Geologic Features
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, a sodium-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.
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 ORES
Economic Features
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, with smaller 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.