The Aluminum Group
Though aluminum is one of the most abundant of all the metals, making some 8% of the crust of the earth, its union with other elements is so firm, that only recently have methods been found for getting the metal free. It was first isolated in 1846, but up to 1890 the extraction of aluminum was so expensive, that it could not be widely used. About that time electrical processes were applied to its extraction, and since then the price has steadily dropped, until now it is under $.20 per pound. It is very malleable, and ductile, and has high tensile strength. Exposed to the air, water or ordinary gases, it does not tarnish; and it is very light, an equal bulk weighing about a third as much as iron. The combination of lightness and strength, and the fact that it is a good conductor of electricity, have made it available for a wide range of uses, such as electrical apparatus, delicate instruments, boats, aeroplanes, and domestic utensils.
It is an essential component of all the important rocks, except sandstone and limestone, and combines to a greater or less degree in a host of minerals. Though present in clays, shales, argillites, feldspars, and micas, it is only from bauxite that it has been successfully extracted. Aside from the small number of simple compounds of aluminum grouped here, it also takes a part in the make-up of a large series of minerals termed silicates, treated a little further on in this book.
It alloys with other metals, especially copper. The union of copper and a small amount of aluminum makes aluminum-bronze, which looks like gold and is used for watch chains, pencil-cases, etc., and also for the antifriction bearings of heavy machinery. A small amount added to steel prevents air holes and cracks in casting.
[Corundum]
Al₂O₃
[Pl. 23]
Occurs in cleavable masses or in hexagonal crystals; hardness 9; specific gravity 4; colorless, red, yellow, blue, or gray; luster vitreous to adamantine; translucent to transparent on thin edges.
Corundum is readily recognized by its hardness, second only to that of the diamond. The crystals may be simple six-sided prisms, hexagonal pyramids or combinations of the two. The cleavage is usually described as parting, for it is by no means perfect, but when it is recognizable it is parallel to the faces of a rhombohedron, and cleavage pieces may appear almost cubic.
When in clear and perfect crystals this mineral is one of the most highly prized of all the gems. Clear and colorless it is known as the “Oriental white sapphire”; when tinged with blue it is the sapphire; when colored yellow, the “Oriental topaz”; when green, the “Oriental emerald”; when purple, the “Oriental amethyst” and when red, the ruby. Sapphires range from colorless to deep blue, the value depending on the shade of the blue, and increasing as the color deepens. The Oriental topaz can easily be confused with the true topaz, which is a much commoner and less valuable gem, but can be distinguished by the hardness, topaz having a hardness of but 8. The name emerald is applied to several green gems, mostly to beryl, which is not so hard and is the true emerald. The Oriental emeralds have a value about the same as diamonds. Rubies of clear and deep color are the rarest of all gems, ranging in value about three times as high as diamonds of equal size. The most sought-for shade is the so-called “pigeon-blood red,” and the value of a stone of this sort is almost dependent on the whim of the buyer. The best of the rubies come from granites or metamorphosed limestones in Burma; the best sapphires from Ceylon, though both of these, and some of the other corundums of gem quality, have been found in North Carolina and Montana.
Around these stones, which have been used so long among the Hindus, Persians, Jews, Egyptians, and Christians, a wealth of lore has been woven. The sapphire was Saturn’s stone, and a talisman to attract Divine favor. Where tradition makes the stone on which the ten commandments were written the sapphire, it is probable that, what was really meant, is lapis lazuli, as is also the case when sapphires are mentioned as building stones for the celestial gates. The ruby in ancient lore is termed “lord of stones,” “gem of gems” etc., and so protected its wearer that he was safe from injury in peace or war.
When corundum is colored brown by impurities of iron, it is termed corundum, when black by greater quantities of iron, it is emery. These varieties are far the commonest form in which corundum occurs, and when ground to finer or coarser powder make the commercial emery. Emery is likely to be found in sands, making so-called “black sands,” where it has accumulated as a result of the weathering to bits corundum-bearing rocks. In some one of its forms, corundum is found in Massachusetts, Connecticut, New York, New Jersey, and all down the Appalachian Mountains, also in Colorado, Montana, California, etc.
[Bauxite]
Al₂O₃·2H₂O
Occurs in grains, or oolitic or clay-like masses; hardness 1-3; specific gravity 2.5; color white to yellowish-white or reddish-brown.
Bauxite never comes in crystals, but is usually in earthy masses, which have resulted from the decomposition of granitic or volcanic rocks, in circumstances where hot alkaline waters were present. This explanation seems to apply especially to the deposits in France, which were first the chief source of the bauxite, and may be applicable to those in Georgia and Alabama. Some of the other deposits, however, do not seem to have had any hot water available, and the deposit appears more like simple decomposition of the underlying rocks by alkaline waters.
In many cases bauxite resembles limonite in being a mixture of two or more aluminum oxides with water of crystallization, such as Al₂O₃·H₂O, Al₂O₃·2H₂O and Al₂O₃·3H₂O. This is particularly true of the bauxite which resulted from the decomposition of rocks by surface water.
Bauxite is the ore from which aluminum is obtained. The deposits are not large, but the United States has its share of them. It is found in Alabama, Arkansas, Georgia, Missouri, Tennessee, and California.
[Cryolite]
Na₃AlF₆
Ice stone
Occurs in pseudo-cubic crystals or massive; hardness 2.5; specific gravity 3; color white; luster vitreous; transparent on thin edges.
Cryolite is a relatively soft mineral, colorless to white as snow; for which reason, and partly also because it comes mostly from Greenland it is called “ice stone.” It is really monoclinic but the inclination of the c axis is so slight, that, unless examined carefully, the crystals appear to be cubic. Until about 1900 great quantities of this mineral were shipped from West Greenland, and from them the metal aluminum was extracted. When bauxite was discovered, it was found to be considerably cheaper to make the aluminum from that mineral, and now cryolite is no longer sought. Aside from its occurrence in Greenland some cryolite is found in Colorado, near Pike’s Peak.