Silver has two important uses—in money and in the arts. As money, it is used in the United States and Europe for subsidiary coinage,—silver coins normally circulating at more than their intrinsic value,—but its greatest monetary use is in India and China, where it has been the basis for the settlement of foreign exchange balances. In China also it is the money standard of the country. In the arts, silver is employed chiefly in the making of articles of luxury, such as jewelry and tableware. In the Orient this use is closely related to its use as money, since the natives invest their savings both in silver jewelry and silver coins. There is some consumption of silver by certain chemical industries, and quantities of increasing importance are used in the form of silver salts by the photographic and moving picture industries. It has been estimated that before 1914 about two-thirds of the new silver produced went into the arts and one-third into money. During the war, however, increasing amounts were used in coinage, and less than one-fifth of the output was used in the arts. Demands for silver for monetary purposes will probably continue to take the larger part of the world's production for some time. In this connection it may be noted that India has adopted a gold standard, but that the conservative habits of the population will doubtless continue to call for large amounts of silver.
About half of the silver production of the world comes from the dry or siliceous silver ores, which are mined solely for that metal and the associated gold; and about half of the production is obtained as a by-product in the mining of other metals, principally copper and lead. The average grades of these ores, in combined values of gold and silver, were referred to on p. 228. While the aggregate amount of silver obtained as a by-product of other ores is large, the percentage of silver in the copper or lead in any mine is ordinarily very small. Consequently the world output of silver depends to a considerable extent upon conditions in the copper- and lead-mining industries.
Of the total world output of silver, normally about 75 per cent comes from North America. Of this the United States and Mexicoeach produce about two-fifths and Canada one-fifth, and minor amounts are produced in Central America. In late years, political disturbances in Mexico reduced that country's production to less than half the normal figure, and the United States took the place which Mexico had held for many years as the leading silver producer. The United States and Mexican supply is obtained from the Rocky Mountain belt, and the Canadian production comes chiefly from the Cobalt, Ontario, district. Outside of North America the principal producing areas are Australia, South America (Peru and to a less extent Bolivia and Chile), Europe (chiefly from Spain, Germany, and Austria-Hungary, but with smaller amounts from all the other countries), and Japan. Thus, while there are sources of silver in many places, the great bulk of the world's output comes from North America. In the financial ownership of mines, including ownership in other countries, the United States controls over half the world's silver, Great Britain about a third, and Germany about a tenth (principally in Mexico).
All the silver mined in the United States is smelted and refined by domestic plants; and in addition much of the Canadian, Mexican, and South and Central American silver is exported to the United States as ore and base bullion, to be treated in this country. The United States is therefore the great silver-selling country of the world.
The great silver-consuming countries are India and China, and normally about a half of the world's output goes to these two countries. This major movement of silver, from America to the Far East, takes place through the London market, since England has been the chief nation trading in the Orient. The balance of the world's silver consumption is widely distributed among the countries of Europe and South America and the United States (which consumes about one-tenth of the total). For the European trade most of the silver also goes through London, which is the great clearing-house and the market where prices are fixed.
In the later years of the war and immediately after, the demands for silver were probably twice the world's output. The resulting rise in price was unprecedented. Silver actually became worth more as bullion than as currency, and in Europe much trouble was experienced because of its withdrawal from currency to be meltedup. This condition was later followed by an equally striking drop in price as supply caught up with demand.
In the United States, as in many other countries, it was desired during the war to accumulate large stocks of gold as a basis of credit for the flotation of government loans, and the export of gold was prohibited. Consequently in the settlement of foreign trade balances, particularly with the nations of the Orient, very large amounts of silver bullion had to be used. Current production proved inadequate, and it was necessary to utilize the stocks of silver dollars in the United States Treasury. To this end the Pittman Silver Act, passed in April, 1918, authorized the melting down and conversion into bullion of 350,000,000 dollars out of the Treasury stock, and the retirement of a corresponding number of silver certificates and the issue of Federal Reserve bank notes. In this manner old stocks of silver, Manila dollars, etc., were called into service—though the stage was not reached, as it was in Germany, where it became necessary to melt down silver plate and ornaments. The silver used for exchange and export was to be replaced by the purchase of bullion from American producers at $1 per ounce, and its coining into new dollars. A minimum price of $1 per ounce was thus established for silver bullion.
The immediate result was to increase the price of silver at the mine; but with the continued rise in demands for silver, the price in the open market went far above this figure, the maximum being reached in 1920 when the price of silver went to $1.39 per ounce. Naturally, but little silver was then offered to the government at the fixed price of $1 under the Pittman Act. With the more recent slump in the general market for silver to a price below $1, offers to the government under the Pittman Act have been renewed.
That part of the silver production which is a by-product of copper production has been low since the war, because of the stagnation in the copper industry. The production from lead ores, on the other hand, was not handicapped by lack of demand for lead. With the restoration of order in Mexico, a presumption of large silver production in that country may be expected. Increases may probably be expected also from new mines in Burma and from Bolivia. On the whole, no large increase in world production can be assumed from present known resources. New discoveries will be necessary to make any considerable change.
Of the mine production of silver in the United States, about two-thirds of the total comes from the states of Montana, Utah, Idaho, and Nevada. Other considerable producers are Colorado, Arizona, California, Alaska, and New Mexico. All the other states together produce less than 5 per cent of the total. The most important single districts are the Butte district of Montana, the Cœur d'Alene district of Idaho, and the Tonopah district of Nevada, supplying respectively about one-fifth, one-eighth, and one-tenth of the country's total silver output.
The most important mineral of silver is the sulphide, argentite or "silver glance." Other minerals which yield a minor percentage of the total silver produced are the silver-antimony sulphides, pyrargyrite or "ruby silver," stephanite or "black silver," and polybasite; the silver-arsenic sulphides, proustite or "light ruby silver" and pearcite; and the silver antimonide, dyscrasite. In the oxide zone the most abundant minerals are cerargyrite (silver chloride) and native or "horn" silver. In addition to these definite mineral forms, silver is present in many ores in an undetermined form in other sulphides, notably in galena, sphalerite, and pyrite. Silver differs from gold in that it is chemically active and forms many stable compounds, of which only the more important have been mentioned.
The fact that half the world's silver is obtained as a by-product in the mining of other metals has been referred to. In the United States about a third of the production comes from dry or siliceous ores, over a third from lead and zinc ores, and a fourth to a third from copper ores. A fraction of 1 per cent of the total is obtained as a by-product of gold placers, and all the remainder is won from lode or hard-rock deposits.
The general geologic features of the silver-bearing copper and lead ores, and of the dry or siliceous gold and silver ores, have been described on previous pages. The Philipsburg district has been referred to in connection with manganese ores, and the Bolivian tin-silver ores will be described in connection with tin. We shall consider here only a few of the more prominent districts which have been primarily silver producers.
The Cobalt district of northern Ontario is the most productivesilver district in North America. The ores are found in numerous short, narrow veins, principally in pre-Cambrian sediments near a thick quartz-diabase sill. Locally they penetrate the sill. Native silver and various silver sulphides, arsenides, and antimonides are associated with minerals of cobalt, nickel, bismuth, lead, and zinc, in a gangue of calcite and some quartz. The ore is of very high grade. The ore minerals are believed to have been deposited by hot solutions emanating from deep magmatic sources after the intrusion of the diabase. The present oxidized zone is very shallow, but may have been deeper before being stripped off by glaciation; it is characterized by native silver and arsenates of nickel and cobalt in the form of the green "nickel bloom" and the pink "cobalt bloom." The silver minerals are distinctly later in origin than the cobalt and nickel in the unoxidized zone, as evidenced by the relations of the mineral individuals when seen under the microscope. This fact, together with the abundance of native silver in the oxide zone, has suggested downward concentration of the silver by surface waters; but recent studies have indicated the probability that some of the silver at least was deposited by the later ascending solutions of magmatic origin.
In the Tintic district of central Utah, Paleozoic limestones have been intruded by monzonite (an acid granitic or porphyritic igneous rock), and covered by surface flows, the flows for the most part having been removed by subsequent erosion. The sediments have been much folded and faulted, and the ore bodies occur as fissure veins which locally widen into chimneys or pipes in fracture zones, accompanied by much replacement of limestone. There is a rough zonal arrangement of the ore minerals around the intrusive, gold and copper minerals (chiefly enargite and chalcopyrite) being more prominent near the intrusive, and argentiferous galena and zinc blende richer at greater distances. Silver constitutes the principal value. The gangue is mainly fine-grained quartz or jasperoid, and barite. The water table is at unusually great depths (2,400 feet) and there is a correspondingly deep oxidized zone, which is characterized by lead and zinc oxide minerals much as at Leadville (p. 219).
The Comstock Lode at Virginia City, Nevada, on the east slope of the Sierra Nevadas, was one of the most famous bonanza deposits of gold and silver in the world. While the richer ore hasall been extracted, lower-grade material is still being mined and the fissure is still being followed, in the hope of some day striking another fabulously rich ore body. The lode occupies a fault fissure parallel to the trend of the range and dipping about 40 degrees to the east, which can be traced about two and a half miles along the strike, with igneous rocks forming both hanging and foot walls. There are no sedimentary rocks in the district. The high-grade part of the vein is several hundred feet in thickness, with many irregular branches; the great thickness has been thought to be at least in part due to the tremendous pressure exerted by growing quartz crystals. The wall rocks have undergone a "propylitic" alteration, with development of chlorite, epidote, and probably sericite, much as at Butte. The ore contains rich silver sulphide minerals and native gold, in a gangue composed almost entirely of quartz. The ore was doubtless formed by hot solutions, but the exact nature of these solutions, whether magmatic or meteoric, has not been proven. The hypothesis was early developed that the ores were deposited by surface waters,—which are supposed to have fallen on the summits of the Sierra Nevadas, to have sunk to great depths where they were heated, enabling them to pick up metallic constituents from the diabase forming one wall of the ore body, and to have risen under artesian pressure along the fault plane, where loss of heat and pressure resulted in deposition. Later studies have emphasized the similarity of the ore-depositing conditions with those in other districts where the ores are believed to have come directly from magmatic sources, and this origin is now generally favored for the Comstock Lode. However, the earlier theory has not been disproved.
The Tonopah, Nevada, district is very similar to the Goldfield district (p. 230). Silver and gold are found in veins and replacements in a series of Tertiary volcanic flows and tuffs, all of which have been complexly faulted. Silver is the dominant constituent of value. The formation of fissures and faults accompanying and caused by the intrusion and cooling of lavas was first clearly shown in this district. Evidences of origin through the work of hot solutions, probably magmatic, are the close association of the ores in place and in time with the igneous rocks—ore deposition in most of the flows having taken place before the next overlying flows were put down,—the presence of fluorine, the nature of thewall-rock alterations, the fact that both hot and cold springs are found close together underground (indicating unusual sources for the hot springs), the contrast in composition between the ores and the country rock, and the general relation of these ores to a large number of similar occurrences in Tertiary lavas in the same general area.
Under weathering conditions, the silver sulphide minerals in general are oxidized to form native silver and cerargyrite, which are relatively insoluble and remain for the most part in the oxide zone. Silver is less soluble than copper and zinc, but more soluble than gold; and to some extent it is removed in solution, particularly where the oxidation of pyrite forms ferric sulphate. Farther down it may be reprecipitated as native silver, argentite, and the sulpho-salts, by organic matter or by various sulphides. The secondarily enriched ores are in a few districts, as at Philipsburg, Montana, the most valuable portions of the deposits. In other cases, sulphide enrichment does not appear to have contributed greatly to the values. The zones of oxide ores, secondary sulphide ores, and primary or protores are in most silver deposits much less regular and much less definitely marked than in the case of copper ores.
The principal uses of platinum are: as a catalytic agent in the contact process for the manufacture of sulphuric acid, and in the making of nitric acid from ammonia; for chemical laboratory utensils that must be resistant to heat and acids; for electrical contacts for certain telephone, telegraph, and electrical control instruments, and for internal combustion engines; in dental work; and for jewelry. In normal times before the war, it is estimated that in the United States the jewelry and dental industries used 75 per cent of the platinum metals consumed, the electrical industry 20 per cent, and the chemical industry 5 per cent. During the war, with the extraordinary expansion of sulphuric and nitric acid plants, these proportions were reversed and the chemical and electrical industries consumed about two-thirds of the platinum. Substitutes have been developed, particularly for the electricaluses, and the demand from this quarter may be expected to decrease.
About 90 per cent of the world's crude platinum produced annually comes from the Ural Mountains in Russia. The deposits next in importance are those of Colombia. Small amounts are produced in New South Wales, Tasmania, New Zealand, Borneo, British Columbia, United States, India, and Spain; and as a by-product in the electrolytic refining of the Sudbury, Canada, nickel ores. The extension of this method of refining to all of the Sudbury ores would create an important supply of platinum. The Colombian output has been increasing rapidly since 1911. Meanwhile the Russian production has declined; and from the best information available, it is not likely that Russia will be able to maintain production for many more years. Estimates of the life of the Russian fields are from 12 to 20 years at the pre-war rate of production.
The platinum situation is commercially controlled by buying and mine-operating agencies,—the French having, before the war, practically dominated the Russian industry, while American interests controlled in Colombia. The situation is further influenced by four large refineries, in England, Germany, United States, and France.
Before the war the United States produced less than 1 per cent of the new platinum it consumed annually. Production comes principally from California, with smaller amounts from Oregon, Alaska, and Nevada. The many efforts which have been made to develop an adequate domestic supply of this metal do not indicate that the United States can ever hope to become independent of foreign sources for its future supplies of platinum.
There is little reason to doubt that the Colombia field, commercially dominated by the United States, holds great promise for the future. The output has come largely from native hand labor, and with the installation of dredges can probably be greatly increased.
During the war, the need for platinum for war manufactures was so urgent and the production so reduced, that restrictions against its use in jewelry were put into force in all the allied countries. The United States government secured quantities of platinum which would have been sufficient for several years' use ifwar had continued. With the cessation of hostilities restrictions on the use of platinum were removed, and the accumulated metal was released by the government from time to time in small quantities; but the demands for platinum in the arts were so great that prices for a time tended to even higher levels than during the war. More recently supply is again approaching demand.
Platinum, like gold, occurs chiefly as the native metal. This is usually found alloyed with iron and with other metals of the platinum group, especially iridium, rhodium, and palladium. Most of the platinum as used in jewelry and for electrical purposes contains iridium, which serves to harden it. Paladium-gold alloys are a substitute for platinum, chiefly in dental uses.
The original home of platinum is in basic igneous rocks, such as peridotites, pyroxenites, and dunites, where it has been found in small, scattered crystals intergrown with olivine, pyroxene, and chromite. Platinum is very dense and highly resistant to oxidation and solution. In the breaking up and washing away of the rocks, therefore, it is concentrated in small grains and scales in stream and beach placers. Of the world production of platinum over 99 per cent has been derived from placers.
The Ural Mountain deposits of Russia are gold- and platinum-bearing placers, in streams which drain areas of dunite rock containing minute quantities of native platinum. The deposits of Colombia and Australasia are placers of a similar character. In the United States small quantities of platinum are recovered from the gold-bearing gravels of California and Oregon, where the streams have come from areas of serpentine and peridotite.
A platinum arsenide, called sperrylite, is sometimes found associated with sulphide minerals in basic igneous rocks. At Sudbury, Ontario, this mineral, together with palladium arsenide, is found in the nickel ores, especially in the weathered zone where it is concentrated by removal of more soluble materials. It has also been found in the copper mines of Rambler, Wyoming. In the Yellow Pine district of southern Nevada, metallic gold-platinum-palladium ore shoots are found in association with copper and lead ores, in a fine-grained quartz mass which replaces beds of limestone neara granitic dike. No basic intrusives are known in the district. The deposit is unusual in that it has a comparatively high content of platinum (nearly an ounce to the ton), and is probably genetically related to acid intrusives. From all these deposits, only small quantities of platinum are mined.
[34]Report of a joint committee appointed from the Bureau of Mines and the United States Geological Survey by the Secretary of the Interior to study the gold situation:Bull. 144, U. S. Bureau of Mines, 1919. See also Report of Special Gold Committee to Secretary of the Treasury, February 11, 1919.
[34]Report of a joint committee appointed from the Bureau of Mines and the United States Geological Survey by the Secretary of the Interior to study the gold situation:Bull. 144, U. S. Bureau of Mines, 1919. See also Report of Special Gold Committee to Secretary of the Treasury, February 11, 1919.
[35]Ransome, F. L., The geology and ore deposits of Goldfield, Nevada:Prof. Paper 66, U.S. Geol. Survey, 1909, p. 193.
[35]Ransome, F. L., The geology and ore deposits of Goldfield, Nevada:Prof. Paper 66, U.S. Geol. Survey, 1909, p. 193.
[36]Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore deposits of Utah:Prof. Paper 111, U.S. Geol. Survey, 1920, p. 195.
[36]Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore deposits of Utah:Prof. Paper 111, U.S. Geol. Survey, 1920, p. 195.
Bauxite (hydrated aluminum oxide) is the principal ore of aluminum. Over three-fourths of the world's bauxite production and 65 per cent of the United States production is used for the manufacture of aluminum. On an average six tons of bauxite are required to make one ton of metallic aluminum. Other important uses of bauxite are in the manufacture of artificial abrasives in the electric furnace, and in the preparation of alum, aluminum sulphate, and other chemicals which are used for water-purification, tanning, and dyeing. Relatively small but increasingly important quantities are used in making bauxite brick or high alumina refractories for furnace-linings.
Aluminum is used principally in castings and drawn and pressed ware, for purposes in which lightness, malleability, and unalterability under ordinary chemical reagents are desired. Thus it is used in parts of airplane and automobile engines, in household utensils, and recently in the framework of airplanes. Aluminum wire has been used as a substitute for copper wire as an electrical conductor. Aluminum is used in metallurgy to remove oxygen from iron and steel, and also in the manufacture of alloys. Powdered aluminum is used for the production of high temperatures in the Thermite process, and is a constituent of the explosive, ammonal, and of aluminum paints.
Deposits of bauxite usually contain as impurities silica (in the form of kaolin or hydrous aluminum silicate), iron oxide, and titanium minerals, in varying proportions. Bauxites to be of commercial grade should carry at least 50 per cent alumina, and for the making of aluminum should be low in silica though the content ofiron may be fairly high. For aluminum chemicals materials low in iron and titanium are preferred; and for refractories which must withstand high temperatures, low iron content seems to be necessary. The abrasive trade in general uses low-silica high-iron bauxites.
The only large producers of bauxite are the United States and France, which supplied in normal times before the war over 95 per cent of the world's total. Small amounts are produced in Ireland, Italy, India, and British Guiana. During the war a great deal of low-grade bauxite was mined in Austria-Hungary and possibly in Germany; but on account of the large reserves of high-grade material in other parts of the world, it is doubtful whether these deposits will be utilized in the future. Bauxites of good grade have been reported from Africa, Australia, and many localities in India. From geologic considerations it is practically certain that there are very large quantities available for the future in some of these regions.
The international movements and the consumption of bauxite are largely determined by the manufacture of aluminum, and to a lesser extent by the manufacture of abrasives and chemicals. The principal foreign producers of aluminum are France, Switzerland (works partly German-owned), Norway (works controlled by English and French capital), England, Canada, Italy, Germany, and Austria. French bauxite has normally supplied the entire European demands,—with the exceptions that Italy procures part of her requirements at home, and that the Irish deposits furnish a small fraction of the English demand.
The deposits of southern France, controlled largely by French but in part by British capital, have large reserves and will probably continue to meet the bulk of European requirements. France also has important reserves of bauxite in French Guiana.
The United States produces about half of the aluminum of the world, and is the largest manufacturer of artificial abrasives and probably of aluminum chemicals. Most of these are made from domestic bauxite. Prior to the war, the United States imported about 10 per cent of the bauxite consumed, but these imports were mainly high-grade French bauxite which certain makers of chemicals preferred to the domestic material. The small production of Guiana is also imported into the United States. Bauxiteis exported to Canadian makers of aluminum and abrasives. During the war period domestic deposits were entirely capable of supplying all the domestic as well as Canadian demands for bauxite, although these demands increased to two and one-half times their previous figure. At the same time considerable amounts of manufactured aluminum products were exported to Europe, whereas aluminum had previously been imported from several European countries.
The United States production of bauxite comes mainly from Arkansas, with smaller amounts from Tennessee, Alabama, and Georgia. The reserves are large but are not inexhaustible. Most of the important deposits are controlled by the large consumers of bauxite, principally the Aluminum Company of America and its subsidiaries, though certain chemical and abrasive companies own some deposits. The Aluminum Company of America also controls immense deposits of high-grade bauxite in Dutch and British Guiana, and further exploration by American interests is under way.
With the return to normal conditions since the war, some of the domestic bauxite deposits probably can not be worked at a profit, a situation which is likely to require the development of the tropical American deposits.
Aluminum is the third most abundant element in the common rocks and is an important constituent of most rock minerals; but in its usual occurrence it is so closely locked up in chemical combinations that the metal cannot be extracted on a commercial scale. In the crystalline form aluminum oxide constitutes some of the most valuable gem stones. Many ordinary clays and shales contain 25 to 35 per cent alumina (Al2O3), and the perfection of a process for their utilization would make available almost unlimited aluminum supplies. The principal minerals from which aluminum is recovered today are hydrous aluminum oxides, the most prominent of which are bauxite, gibbsite, and diaspore—the aggregate of all these minerals going commercially under the name of bauxite.
Prior to the discovery of bauxite ores, cryolite, asodium-aluminum fluoride obtained from pegmatites in Greenland, was the chief source of aluminum. It is only within about the last thirty-five years that bauxite has been used and that aluminum has become an important material of modern industry. Cryolite is used today to form a molten bath in which the bauxite is electrolytically reduced to aluminum.
Bauxite deposits in general are formed by the ordinary katamorphic processes of surface weathering, when acting on the right kind of rocks and carried to an extreme. In the weathering of ordinary rocks the bases are leached out and carried away, leaving a porous mass of clay (hydrous aluminum silicates), quartz, and iron oxide. In the weathering of rocks high in alumina, and low in iron minerals and quartz, deposits of residual clay or kaolin nearly free from iron oxide and quartz are formed. Under ordinary weathering conditions the kaolin is stable; but under favorable conditions, such as obtain in the weathered zones of tropical climates, it is broken up, the silica is taken into solution and carried away, and hydrous aluminum oxides remain as bauxite ores. This extreme type of weathering is sometimes called lateritic alteration (see pp. 172-173). Impurities of the bauxite ores are the small quantities of iron and titanium present in the original rocks, together with the kaolin which has not been broken up. The deposits usually form shallow blankets over considerable areas, with irregular lower surfaces determined by the action of surface waters—which work most effectively where joints or other conditions favor the maximum circulation and alteration. A certain degree of porosity in the original rock is also known to favor the alteration. A complete gradation from the unaltered rock through clay to the high-grade bauxite, with progressive decrease in bases and silica, concentration of alumina and iron oxide, and increase of moisture and pore space, is frequently evident (see Fig. 13). The bauxite is earthy, and usually shows a concretionary or pisolitic structure similar to that observed in residual iron ores (p. 172). Near the surface there may be an increase in silica,—probably due to a reversal of the usual conditions by a slight leaching of alumina, thus concentrating the denser masses of kaolin which have not been decomposed.
The Arkansas bauxite deposits, the most important in the United States, are surface deposits overlying nepheline-syenite,an igneous rock with a high ratio of alumina to iron content. The most valuable deposits are residual, and some parts have preserved the texture of the original rock, though with great increase in pore space; most of the ore, however, has the typical pisolitic structure. Near the surface the pisolites are sometimes loosened by weathering, yielding a gravel ore, and some of the material has been transported a short distance to form detrital ores interstratified with sands and gravels. The complete gradation from syenite to bauxite has been shown.
Figure 13Fig. 13.Diagram showing gradation from syenite to bauxite in terms of volume. The columns represent a series of samples from a single locality in Arkansas. After Mead.ToList
Fig. 13.Diagram showing gradation from syenite to bauxite in terms of volume. The columns represent a series of samples from a single locality in Arkansas. After Mead.ToList
In the Appalachian region of Tennessee, Alabama, and Georgia, bauxite occurs as pockets in residual clays above sedimentary rocks, chiefly above shales and dolomites. Its origin has probably been similar to that described.
The bauxite deposits of southern France occur in folded limestones, and have been ascribed by French writers to the work of ascending hot waters carrying aluminum sulphate. They present some unusual features, and evidence as to their origin is not conclusive.
At the present time bauxite is doubtless forming in tropical climates, where conditions are favorable for deep and extreme weathering of the lateritic type. The breaking up of kaolin accompanied by the removal of silica is not characteristic of temperate climates, though many clays in these climates show some bauxite. It is possible that, at the time when the bauxite deposits of Arkansas and other temperate regions were formed, the climate of these places was warmer than it is today.
In studying the origin of bauxites, it should not be overlooked that they have much in common with clays, certain iron ores, and many other deposits formed by weathering.
Antimony is used mainly for alloying with other metals. Over one-third of the antimony consumed in the United States is alloyed with tin and copper in the manufacture of babbitt or bearing-metal. Other important alloys include type-metal (lead, antimony, and tin), which has the property of expanding on solidification; "hard lead," a lead-antimony alloy used in making acid-resisting valves; Britannia or white metal (antimony, tin, copper, zinc), utilized for cheap domestic tableware; and some brasses and bronzes, solders, aluminum alloys, pattern metals, and materials for battery plates and cable coverings. Antimony finds a very large use in war times in the making of shrapnel bullets from antimonial lead. Antimony oxides are used in white enameling of metal surfaces, as coloring agents in the manufacture of glass, and as paint pigments; the red sulphides are used in vulcanizing and coloring rubber, as paint pigments, in percussion caps, and in safety matches; and other salts find a wide variety of minor uses in chemical industries and in medicine.
Antimony ores vary greatly in grade, the Chinese ores carrying from 20 to 64 per cent of the metal. The presence of arsenic and copper in the ores is undesirable. Several of the more important antimony districts owe their economical production of that metal to the presence of recoverable values in gold. Some lead-silver ores contain small quantities of antimony, and "antimonial lead," containing 12 to 18 per cent antimony, is recovered in their smelting.
China is by far the most important antimony-producing country in the world, and normally supplies over half the world's total. Chinese antimony is exported in part as antimony crude (lumps of needle-like antimony sulphide), and in part as antimony regulus, which is about 99 per cent pure metal. France was the only other important source of antimony before the war (25 to 30 per cent of the world production), and Mexico and Hungary produced small amounts. The large demand for antimony occasioned by the war, besides stimulating production in these countries, brought forth important amounts of antimony ore from Algeria (French control) and from Bolivia and Australia (British control), as well as smaller quantities from several other countries. Of the war-developed sources, only Algeria and perhaps Australia are expected to continue production under normal conditions.
Before the war, antimony was smelted chiefly in China, England, and France, and to a lesser extent in Germany. British and French commercial and smelting interests dominated to a considerable extent the world situation, and London was the principal antimony market of the world.
During the war Chinese antimony interests were greatly strengthened, and facilities for treating the ore in that country were increased. Japan also became important as a smelter and marketer of Chinese ore, and increasing quantities of antimony were exported from China and Japan directly to the United States. English exports ceased entirely and were replaced in this country by Chinese and Japanese brands.
The United States normally consumes about one-third of the world's antimony. Before the war the entire amount was secured by importation, two-thirds from Great Britain and the rest from the Orient, France, and other European countries. Domestic production of ore and smelting of foreign ores were negligible. (These statements refer only to the purer forms of antimony; the United States normally produces considerable amounts of antimonial lead, equivalent to somewhat less than 5 per cent of the country's total lead production, but this material cannot be substituted for antimony regulus in most of its uses.)
During the war, under the stimulus of rising prices, mining of antimony was undertaken in the United States and several thousand tons of metal were produced—principally from Nevada, withsmaller amounts from Alaska, California, and other western states. The great demands for antimony, however, were met chiefly by increased importation. Imports were mainly of regulus from Chinese and Japanese smelters of Chinese antimony; but about a third was contained in ores, including most of the production of Mexico which had formerly gone to England, and about 15 per cent of the Bolivian output. Antimony smelters were developed in the United States to handle these ores.
At the close of hostilities there had accumulated in the United States large surplus stocks of antimony and antimonial materials. With a very dull market and low prices, domestic mines and smelters were obliged to close down. The dependence of the United States on foreign sources of antimony and the importance of the metal for war purposes led to some agitation for a protective tariff—in addition to the present import duty of 10 per cent on antimony metal—in order to encourage home production (see pp. 365-366, 393-394).
In summary, the United States is almost entirely dependent upon outside sources for its antimony, although there are inadequately known reserves in this country which might be exploited if prices were maintained at a high level. The future of United States smelters is problematical. China, the world's chief source of antimony, at present dominates the market in this country, largely due to the low cost of production and favorable Japanese freight rates.
The antimony sulphide, stibnite, is the source of most of the world's production of this metal. Antimony oxides, including senarmontite, cervantite, and others, are formed near the surface, and in some of the deposits of Mexico and Algeria they supply a large part of the values recovered. Jamesonite, bournonite, and tetrahedrite (sulphantimonides of lead and copper), when found in lead-silver deposits, are to some extent a source of antimony in the form of antimonial lead.
Stibnite is found in a variety of associations and is present in small quantities in many types of deposits. In the commercial antimony deposits, it is in most cases accompanied by minor quantities of other metallic sulphides—pyrite, cinnabar, sphalerite,galena, arsenopyrite, etc.—in a gangue of quartz and sometimes calcite. Many of the deposits contain recoverable amounts of gold and silver.
The deposits of the Hunan Province of southern China occur as seams, pockets, and bunches of stibnite ore in gently undulating beds of faulted and fissured dolomitic limestone. In the vicinity of the most important mines no igneous rocks have been observed, and the origin of the ores has not been worked out.
In the Central Plateau of France the numerous antimony deposits are stibnite veins cutting granites and the surrounding schists and sediments. An origin related in some way to hot ascending solutions seems probable.
The deposits of the National district of western Nevada, the most important war-developed antimony deposits of the United States, consist of stibnite veins with a gangue of fine-grained drusy quartz, cutting through flows of rhyolite and basalt. They are intimately related to certain gold- and silver-bearing veins, and all are closely associated with dikes of rhyolite, which were the feeders to the latest extrusion in the district. The wall rocks have undergone alteration of the propylitic type. These relations, and the presence of the mercury sulphide, cinnabar, in some of the ores (see pp. 258-259), suggest an origin through the work of ascending hot waters or hot springs. These waters probably derived their dissolved matter from a magmatic source, and worked up along vents near the rhyolite dikes soon after the eruption of this rock.
In the weathering of antimony deposits, the stibnite usually alters to form insoluble white or yellowish oxides, which are sometimes called "antimony ocher." These tend to accumulate in the oxide zone through the removal of the more soluble accompanying minerals. Secondary sulphide enrichment of antimony deposits, if it occurs at all, is negligible.
About two-thirds of the arsenic consumed in recent years has been used in agriculture, where various arsenic compounds—arsenic trioxide or "white arsenic," Paris green, lead arsenate,etc.—are used as insecticides and weed killers. Arsenic compounds are also used in "cattle-dips" for killing vermin. The only other large use of arsenic is in the glass industry, arsenic trioxide being added to the molten glass to purify and decolorize the product. Small quantities of arsenic compounds are used in the preparation of drugs and dyeing materials, and metallic arsenic is used for hardening lead in shot-making.
The principal arsenic-producing countries are the United States, Germany, France, Great Britain, Canada, and Mexico. Spain, Portugal, Japan, and China are also producers, and recent trouble with the "prickly-pear" pest in Queensland, Australia, has led to local development of arsenic mining in that country. For the most part, European production has been used in Europe and American production in the United States.
Arsenic is recovered almost wholly as a by-product of smelting ores for the metals. The potential supply is ample in most countries where smelting is conducted, but owing to the elaborate plant required to recover the arsenic, apparatus is not usually installed much in advance of the demand for production. Rapid expansion is not possible.
Before the war the arsenic needs of the United States (chiefly agricultural) were supplied by a few recovery plants in the United States, Mexico, and Canada. Several large smelters had not found it profitable to install recovery plants, as the market might have been oversupplied and prices were low. During the war, with the extensive demand for insecticides for gardening, there was a considerable deficiency of arsenic supplies. With rising prices production was stimulated, but was still unable to meet the increased demand. This situation resulted in regulation of the prices of white arsenic by the Food Administration.
Production of arsenic in the United States comes chiefly from smelters in Colorado, Washington, Utah, Montana, and New Jersey. Small amounts are produced by arsenic mines in Virginia and New York. A Mexican plant at Mapimi has been shipping important quantities to the United States. The plant at Anaconda, Montana, is expected to produce an ample supply in the future.
The United States is entirely independent in arsenic supplies and will probably soon have an exportable surplus. Export trade,after the reconstruction period, will probably meet competition from France and Germany where production was formerly large.
Arsenic-bearing minerals are numerous and rather widely distributed, but only a few of them are mined primarily for their content of arsenic. Arsenopyrite or "mispickle" (iron-arsenic sulphide) has been used intermittently as a source of white arsenic in various places,—notably at Brinton, Virginia, and near Carmel, New York. The former deposits contain arsenopyrite and copper-bearing pyrite impregnating a mica-quartz-schist, adjacent to and in apparent genetic relation with aplite or pegmatite intrusives. In the latter locality arsenopyrite is found associated with pyrite in a gangue of quartz, forming a series of parallel stringers in gneiss close to a basic dike.
The orange-red sulphides of arsenic, orpiment and realgar, are formed both as primary minerals of igneous source and as secondary products of weathering. They are rather characteristic of the oxide zones of certain arsenical metallic ores, and are believed in many cases to have formed from arsenopyrite. They are mined on a commercial scale in China.
The great bulk of the world's arsenic, as previously stated, is obtained as a by-product of smelting operations. The enargite of the Butte copper ores (pp. 201-203) contains a considerable amount of arsenic, a large part of which will be recovered from the smelter fumes by new processes which are being installed. The gold-silver ores of the Tintic district (pp. 235) also yield important amounts, the arsenic-bearing minerals being enargite and tennantite (copper-arsenic sulphides) and others. The silver ores of the Cobalt district of Ontario (pp. 234-235), containing nickel and cobalt arsenides, produce considerable arsenic. Many other metallic ores contain notable amounts of arsenic, which are at present allowed to escape through smelter flues, but which could be recovered under market conditions which would repay the cost of installing the necessary apparatus.