy, crystal-faced material from veins. See page 11.

Dolomite crystals have a pearly luster and are usually pale pink in the Joplin district. Their faces are commonly curved but where broken show glistening to pearly cleavage faces. These properties assume more significance in mineral determination as one becomes familiar with mineral collections, but the non-technical person can rely on the acid test.

With the above information in mind, one may proceed with certainty to identify a layer of dolomite from a quarry or hillside, or a crystal of it in a hand specimen. First, determine that it is scratched readily with a knife blade or iron nail. Anything too hard to be scratched by steel is neither calcite nor dolomite. Second, scrape a small mound of powder on the lump specimens. Third, apply one or two drops of cold dilute acid to the lump near the powder and allow the acid to run into the powder. If the _lump_ effervesces _freely_ the specimen is _calcite_ mineral or limestone rock. If the _lump_ does _not effervesce freely_ but the _powder does_, it is _dolomite_ mineral or dolomite rock, dolomitic limestone. If neither lump nor powder effervesce it is neither calcite (ordinary limestone) nor dolomite (dolomitic limestone). In the latter case, it may be gypsum, barite, Shale, weathered chert, clay, or fire clay, or other rock.

The composition of dolomite is calcium-magnesium carbonate, CaMg(CO₃)₂, and when pure runs about 54½ per cent calcium carbonate and 45½ per cent magnesium carbonate. However, dolomite is _not a mechanical mixture_ of the two carbonates; it is a single crystalline compound wherein the calcium and magnesium are securely interlocked within the arrangement of the atoms. For that reason, the extraction of magnesium metal or other magnesium compounds from dolomite is so difficult and costly that other magnesium minerals, although not nearly so abundant and accessible to industry as dolomite, have been processed to obtain the lightweight metal magnesium.

The thick beds of Missouri dolomitic limestone (and some fairly pure dolomite) have been used chiefly as agricultural stone for soil sweetening, for building stone, gravel, and other purposes to which rough stone is put.

Shale

Shale is a compressed, and layered or laminated clay or mud rock. Consequently it will return to mud if it is wetted with water and rubbed. This may serve as a test for shale. It may occur in thick layers or formations, five, ten to fifty or more feet in thickness, and it ranges downward to paper-thin partings between beds of limestone. It is also commonly associated with coal beds. The color of shale varies from light gray to black, or it may be tan, yellow, red, rust, purplish, or green. It is platy, and these thin plates or laminae, piled on each other, make up the shale bed.

Some shales are hard, tough, and strong enough to serve as temporary mine roofs. Hard shales are sometimes called “slate” but this name is technically incorrect. _True slate_ is a metamorphic rock, composed chiefly of the mineral mica in very fine flakes, and will resist the action of water (weathering) for a long time. Therefore, it is a good roofing material for buildings, whereas shale is composed chiefly of clay minerals, and despite the strength and compactness of the more “slaty” varieties soon disintegrates in water. Missouri “slaty” shale would not serve as satisfactory roofing material.

The red “burned” shale found on burned-out coal mine dumps is called “shale” locally. It is, of course, shale which has been fired more or less to the condition of building brick by the hot burning waste coal. The same original shale could be crushed, molded into brick, “burned” in a kiln, and become a satisfactory building brick. The “burned shale” of the coal mine dumps is used in many places as a drive-way covering.

“Soapstone” is a name applied by some persons to some soft, slippery to greasy shales, but this name is incorrect in a technical sense. True soapstone is a metamorphic rock (shale is sedimentary) which is composed chiefly of the mineral talc. Soapstone occurs abundantly in certain parts of the Appalachian Mountains but is exceedingly sparse in Missouri.

The chief commercial uses of shale are in the manufacture of common brick, building brick, building tile, drain tile, sewer pipe, Portland cement, and other ceramic products. Many shale beds and occurrences are technically suitable for these uses but have no real commercial value because other necessary factors are lacking. In order to make brick, tile, or cement there must be sufficient fuel available at low cost, low-priced bulk transportation of the raw and finished products, available labor, capital for the erection of a plant, and above all a large near-by, dependable market for the manufactured product. The value of a shale deposit, therefore, depends as much upon outside conditions as upon the properties of the rock (shale) itself.

The shales of Missouri were formed from deposits of mud that settled out in sea water which in the past covered this state. Fossil remains of sea-living organisms which are preserved in the shale give evidence of the marine conditions once existent here. Like the muds that are accumulating along the Atlantic coast and in the Gulf of Mexico, where the Mississippi River is discharging its load of silt and clay, so did mud form layers on the bottom of geologically ancient interior seas. In some cases sand was later washed in and covered the mud; in other cases limestone-forming material (like off the coast of Florida today) was deposited on top of the mud. The weight of the overlying beds and the slow movement which raised the sea bottom up to land squeezed out the excess water, compressed and compacted the muds into thin layers, and brought about the shale rock which is exposed to us today.

Black muds, rich in humus and other organic material, formed black shales; red and yellow clays colored by red and yellow iron oxides (iron rusts) formed red and yellow shales; and sandy muds were compacted into gritty, sandy shales. All of them were derived from eroding land and soils just as today our eroding soils contribute to the formation of more shale now in the long, slow process of formation.

The chemical composition of an average shale is not simple, as is shown by the subjoined composite analyses of sedimentary rocks taken from U. S. Geological Survey Professional Paper No. 127.

78 shales 253 sandstones 345 limestones

SiO₂ 58.11 78.31 5.19
Al₃O₂ 15.40 4.76 .81
Fe₂O₃ 4.02 1.08 .54
FeO 2.45 .30
MnO 2.44 1.16 7.89
CaO 3.10 5.50 42.57
Na₃O 1.30 .45 .05
K₂O 3.24 1.32 .33
H₂O+ 3.66 1.32 .56
H₂O- 1.33 .31 .21
CO₂ 2.63 5.04 41.54
TiO₃ .65 .25 .06
P₃O₅ .17 .08 .04
SO₃ .65 .07 .05
Organic carbon .80 — —
(100.) (100.) approx. (100.)

Many persons upon learning that average shale, and even “clay dirt,” may contain 15% alumina, Al₂O₃ (equivalent to almost 8% metallic aluminum), become thoughtlessly and erroneously enthusiastic about aluminum ore possibilities on their farms or properties. The aluminum is there all right, but it is so securely combined with silica and other elements that the cost of extraction is now greater than the price of aluminum obtained from less abundant ores. Until chemists find a method of extraction of the metal from ordinary clay or shale that can be carried out at considerably less expense than is now possible, the vast quantities of clay and shale on the earth’s surface must be considered a distant reserve of a prohibitively high cost aluminum.

Missouri possesses a little bauxitic clay in the southeastern part of the state but unfortunately does not contain deposits of high grade bauxite, the chief ore of aluminum, and so does not contribute to the aluminum production of the United States (see the discussion under DIASPORE CLAY). Arkansas is a leading producer of bauxite, but the geological conditions present in that bauxite locality are so different from Missouri geology that little hope is held for finding bauxite in Missouri, except possibly in the extreme southeastern part.

Fire Clay

Fire clay resembles shale in that it is also a clayey rock and becomes muddy upon wetting and rubbing. It differs from shale at sight in that it (fire clay) is not laminated like shale, but occurs instead in a massive structure which is relatively uniform throughout. Fire clay fractures naturally into blocky or irregular fragments ranging in size from boulders to rough flakes, whereas shale weathers into layered, platy chips.

Shales are commonly buff, yellow, reddish, greenish, or brown in addition to gray in color, whereas good useable fire clay predominates in white, cream, and gray to almost black (if much organic matter is contained in it). Shale is ordinarily gritty with hard sand particles, but most good Missouri fire clay contains only a small amount of sand. Of course, fire clay may grade into sandstone through a sandy clay phase, but this part would not be confused with a layered, gritty shale.

The really determining characteristic of fire clay is its resistance to melting under high temperature. The most positive test for this property is to heat the fire clay to a white heat in comparison with standard preparations (Pyrometric Test Cones) whose fusion temperatures are known. Most of Missouri fire clay will withstand a clean oxidizing heat of over 3000° Fahrenheit without melting.

Clay minerals originate, in general, from the weathering of previously existing silicate rocks and have therefore been called, on occasions, “rotted rocks.” The writer has long insisted that clays, particularly fire clays, should be thought of instead as purified or refined rocks. The original silicate rocks and minerals, which were rich in constituents melting at low temperatures, have been soaked, leached, and washed by chemically active ground water and rain water until many of the undesirable elements have been carried away, leaving a refined material which we use and know as fire clay. Missouri possesses one of the largest reserves of finest quality fire clay in the world. Special bulletins on Missouri fire clay are published by the State Geologist, Rolla, Missouri, and may be obtained from his office.

Persons who have undeveloped fire clay deposits on their property frequently ask advice on whom to contact and how to arrange for sale of their fire clay, with the expectation of a fair return and fair treatment. The writer recommends in such cases that the owner of the clay dig into his deposit to obtain a fresh, clean, _representative_ specimen of his fire clay (about one pound) and send it to one or more of the large substantial fire brick or refractories companies operating in Missouri. Obviously the company located nearest the deposit, or with the lowest-cost shipping facilities, will be in a favored position to purchase the clay. If the individual is skeptical about the trustworthiness of the company’s report, he may send opposite parts of the sample lumps to competitive companies. Of course, the individual may have his clay tested by an independent laboratory at his own expense, but this is ordinarily a useless, costly experience because a company will duplicate those tests in its own laboratory before purchasing the clay. If the refractories companies find the clay useful to them they will proceed with negotiations. If the clay is of inferior quality or if it is not needed by the particular company at _that time_, even though of acceptable quality, usually the company will return a truthful report at no cost to the clay owner.

The same general advice is given in regard to the development of any mineral deposit which the holder may have. The caution about obtaining a representative sample is especially to be emphasized. It applies to the metallic ores, mineral water, and common rock as well as to fire clay.

Plastic Fire Clay

Plastic fire clay forms a sticky, soft mass when wetted and kneaded with water, and will bond together other clays or rocks. Large plastic fire clay deposits occur in Audrain, Callaway, and St. Louis counties, and lesser quantities are known in Boone, Osage, Gasconade, and Phelps counties. The larger deposits assume a blanket shape with a highly irregular lower surface.

Flint Fire Clay

Flint fire clay is very fine-grained, smooth or slick, and breaks with a shell-like (conchoidal) fracture. It varies in color from white to black, but most flint fire clay mined is near to white. It is relatively non-plastic—that is, does not readily slake or form a sticky mass when worked a little in water. In fact, flint fire clay has been used locally as road surfacing because it does not become very muddy and sticky. Of course, it is inferior to black-top or concrete road surfaces and has too high a commercial value now to be used extensively as road metal.

A hard, white variety of flint fire clay which breaks with numerous conchoidal fractures in appropriate shaped fragments has been called locally “pop-corn flint.” This clay, and other sand-free flint clay, when crushed between one’s teeth “goes to water” in the mouth. Many clay miners use the chewing test to establish the freedom of their clays from gritty sand, which renders flint clay inferior in quality.

Flint fire clays occur geologically in old land depressions and in roughly funnel-shaped pits surrounded by an enclosing layer of sandstone, the whole lying within limestone country rock. The most prominent flint fire clay deposits are found in Callaway, Warren, Lincoln, Osage, Gasconade, Maries, Franklin, and Phelps counties.

Diaspore Clay

Diaspore clay is a harsh, usually porous, earthy type of clay which has been found in Warren, Osage, Gasconade, Maries, Franklin, Phelps, and Crawford counties in Missouri. Some diaspore clay is mealy, or finely granular, some is chalky to compact, and much of it is more or less oolitic. Oolites (oolitic structure) are small rounded bodies varying in size from about bird shot to BB shot size, and those in diaspore may be solid or hollow. Their hollow structure contributes to the porous condition in diaspore clay. See page 29.

It is almost impossible to write a description of diaspore clay which can be used to determine it because the clay has so few individual characteristics. A person familiar with diaspore clay, however, can recognize it at a glance. Probably diaspore clay will not be found outside the counties listed above, and within those counties many persons know the clay from contact with the commercial production of it.

Diaspore clay occurs in old sink-hole, funnel-shaped pits which formed in the dolomite (limestone) underlying that region. A sandstone layer which lines the pit and commonly stands somewhat above the level of the clay because of the sandstones superior resistance to weathering is known as the “rim rock” of the pit. The diaspore clay may be thought of as an extra-refined type of fire clay from which silica has been leached during prolonged solution in swamps and ground water and the more stable alumina (Al₂O₃) left behind as the refined product.

Pits in the diaspore region may contain from a few truck loads of clay to over 50,000 tons of it, but a pit which produces 10,000 tons of good clay is a valuable and not very common deposit. A small fortune falls to the landowner who finds a large diaspore deposit (pit) on his farm, for royalty rates at this time are not less than $1.00 per ton for first grade, 70% Al₂O₃ clay.

Because of the high value of diaspore, a highly competitive prospecting, leasing, mining, and brokerage business has developed in the diaspore region. Practically all of the thrills, hopes, disappointments, and good fortunes that are associated with oil booms are found in this business and clay area; clay pits are only smaller in scale than wild oil gushers. Clay scouts work in secret, mining leases are contested in court, rumors fly fast in English, German, and German-Swiss over the country telephones, prospecting results may be hidden, personal pressure may be brought to influence a deal, and speedy salesmanship is employed when an exciting find is in the offing. When the legends, traditions, and facts of the diaspore region are collected and recorded, an interesting and essential part of Missouri history will have been written.

Missouri has the only locality in the entire world where relatively pure diaspore clay is now mined in commercial quantities. Because of its extreme resistance to fusion under very high temperatures, diaspore has been called the “aristocrat of fire clays.” Diaspore contains a higher percentage of aluminum than does bauxite, the chief ore of aluminum, but because of diaspore’s extremely refractory nature it is less easily reduced to aluminum metal than is bauxite, and therefore finds a more specialized use in the manufacture of refractory and super-refractory brick and tile which may even be used in furnaces to calcine aluminum ore. Where resistance to very high temperature has been required, diaspore super fire brick has been remarkably useful.

Burley Clay

Burley clay is a fire clay intermediate in alumina content between flint clay and first quality diaspore. It takes its name from the oolites (rounded pellets of diaspore) which are scattered through a flint clay base and which were called “burls” by the early clay miners. As the relative number of diaspore oolites increase, an otherwise flint clay becomes burley-flint, then typical burley, and finally grades into second quality and first quality diaspore. Clay in any stage of the variation may be found in some part of the diaspore region or pits. Most of the remarks written on diaspore apply as well to burley clay.

Sandstone

Sandstone is a rock made of sand-size particles more or less well cemented. It is recognized by the grains of sand which are dislodged or scratched loose when the rock is broken, or when scraped with a piece of steel or another hard rock. The old-fashioned grindstone is a sandstone nicely cemented by nature.

Sandstone occurs in thin layers to thick massive beds and deposits which may exceed fifty feet in thickness. In addition to possessing horizontal bedding and parallel bedding planes, some sandstone displays beautiful, intricate cross-bedding or cross-lamination.

The grains of sand composing the stone may be either angular or rounded. They may sparkle in the light from reflections from their crystal faces, or they may have dull, frosted surfaces. Sandstones are ordinarily nearly white in color except where the grains are covered with coatings of yellow or red iron oxide (rust).

The grains themselves are predominantly particles of the mineral quartz, although any rock or mineral of sand size may be present in sandstone. The quartz (see discussion of quartz on page 41) may have been derived from pre-existing sandstones or more directly from granite, porphyry, or other igneous rocks in which quartz crystallized when the hot liquid rock solidified. Today quartz grains which are weathering out of igneous rocks and sandstones are being carried by the Missouri river and tributaries to the Mississippi river and thence to the ocean, where extensive deposits of sand are accumulating, probably destined to become widespread beds of sandstone.

The grains of sand may be broken and become angular during their long trip to the ocean, or they may become rounded by rubbing against each other. If they exist in sand dunes, blown about by the wind before being cemented into rock, the grains usually become somewhat rounded. Even after sandstones are buried beneath other rocks, silica, which is carried in solution by ground waters percolating through the sandstone, may crystallize out on the sand grains and restore some brilliant, angular crystal faces to the otherwise rounded grains.

Cementation of loose sand to more or less firm sandstone is due to the presence of clay, iron oxides, or calcite (mineral of limestone) which may be deposited with the sand. All of these cements are softer and weaker than quartz, thereby being broken first and freeing the harder quartz when the rock is scratched or crushed.

A variety of very hard sandstone called quartzite is one that is so strongly cemented that it breaks through the sand grains instead of around them as is the case with ordinary sandstone. This condition is brought about by their being cemented with silica (chemically the same as quartz), which makes for essentially uniform hardness throughout the rock.

Quartzites are, as previously noted, extremely hard, and resist abrasion and chemical weathering. Reddish quartzite boulders occur rather abundantly north of the Missouri River in the glacial clay, sand, and gravel which overlie the sedimentary rocks that form the bed rock or country rock there. Locally, the hard, red quartzite boulders may be called “red niggerheads”, although the term “niggerhead” is more often applied to black or dark greenish black boulders of basalt (see page 48) also present in the glacial drift. It is to be recalled that the distinguishing hardness of quartzite is due to the hardness of the quartz grains plus the equal hardness of the silica cement.

Asphaltic sandstone is a sandstone impregnated with a bituminous residue from the evaporation of petroleum which once occupied the pores of the rock. It has been reported from more than a dozen counties in western Missouri, but the most extensive deposits are probably in Barton, Vernon, and Lafayette counties.

Attention has been directed to the origin of sandstones from ocean deposits of sand and from sand dunes, but it should be recognized also that river channels and stream valleys which contain deposits of sand (such as those on floodplains, river bottoms, and sand bars) may be covered, and the sand consolidated to sandstone. Many years ago, even long ago geologically, a large river, almost comparable in size to the Missouri river, occupied a channel which is now represented by a long narrow sandstone deposit extending from a little north of Clinton through Warrensburg to Lexington and then east through Moberly almost to Paris. Smaller channel sandstones are abundant in other areas in Missouri.

The sandstones of the so-called Roubidoux formation, which occurs in south central Missouri, commonly show well-preserved ripple marks on the rock slabs. These marks were formed exactly as their name suggests—in sand which was thrown into ripples by the shallow water in which it accumulated and was covered and cemented so as to retain the ripple forms.

Sandstone is used for building stone, walks, grindstones, furnace linings, and rock gardens. Large quantities are mined each year near Pacific, Festus and Crystal City, Klondike, and Hermann, for the manufacture of glass and other uses. Common glass is a cooled melt of relatively pure silica sand, soda ash, and lime. Asphaltic sandstone is used in road building. Sand-lime brick are made of sand. Sand is used as a molding material for metal castings, a parting substance between brick in kilns, and in large quantities in concrete and mortar mixtures.

Chert, Flint

The names chert and flint have in some regions been used for the same hard, fine-grained rock found so abundantly in Missouri, but correct usage employs chert for the white and gray varieties, and flint for the black variety. Flint may be thought of as slightly impure chert, a chert which is colored black by a small amount of pigment, usually fine carbon, or perhaps iron sulphide, scattered through it like fine dust.

Chert is characterized by being harder than glass, brittle, very fine-grained, and by breaking with a smooth, rounded or hollowed clam shell-like (conchoidal) fracture and sharp edges. It was used by Indians to make arrow heads. It accumulates in abundance both in stream beds as gravel which has been more or less rounded by wear, and on the hillsides within the soil and sub-soil. Yellow and red iron oxides may stain and penetrate weathered chert gravel so that it becomes reddish, rusty, tan, yellow or brown.

Chert remains abundant because of its extreme resistance to weathering. It is so hard that stream action wears it only very slowly. Its chemical composition is silica, SiO₂, a substance which is but little affected chemically by ground water. Where chert has contained fine grains of calcite scattered through it, the calcite may be removed in solution, leaving pores, and a zone of porous, light weight, tripolitic chert, harsh to the feel and enveloping an unaltered interior (See WEATHERED CHERT). Not uncommonly, fossil remains of calcite which were embedded in chert have been dissolved, leaving their hollow impressions.

Chert in Missouri originally occurs chiefly in limestone formations, where it is found as nodules, lenses, stringers, and irregular forms in and between the limestone beds. Chert and flint may be deposited directly from silica in solution, or they may replace (substitute for) wood, fossils, or older rock where silica-bearing solutions contact and react with the replaced substance. For example, petrified wood usually is wood which has been replaced molecule by molecule with silica. This statement applies equally to the brightly colored petrified wood in the Petrified Forest in Arizona and to that with comparatively drab coloring in Missouri. Many other siliceous fossils, notably animal remains, are replacements of calcite (limestone) by silica.

In anticipation that the reader may have difficulty understanding how silica may go into solution if chert (silica) is hardly attacked by the weathering process, it should be explained that silica is freed in solution predominately during the weathering of complex silica-combinations, silicates, rather than from uncombined silica. For instance, feldspar and pyroxene from granite or gabbro weather in ground water to a soil-forming clay mineral and release some silica in solution in the ground water. After this silica is redeposited in an uncombined form, like chert, it becomes highly insoluble.

An observation in regard to flint is that the metallic “flints” which are used to ignite gas burners or cigarette lighters are not black chert, SiO₂. Instead, they are special alloys containing rather uncommon elements which possess the useful characteristic of emitting a brilliant hot spark when harshly scratched.

Chert is the chief source of natural gravel in Missouri because it accumulates in stream beds and on hillsides on account of its resistance to weathering. The piles of “chats” in the Joplin region, containing thousands of tons of crushed chert, have been used in part in road surfacing material.

Weathered Chert

Weathered chert, or leached chert, is a white to gray, or yellowish, porous, light-weight, harsh to feel, chalky-appearing rock which occurs over much of the southern half of Missouri. It does not effervesce in acid. Usually it occurs as a zone from a fraction of, to more than an inch in thickness, about a denser core of hard, compact chert (flint), or makes up an entire small rock fragment or gravel.

It develops as a relatively insoluble residue left when the more soluble rock material in association has been leached away during the weathering process. Its composition approaches pure silica. It has no established use and no commercial value.

“Kaoleen”

“Kaoleen” is a term used locally in part of south-central Missouri to refer to a chalky, white to tan or buff, porous weathered chert, but the name should be dropped because it is unnecessary (use weathered chert), confusing, and not recognized elsewhere. Most probably the term arose in corruption of the word _kaolin_, which is the name for a true, high-quality clay, to which the leached and weathered chert bears a slight resemblance. Kaolin has the chemical composition of clay (hydrous aluminum silicate), whereas “kaoleen” is impure silica. See the discussion on Weathered Chert.

Tripoli

Tripoli occurs in the vicinity of Seneca, Newton County, Missouri. It is a light-weight, porous, white to creamy, siliceous rock, which may be scratched because of its softness. Tripoli represents the porous insoluble residue of an earlier rock, which was composed of skeletal insoluble silica and interstitial soluble calcium carbonate (calcite), the latter having been dissolved away by ground water. Tripoli has a chalky appearance but is totally unlike chalk chemically. Tripoli is nearly pure silica, whereas chalk is calcium carbonate. Any tripoli-like rock found in Missouri outside the region of tripoli mines is likely to be a fragment of weathered chert which is described above.

Tripoli has been used as an abrasive, a polishing agent, a parting material in molding sand, and a filter rock.

Agate

Agate is a banded variety of chert. Although the chemical composition of agate is SiO₂, the same as chert, a microscopically fibrous part of it having a waxy luster or varying in color or translucency may give the appearance to the rock that we associate with the name agate. The mineral name chalcedony is given to the fibrous, waxy material.

Typical agates are most abundant in Missouri in the glacial and stream gravels in the northern part of the state, although part of the Potosi drusy quartz and chalcedony in the southeast is also prized. The large gravel pit near LaGrange, in the northeast, has furnished many beautiful specimens, not only of agate, but also of petrified wood and fossils.

Missouri lapidists and collectors of semi-precious stones find plenty of interesting raw material within their own state.

Jasper

Jasper is chert which is colored red or yellowish brown by iron oxides.

Granite

Granite is a granular (coarse-grained) rock which has a glassy luster and is too hard to be scratched appreciably by steel. It may be white to gray, tan, brown, or pink to red in color, but pinkish to red granite predominates in Missouri. Some black stone, referred to locally as “black granite,” is usually a variety of gabbro. Most Missouri granite is coarse-grained, so that the constituent mineral grains—quartz, feldspar, and (less frequently) mica—can be readily recognized by anyone familiar with those minerals. It makes up many of the mountains and hills in Iron, Madison, and St. Francois counties and adjacent regions. North of the Missouri River, or where the glacial deposits remain, granite boulders may occur in the sandy and clayey glacial drift.

The mineral quartz is recognized in granite by its glistening, oily luster, really more brilliant than the luster of glass, and by its curved to irregular broken surface. Furthermore, the brilliant luster of quartz is not dulled by exposure to weather.

The mineral feldspar, in granite, has a glassy luster on the tiny flat cleavage faces where the individual grains are broken. Where weathered, feldspar becomes dulled, and chalky to dusty or clayey. Fresh feldspar may be glassy, white, buff, pink, red, in intermediate shades in color. With the mica, it imparts most of the color to granite.

Mica is recognized by its softness and its ability to be split very easily into tiny flakes. Other minerals may be found in granite under the microscope, but they have little importance or significance.