The Common Rocks and Minerals of Missouri

Chapter III: Introduction (2)

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Granite is an intrusive igneous rock; that is, it solidified from a hot liquid state (like lava) in a large body, beneath, or surrounded by pre-existing rocks. Because of slow solidification a coarse-grained texture was developed. In southeastern Missouri where granite is now exposed at the surface (for example, the Elephant Rocks State Park at Graniteville), that granite was covered originally by hundreds of feet of rock at the time it solidified from a liquid. During the millions of years which have elapsed since the granite solidified, its cover and the upper part of the granite have been eroded away by streams and rain after weathering to soil material. In fact, the ocean has covered the area several times during its long geological history.

Missouri has a fine quality of granite in large quantity in southeastern Missouri. Granite is used for building, structural and monument purposes (see discussion under marble), for rubble stone, rip-rap, ballast, gravel, paving blocks, crushed chicken gravel, and for other specialized uses where favorably located. Chemical analyses of granite and porphry, taken from Missouri Geological Survey Report, Volume VIII, 1895, follow.

Porphyry 6 miles east Granite 6 miles east
of Ironton of Ironton

SiO₂ 71.88 72.35
Al₂O₃ 12.88 13.78
Fe₂O₃ 3.05 1.87
FeO 1.05 0.36
CaO 1.13 0.87
MgO 0.33 0.42
K₂O 4.46 4.49
Na₂O 4.21 4.14
P₂O₅ 0.15 0.13
TiO₃ 0.22 0.44
Ignition loss 0.26 0.54

The glacial granite boulders found in central to northern Missouri also solidified as intrusive rock in the northern United States or in Canada. After being exposed at the surface they were picked up and carried down by the geologically recent, continental ice sheet (glacier) that moved down from Canada to across the northern half of Missouri. Scratches and grooves may have been cut in some of these boulders, or flat faces scoured and planed off as they were scraped against other hard rocks. Quartz gravel is usually present, often in abundance, in glacial deposits. Small specimens of native metallic copper, which come from near Lake Superior, have been found in Missouri glacial deposits. Even diamonds from an unknown source in the north were carried by the ice down into the United States. The history of the glaciation is a spectacular account of changes which our continent has undergone in the geological past.

Quartz

Quartz is a mineral of wide-spread occurrence which is characterized by the following properties: (1) it is considerably harder than glass or steel, (2) it has a high luster, glassy to oily, (3) it breaks with an irregular or rough glistening fracture, and (4) it crystallizes in six-sided crystals when it grows unobstructed. Ordinary acids do not attack quartz, and it is relatively unaffected by chemical weathering in Missouri. Its composition is silicon dioxide, SiO₂.

Quartz occurs in granite as the lustrous, partially rounded grains which constitute perhaps 20% of the rock (feldspar makes up most of the more opaque remainder which breaks with many small flat faces), and is recognized in the small glistening grains in the porphyry. Hence it is an important igneous rock-forming mineral.

Sandstone in Missouri is made almost entirely of quartz grains which have been broken, worn, and more or less rounded during their long travel history. The sandstone formation quarried and mined near Pacific and Crystal City, named the St. Peter sandstone formation, is an outstandingly pure quartz sandstone and therefore usable in the glass industry. It is obvious that quartz is an important constituent of sedimentary rocks.

Further, in the sedimentary dolomite formation near Potosi, fine to coarse quartz crystals line the surfaces of cavities and pockets in the stone. This cavity coating of quartz which reflects light brilliantly from the many small crystal faces is called drusy quartz by the mineralogist, but is locally and popularly known as “blossom rock.” Thousands of pounds of “blossom rock” are sold each year for rock gardens and various ornamental purposes.

In northeastern Missouri quartz crystals line the hollow, more or less spherical bodies called Geodes, which vary in size from small nuts to melons, and weather out of the so-called Warsaw formation. Other types of hollow cavities in many Missouri rocks may contain quartz growing inward from their walls.

Missouri chert is composed primarily of quartz in microscopically fine grains; likewise, agate and petrified wood may contain abundant quartz. Other varieties are rock crystal, rose quartz, amethyst, false topaz, bloodstone, carnelian, and onyx.

Quartz crystallized in an igneous rock as the hot fluid cooled through its “freezing” temperature interval, which was probably not below 1000°F. In the cases of the quartz in geodes, the drusy quartz, or that in cavities within petrified wood, quartz crystals grew from ground water solutions which must have carried very low concentrations of silica in solution, and whose temperatures did not depart far from that of rocks buried at various depths today. Although quartz is a very common and abundant mineral, our specific knowledge about its transport and deposition is surprisingly meager.

Quartz crystals are used in large quantities in radio apparatus where it is necessary to maintain very close control on the tuning of a circuit. This use requires quartz of highest quality and crystals above minimum size, which have never been found in Missouri and probably are not present. Silica production from this state is in its sandstone, tripoli, chert chats, and rock garden ornamental stone.

Feldspar

Feldspar is a white to pink or red mineral having a glassy luster on its flat broken surfaces (cleavage faces). It will scratch window glass.

It is the most abundant mineral in granite and usually controls the color of that rock; for example, the red granite at Graniteville contains red feldspar, and the pink-gray granite in the Knoblick region has feldspar of those colors. Small bodies or bands of very coarse feldspar, quartz, and mica (pegmatite dikes) which cut the granite may contain crystals of feldspar large enough to be recovered as small, hand specimens, but otherwise it does not occur in coarse fragments. The recognizable crystals, or phenocrysts, in the porphyry are mainly feldspar.

Feldspar is really a family name for a group of several minerals, all of which are crystallized in the igneous rocks. The potassium (potash)-containing varieties, named orthoclase and microcline, occur in the granite and porphyry, whereas plagioclase, a calcium-sodium (lime-soda) feldspar is in gabbro, diabase, and basalt.

Plagioclase commonly has a thin, lath shape, is a shade of gray, and makes up the lighter colored part of the greenish to dark gray igneous rocks. Further differences between it and orthoclase may interest the mineralogist but are of little concern to the non-technical person.

Pulverized feldspars are used extensively in the ceramic industries, but Missouri does not have any productive deposits. Under natural, long-time weathering processes feldspar usually decomposes to clay which may be used technically, but the usual fate of it is soil formation.

Mica

Mica, incorrectly called isinglass, is an elastic, fairly soft, platy mineral, which may be split into flakes of paper thinness. The relatively clear variety is called muscovite, and the brownish black to black variety is biotite, both being members of the mica family. They may occur in Missouri in small grains in the igneous rocks, except that muscovite may be present in sandstone, where it was deposited along with the quartz sand.

Mica is used chiefly as insulating material in the electrical industry where large sheets are required. Another use is as window or chimney material in stoves or lanterns. Missouri has no mica which is satisfactory for these purposes.

Porphyry, Rhyolite, Rhyolite Porphyry

Porphyry and granite are the two most abundant igneous rocks in southeastern Missouri (Iron, Madison, and St. Francois counties, and adjacent country). The porphyry there is a compact, very fine-grained, almost glassy, hard, brittle rock that varies in color from light gray through pink and red to dark purplish red and almost black. It always breaks with a horny, flinty fracture. Small mineral crystals of glistening quartz and usually reddish feldspar are generally scattered throughout the dense background (groundmass). The crystals are commonly about one-sixteenth of an inch in cross section and ordinarily constitute from about ten to twenty per cent of the rock. Other names, somewhat more specific than simple porphyry, which are applied technically to certain phases of the rock are rhyolite, and rhyolite porphyry.

The porphyry of southeastern Missouri is igneous rock which in the main poured out as lava flows, millions of years ago. Volcanic dust or “ash” was erupted during the same period, and layers of it, now strongly cemented, are found in association with the flow rock.

The Missouri rhyolite porphyry has about the same chemical composition (see page 40) as Missouri granite, but whereas granite is coarse-grained, the porphyry has an extremely fine-grained to almost glassy ground-mass. This difference in texture (grain size) is due to the difference in rate of solidification. The porphyry lava flows chilled and solidified very rapidly, thereby freezing the liquid to glassy and extremely fine-grained rock, except for the scattered larger crystals (phenocrysts) which had developed prior to eruption. Granite, on the other hand, solidified very slowly under a thick cover of rock which acted as a heat insulator, and during the long time of solidification large or coarse grains of minerals could grow and develop by crystallization so that a coarse-textured rock (granite) was formed.

The relative ages of the Missouri igneous rocks are of interest to geologists and to most persons who recognize the different types within a small area. It has been found that the prophyry was invaded by the granitic liquid, that both the porphyry and granite were cracked after solidification, and that liquid basalt rose and filled the cracks. Hence the porphyry is the oldest, the granite next in age, and the basalt is youngest. In fact, it may be mentioned in passing that some basalt and allied dikes have been found cutting through the sedimentary sandstone, shale, and limestone which overlie the igneous granite and porphyry and are much younger.

Missouri porphyry has little use or value other than of bulk or crushed stone.

Basalt

Basalt is a fine-grained, dark-gray, dark-green, or greenish-black rock which is hard enough to be scratched with difficulty by steel. It originated by the solidification of lava. Today, basalt rock is forming where lava at the Hawaiian volcanoes solidifies.

The relatively small amount of basalt in southeastern Missouri solidified mostly in cracks within other rocks through which it rose. Those occurrences—that is, fillings in nearly vertical cracks—are called dikes. The basalt dikes in southeastern Missouri have been exposed by the weathering and erosion of rocks which previously covered them.

In northern Missouri, boulders of basalt may be found in deposits of glacial clay, sand, and gravel (glacial drift), where they were left after the melting of the great ice sheet which brought the basalt down from ancient dikes and igneous bodies cropping out in the northern United States and Canada. Many of the boulders have been rounded by weathering, and their shape, together with their dark color, has stimulated the local name “niggerhead” for them.

Basalt is a strong, tough, well-knit rock that will withstand heavy blows from a sledge hammer, which usually rebounds upon striking. Except for use as rubble stones, basalt has no commercial value. It weathers characteristically to a yellowish, brownish, or reddish surface coating of iron oxide and clay.

Gabbro and Diabase

Gabbro and Diabase are dark-colored, coarse-grained, hard igneous rocks, which may be found in the granite and porphyry regions of southeastern Missouri and as separate boulders in the glacial deposits north of the Missouri River. Both resemble basalt, which has been described in detail elsewhere, except that basalt is fine-grained, whereas gabbro and diabase are coarse-grained (separate grains easily distinguished without a magnifying glass). The layman is ordinarily not concerned with the technical differences between gabbro and diabase, which appear about the same. Both contain plagioclase feldspar (see FELDSPAR) and a dark green mineral of the pyroxene family.

Gabbro and Diabase are sometimes called “black granite.” Their chief use is as bulk or rubble-stone, although special varieties may be used for building purposes.

Coal

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.

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.

Pyrite and Marcasite

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.

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 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.

Gneiss

Gneiss is a hard, granular rock which exhibits a coarsely banded structure (resulting from metamorphism). The bands are evident because of color differences due to different mineral content; those dark in color are commonly rich in dark mica (biotite) or hornblende (a dark green to black, hard mineral), whereas the light bands contain feldspar and quartz. Many gneisses have about the same mineral composition as granite; hence, for our nontechnical purposes, a banded rock, otherwise granite-like, is a gneiss.

Gneiss is a metamorphic rock, a _changed_ rock. The banded structure was developed by a combination of very high pressure, high temperature, and solutions acting on a previously existing rock in essentially a solid condition. The original rock may have been an igneous or sedimentary rock which has been crushed or made to flow into bands, or has been re-crystallized. The tremendous pressure which operated during the banding of most gneisses also crumpled square miles of rock thousands of feet thick into folded and broken (faulted) mountains. True slates, marbles, and some quartzites are formed from soft shales, limestones, and sandstones, respectively, in the metamorphic process.

Almost no metamorphic rock of this regional type crops out in Missouri, but the boulders of gneiss which are found in the glacial deposits were picked up in Canada or the northern United States and carried to Missouri by a continental glacier thousands of years ago.

Except for use as bulk stone or possible structural purposes the gneiss in Missouri has no value. The glistening yellowish mica sometimes seen in gneiss is not gold, of course, and is likewise valueless.

Hematite

Hematite (“keel”) is a heavy, red to purplish red, dull to glistening mineral which leaves a red mark or streak when rubbed on a hard white rock (like chert) or on unglazed porcelain. This red color of hematite coating or stain is responsible for our red clays, red soils, red iron rust, reddish creek-water, and almost every bit of natural, red mineral matter in Missouri. Hematite is iron oxide, Fe₂O₃, and has a close associate, limonite, which is yellow to brown in color, and has the chemical composition Fe₂O₃·nH₂O. The two are mentioned together here because they are commonly associated in nature, where they can be recognized in mixture by the yellowish red or reddish brown colors on rocks or soils. Individual discussion is given limonite under its heading, but its relationship to hematite is repeated here for obvious reasons.

Hematite varies in hardness enough that some specimens can be scratched easily with iron, whereas others are almost as hard as that metal itself. Where clay occurs mixed with hematite, as in paint ore, it may be quite soft, but “blue kidney ore” is usually hard.

Hematite is the ore (iron ore) mineral at Iron Mountain and Pilot Knob mining districts and in the various sink-hole mines or pits in south central Missouri. Scattered boulders of hematite occur in non-commercial quantities within a shaly layer (lower part of Pennsylvanian age rocks) which crops out extensively in central Missouri, and the finding of these boulders has at times, unfortunately, stimulated short-lived hopes of locating a valuable deposit of iron ore. Flaming red soil or mountains of red solid rock (as are present in western United States) may be colored by less than five per cent iron oxide and are in no sense iron ore because the iron is not concentrated. Iron is the fourth most abundant element in the earth’s crust, but workable iron mines and deposits are few and far between. To be commercially valuable an iron ore deposit must contain tens of thousands of tons and be relatively free from impurities, notably sulphur and phosphorus. Hence, not many Missouri farms are locations of iron ore deposits.

The origin of Missouri hematite is about as diverse as its occurrences. Hot iron-rich solutions coming from an igneous source below are believed to have introduced the hematite in the Iron Mountain-Pilot Knob area, but the sink-hole hematite resulted from the oxidation of iron sulphide. Weathering of older iron-containing rocks and minerals gave rise to the coloring hematite seen on our sub-soil and surface rock.

Hematite is used as a polishing agent, as a pigment in paint, and, of course, as an ore of metallic iron. In the smelting of iron from hematite the ore is mixed in a huge, chimney-shaped blast furnace with coke (from coal) and limestone. Air is blown into the furnace as into a blacksmith’s forge; and the coke and gasses, burning at an incandescent heat, take the oxygen from the Fe₂O₃, leaving metallic iron which melts and is run out of the furnace at periodic intervals. Thus the smelting process is the opposite of the rusting process. The impurities and cinder run out as molten slag.

Limonite

Limonite is a heavy, yellow to brown, or brownish black mineral which always leaves a yellow to brown mark or streak when rubbed across a hard white rock or unglazed porcelain. It usually has a dull luster on a broken surface, and may vary from thumb-nail hardness to almost that of steel. The distinguishing test is its yellow to brown streak.

In composition limonite is iron oxide which contains more or less water chemically combined, Fe₂O₃·nH₂O. That is, it may be dried bone-dry at the temperature of boiling water, but upon heating to redness the additional, chemically held water will be driven off.

Limonite is ordinarily formed from the weathering of other iron-containing minerals (pyrite, for example) and is therefore a wide-spread mineral in surface rock, in films on pools of water, and in soil, all of which it colors yellow to brown. In fact, almost all of the yellow to brown inorganic mineral color and stain seen in nature is that of limonite.

Commercial deposits of limonite occur in southeastern Missouri where large boulders, discontinuous and irregular lenses or beds, pipes, nodules, and gravel to clay-sized particles of the mineral are associated with the cherty, gravelly residual clay. Usually the ore is crushed, hand-sorted, and washed preparatory to concentration for shipment to a furnace or for use in cement manufacture. As in the case of hematite, unless one has a deposit amounting to thousands of tons of ore it has little commercial value, and unless the mineral is relatively pure it can not be used.

Paint Ore or Red Ochre

An intensely red-colored, clayey iron ore has been mined for paint pigment in several deposits in south central Missouri. It occurs in sink hole deposits like those containing fire clay. Brown ocher may be available from southeast Missouri.

Iron Band Diaspore

Shells of red or reddish brown iron oxide occur about cores of diaspore clay in some of those deposits south of the Missouri River. Previously this material had no value, but in the last few years it has been purchased for and shipped to a cement company, which used it in the manufacture of cement. Diaspore clay is discussed elsewhere in this pamphlet.

Manganese Ore

Several manganese minerals make up the manganese ore which occurs to a limited extent in southeast Missouri, principally in Shannon, Reynolds, Carter, Iron, and Madison counties. Although the Missouri manganese minerals are usually heavy, black or nearly so, and have a black or brownish-black mark or streak, the identification of the individual minerals is difficult and should be left to a technically trained mineralogist.

Manganese minerals are used in the chemical industry and in the production of certain kinds of iron. A special report on the manganese deposits of Missouri is available at the Missouri Geological Survey, Rolla, Missouri.

Galena

Galena (“lead”) is a heavy, soft, somewhat brittle ore of lead. It has a brilliant metallic luster, and silvery gray color on a freshly broken surface. Where weathered it appears dull gray. It can be scratched with a knife, and breaks with surfaces at 90°, forming cubes. The unbroken, original crystal form of galena which has grown unobstructed in a vein opening is commonly cubical in habit or a modification thereof. It leaves a dark, lead-gray to black mark or streak when rubbed across unglazed porcelain or chert.

Galena is lead sulphide, PbS, and when pure contains 86.6 per cent lead and 13.4 per cent sulphur. Small amounts of silver may also be present.

Galena commonly occurs in Missouri as a cavity filling in crushed limestone or chert, or as a replacement in limestone or dolomite, or in shale, so that a large quantity of practically worthless enclosing rock (gangue) must be taken out in order to obtain the desired galena. If a person desires to estimate the value of his galena (lead) prospect by having an analysis or assay made of his ore, he must include in his sample the gangue rock that would of necessity have to be taken out when mining his ore. Too often persons carefully select for analysis a choice galena specimen which may run over 80% lead, only to find that as a practical mine product it would be reduced to less than 5% lead in all the rock which also would have to be taken out.

After a galena ore is mined it is customarily crushed and the galena removed from the gangue by a gravity-separation process which takes advantage of the difference in “heaviness” (specific gravity) between galena (7.5) and limestone (calcite) or chert (2.7-2.6), or by a froth flotation process in which the galena is preferentially wetted and carried off by an oily froth or foam. The galena concentrate is roasted to burn out the sulphur, reduced by carbon, and smelted to metallic lead. The origin of some Missouri lead deposits is debatable, but the writer believes the most reasonable explanation to be that warm, chemically active waters arose from an igneous body below and carried to the place of deposition the lead which they held in solution.

Missouri is one of the leading producers of lead in the world from its Flat River, Fredericktown, Joplin, and central Missouri districts, from which in 1941 lead concentrates having a value of over $15,000,000 were produced.

Sphalerite

Sphalerite (locally called Jack, Rosin Jack, Black Jack, Ruby Jack, Zinc, Rosin Spar) is a tan-brown, resinous, brown or brownish black mineral having a very high luster on its broken (cleavage) surfaces. Much of it so strongly resembles lump rosin that the term “Rosin Jack” is truly descriptive. Less commonly, a ruby red variety occurs as crystals perched on other sphalerite or on waste rock. Sphalerite is readily scratched with steel. Its chemical composition is zinc sulphide, ZnS—zinc 67 per cent, sulphur 33 per cent—and it is an important ore of zinc.

Sphalerite occurs abundantly in the mining district of southwest Missouri, but small, non-commercial amounts of it have been found through an area extending even north of the Missouri River. At the mines, after the ore and rock are taken out, they are crushed and separated, the ore going to the smelter and the rock to tailings piles. Under the old milling process employed in south_western_ Missouri, thousands of tons of coarse tailings, largely chert, were poured onto huge “chat” piles, many of which remain as a low-priced by-product for some one to put to use. This chat differs mineralogically from the south_eastern_ Missouri chat, which is largely dolomite.

Barite (“Tiff”)

Barite (“Tiff” in south_east_ Missouri, Heavy Spar, Barytes) occurs in Missouri predominantly as a white, quite heavy, soft, non-metallic mineral which has a high luster on a freshly broken surface. Slightly bluish “glass” barite or “glass tiff” has been found in smaller quantity with the more abundant, opaque white material. The glassy barite may superficially resemble calcite or selenite gypsum, but in distinction, barite breaks or cleaves to surfaces joining at right angles and does not effervesce with acid, whereas calcite does effervesce in acid and cleaves at oblique angles (rhombohedral cleavage). Gypsum is so soft that it can be scratched very easily with the thumb-nail, whereas barite is scratched with difficulty, if at all, by the thumb-nail. Notably, again, barite is “heavy,” with a specific gravity of about 4.5, whereas calcite, gypsum, limestone, and chert are “lighter,” with a specific gravity of about 2.6 to 2.7. Barite has the composition barium sulphate, BaSO₄, of which barium oxide constitutes 65.7 per cent.

Barite occurs in abundance in the Jefferson-Washington counties district, which furnishes about 80% of Missouri production. Other production comes from near Houston, Texas county, and from the central district—Miller, Moniteau, Morgan, Cole counties, and adjacent territory. In the Jefferson-Washington counties district, it is dug from residual clay over dolomite and is run through washing and concentrating mills which remove the clay and lighter waste rock. Most of the central district production comes from old sinkhole deposits, the ore being also crushed, washed, and concentrated in preparation for shipment. Missouri barite which was produced during 1941 had a value of over $1,300,000 and constituted about 40% of the total United States productions.

Barite is used as a paint pigment and extender, as a flux, as a source of barium in the chemical industry, as a filler in rubber, paper, oil cloth, textiles, and leather, and as a heavy substance in oil well drilling mud. The largest single use is in the manufacture of lithopone paint.

Gypsum

Gypsum is a soft mineral which can be scratched easily with the finger or thumb-nail. It may be glassy or transparent, or may grade into an opaque white body, possibly stained by iron oxide, but it is always very soft. Of the three varieties of gypsum—selenite, alabaster, and satinspar—only the first two have been found in Missouri by the writer. The chemical composition of gypsum is CaSO₄·2H₂O.

The chief use of gypsum is in the manufacture of plaster of paris, during which it is pulverized and heated to drive off part of the water of crystallization so that its composition corresponds to CaSO₄·½H₂O. This powder, when mixed with water and poured into a mold, heats and sets; that is, it hardens by taking up enough of the water to restore its original composition.

Although thick, wide-spread beds of gypsum occur in other localities, probably most of the gypsum in Missouri has been secondarily formed, as from the reaction of sulphuric acid from oxidizing pyrite on calcite; and its quantity is limited to small crystals, veins, and crusts in or on other rocks. Gypsum may be an impurity in coal, and some beautiful crystals a few inches long have been found in weathering clay deposits. It therefore cannot be considered as a commercially valuable mineral of this state.

Meteorites

Meteorites, the rock-like specimens which have come to our earth as sparkling meteors in the sky, are perhaps the most prized specimens which the average collector hopes to find, and perhaps more specimens are mistaken for meteorites than for any other geological substance. Meteorites are rare and not easy to find; they are also not easy to determine.

The iron variety is usually a heavy, roughly-pitted, brown, tough, metallic, nickel alloy of iron. Therefore, a positive chemical test for nickel is usually strongly suggestive of a meteoric origin, but confirmation almost requires that a surface be polished and etched with dilute acids to bring out typical and characteristic structures.

The stony variety of meteorites usually contains a rock-forming mineral called olivine, beneath its pitted brown surface. In case of either variety, since special equipment is required for final testing and determination, it is recommended that this be done at a laboratory appropriately equipped.

Gold

Gold is not known to occur in Missouri, except for very small quantities which have been carried into the state with the glacial deposits in the north half. Miners have searched carefully, and geologists have studied Missouri rocks intently, comparing them with the gold veins of the western states, but they find no promise of a gold deposit in Missouri. We have been favored with other geological products, but it is a waste of time to search for gold in Missouri.

Silver

Silver has been recovered from ore in the Silver Mines area in Madison county and from the galena of southeastern Missouri. Except for occurrences within the igneous rock area and the lead mining regions, geologists do not expect to find additional silver ore deposits.

Diamonds

No diamond has ever been found in native Missouri rock. It is possible for diamonds to have been carried into the state with the glacial deposits in the northern part, but the probability of finding one, if it did come in, is extremely remote.

Diamonds do occur in one part of Arkansas, but those rocks are strikingly different from all Missouri rocks except in a few localities, having small areas about the size of one’s house, in the southeastern part of the state. The writer has received quartz and calcite crystals for testing from persons who hoped they might be diamonds. It is almost a foregone conclusion that diamonds do not occur in Missouri.

A diamond may be recognized by its extreme hardness. It is the hardest substance known, natural or artificial, and will scratch any known substance; but it, in turn, is scratched only by another diamond. Acids do not affect diamonds in the least.

Uranium Minerals

Three uranium-containing minerals, tyuyamunite (pronounced tyew-yuh-moon-ite), possibly carnotite, and metatorbernite, have been found in Missouri but none has been mined commercially. Tyuyamunite and carnotite are canary yellow powdery minerals so similar in appearance they can be differentiated only by chemical and x-ray properties. Both minerals contain uranium, vanadium, oxygen, water, and one other element, which, if it is calcium, the mineral is tyuyamunite, but if it is potassium the mineral is carnotite. The canary yellow color referred to is distinctly different from the brownish or reddish yellow color of iron oxide minerals. These yellow uranium minerals have been found near Ste. Genevieve along cracks in limestone and in the black shale above the limestone, and in dark, sandy shales near Shelbina, and elsewhere north of the Missouri River.

Black shales (high in organic matter) of marine origin are the most highly radioactive, whereas black shales deposited on land (as with coal), and all shales of other colors are usually lower in radioactivity.

Metatorbernite is a soft, pale apple-green scaly mineral that has been found in paper-thin cracks in flint fire clay deposits. It contains uranium, copper, phosphorus, oxygen, and water. All of these uranium minerals activate a Geiger counter.

MISCELLANEOUS ROCK STRUCTURES

Concretions

A concretion is an aggregate of inorganic matter in the shape, roughly, of a ball, disc, rod, or irregular nodular body. Usually the aggregation or accumulation started around a small center grain or particle and continued in the growth of layers about it like the shells of an onion, or in the growth of needle-like fibers which radiate from the center like pins stuck into a spherical pin cushion. Concretions vary in size from buck-shot (buck-shot concretions in the soil) to oddities ten or twenty feet in diameter, or even longer in elongate forms. The variety about one-sixteenth or one-thirty-second of an inch in diameter is called an oolite (pronounced oh-oh-lite). Some chert of southern Missouri, most of the diaspore and burley clay, and a limestone cropping out near Louisiana, Missouri, are made up partly to almost entirely of oolites. See page 29.

Concretions may be composed of pyrite, calcite, limonite, chert, cemented sandstone, or even cemented clay. They are usually recognized by their structure after the previously enclosing rock has been eroded. Thus pyrite, limonite, or calcite (limestone) concretions remain after shale has softened and washed away, chert remains after limestone, and strongly cemented “irony” (limonite or hematite) or siliceous sandstone concretions may be found on the outcrop where the softer or less resistant host rock has been carried off. The irregular-shaped, intergrown, nodular limestone concretions (sometimes called “loess-kinder”, or loess dolls) in the upper part of loess deposits along the Missouri and Mississippi Rivers can be found remaining on rain-washed slopes. Limy, mudstone concretions and brown iron carbonate concretions are abundant in certain localities in northwestern and southeastern Missouri, where they are used as oddities in rock gardens or walls.

Some concretions are formed at the same general time as the surrounding rock accumulates, but others may be formed years after the surrounding rock has been buried or removed from the environment of its formation. In either case, deposition of the mineral matter follows the pattern of addition or “growth” from inside out. This growth, of course, does not involve a life process like that of a plant or animal. If two or more centers of deposition occur close together, the several growing concretions may touch, intergrow, and develop some weird forms, suggesting organic growth. However odd these curiosities may become, there is no question that they are not fossils, or evidence of life. Probably concretions excite the interest of persons more than any other rock structure.

Ground water, carrying calcium carbonate, silica, or iron compounds in solution, is a great concretion builder. It percolates through sandstone or other permeable rock and slowly leaves behind enveloping layers or additions of mineral matter, until a concretion is formed, to remain hidden from view until its host rock is softened and removed by the action of the weather.

Geodes

A geode is usually a hollow, more or less spherical or ball-shaped shell of mineral and crystal growth which has formed within surrounding rock. Missouri geodes commonly vary in size from hickory nuts to small watermelons, although neither direction of variation is limited. They weather out abundantly at several localities in northeast Missouri from the so-called Warsaw formation, a limy shale. Here they are dark brown, rough and irregular on the outside, but where broken open show many brilliant, glistening faces of intergrown quartz crystals. Less frequently calcite, chalcedony, kaolinite, and rarely millerite (nickel sulphide) may occur in Missouri geodes.

The minerals and crystals of a geode grow inward from the walls of a cavity in the rocks. The mineral matter is carried there in solution by ground water and crystallizes out very much more slowly but in the same manner that sugar or salt crystals develop in a saturated solution of those substances. If crystal growth continues until the geode is solid, it may bear superficial resemblance to a concretion, but the latter structure is one which has grown outward. The idea of “growth” in either case is that of mineral crystallization and enlargement, but does not in the least involve life like that of a plant or animal. Geodes have no value or use other than for ornamental purposes.

Fossils

Fossils are also found and collected by persons who are interested in rocks and minerals. The varied remains of plants and animals long since petrified or replaced by mineral matter have stimulated the curiosity and become a source of enjoyment to many persons, from those who merely give a passing glance to the peculiar organic structures in the rocks to those who make a serious hobby or business of collecting and classifying the unreplaceable heritage from the ancient rocks. Fossils are interesting in part because of their variety, for they include petrified wood, shells like those of oysters, fish teeth, foot-prints, amber, dinosaur eggs, coal, imprints of fern leaves, of insects, and of fishes, and the bones of small and gigantic dinosaurs and elephants. In fact, a fossil is any evidence of life in the geologic past preserved in the rocks. Missouri rocks furnish fossils ranging in size from microscopically small fish teeth to the big skeletal remains of the mastodon, an ancestor of the elephant; but the most common ones are the structures and shells of ancient clams, corals, brachiopods, crinoids, and trilobites.

The accompanying photographs illustrate a few fossils that may be found within our state, but a thorough, non-technical treatment of Missouri fossils is available in a companion volume to this booklet, “The Common Fossils of Missouri” by Prof. A. G. Unklesbay, Missouri Handbook No. 4.

The study of fossils, or paleontology, is a fascinating branch of geology which extends far beyond the recognition and cataloging of the specimens. It has been found that certain particular fossils occur in rocks of the ages which produce petroleum, and the search for that valuable substance has been directed in many instances by the fossil content of the rocks. Rocks of different ages carry different fossil assemblages, and a man skilled in paleontology utilizes the fossils in dating geologic history like the page numbers in a book of human history. Further, any student of present-day animals and plants is aided in his understanding of them if he knows the fossil record of their ancestors of the long geologic past.

Arrow Heads and Other Indian Artifacts

Arrow heads, scrapers, rock knives and saws which were left by the Indians who formerly lived in Missouri may be found in moderate abundance in many parts of the state. Usually these artifacts are chert in its various colors, white, gray, mottled, reddish, or black (flint). See the discussion of chert on page 34. Chert, because of its conchoidal fracture, lack of cleavage, resistance to chemical weathering, and superior hardness, is an exceptionally useful rock for making tools and weapons.

Hammers and axes of basalt, and arrow heads of rhyolite are less abundant than the chert artifacts.

THE ROCKS OF MISSOURI

Geologists classify rocks into these groups: igneous, sedimentary or metamorphic. Representatives of all three have been described in the preceding pages.

Sedimentary rocks are those whose particles settled down through the air or water to form rocks in layers or beds; hence layered, bedded, or so-called stratified rocks are sedimentary rocks. For instance, the sand and mud settling out of the Gulf of Mexico (or ocean) after being brought in by the Mississippi River is on its way toward becoming sandstone and shale. Limestone is forming off the coast of Florida now. All of these rocks are accumulating in layers. Where one sees regularly-layered or stratified rocks in streams, road cuts, quarries, bluffs or hillsides, he expects them to be sedimentary rocks. The sedimentary rocks that have been described herein include:

Limestone
Dolomite
Chert
Shale
Fire clay
Flint fire clay
Diaspore and Burley clay
Sandstone
Quartzite
Travertine
Coal

Igneous rocks are those which solidified from a hot liquid which was either forced into older surrounding rocks (intrusive) or discharged on the earth’s surface as a lava flow or products from a volcano (extrusive).

The examples given below illustrate the two types. Everyone knows about the extrusive forms from accounts of present-day volcanoes and occasional lava flows, like those of Vesuvius, Paricutin, and Mauna Loa. An intrusion was injected beneath the Yellowstone Park area years ago, and its heat, with steam and gases, is contributing to the unusual natural features which are found in the park and which make it famous.

Igneous rocks in Missouri are:

Granite
Porphyry, Rhyolite, Rhyolite porphyry
Gabbro and Diabase
Basalt

Metamorphic rocks are rocks which have been changed through the effects of tremendous pressure (enough to raise mountains) and high temperature while in the solid state. In most cases a banded rock results. The metamorphic rocks mentioned in this booklet are:

Gneiss
Marble

No doubt it has become apparent to the reader that rocks ordinarily occur in great quantities, that they are composed of multitudes of grains (mineral grains), and that their properties and compositions vary with the different minerals which are present in the grains of the rock. A _rock_ can, therefore, be different from a _mineral_. In fact, a rock may be defined as “an aggregate of mineral particles,” or more broadly “a typical part of the earth.” To focus closer attention on minerals we may discuss them for their own sake below.

MINERALS OF MISSOURI

A mineral is characterized by a constancy of composition and of properties which sets it apart from rocks which vary widely. Minerals may be metallic, like pyrite, or non-metallic, like barite; they may be ore, like galena, or rock-forming, like quartz or feldspar; they may show crystal faces, or they may be fragments with rounded or broken surfaces. A favored definition teaches that “a mineral is a naturally occurring, inorganic substance having a definite chemical composition and definite physical properties, within limits.” The Missouri minerals listed herein include:

Quartz
Mica
Calcite
Dolomite
Sphalerite
Galena
Barite
Marcasite
Carnotite
Tyuyamunite
Pyrite
Hematite
Limonite
Gypsum
Pyroxene
Orthoclase feldspar
Microcline feldspar
Plagioclase feldspar
Metatorbernite

GEOLOGIC VALUES

Although the emphasis in this pamphlet has been on the recognition of Missouri rocks and minerals, it is not out of order to consider the broad values that they contribute to our civilization. Their use as building materials has been noted, but it should be further recognized that as our timber is being rapidly depleted more and more structures will be built out of earth materials. Missouri possesses a wealth of beautiful limestone that is serviceable and readily quarried. Where limestone is not near, there is usually shale or glacial clay which can be used in the manufacture of brick and tile. Permanency will be the keynote of the rock and ceramic structures. Gravel and sand are abundant in Missouri for concrete and other varied uses.

The soil is Missouri’s most valuable earth material. Hundreds to thousands of years of normal weathering are required to develop the rocks and minerals and texture of the inorganic fraction of the soil. We should preserve it and prevent disastrous soil erosion.

Aside from these more tangible values, a fascinating and instructive hobby can be made of collecting, arranging, and studying rocks and minerals. One gains a fuller understanding and appreciation of nature from their study. The orderliness, constancy, and interrelation within the rock and mineral “world” is a restful contrast to the one which man often keeps in turmoil. The beauty of a glistening crystal or a polished mineral or stone is as inspiring as a lovely flower, yet it lasts and lasts through centuries, a veritable “rock of ages.”

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The Common Rocks and Minerals of MissouriChapter III: Introduction (2)

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