13 ALUMINIUM Al (Latin: alumen = alum)
Aluminium is a shiny white soft lightweight corrosion-resistant metal of relative density 2.70. Aluminium is a highly electropositive element, and strong lithophile. The surface forms an extremely thin but protective oxide film in air that prevents further tarnishing. This very thin and insulating oxide film (with a very high dielectric strength) is responsible for the high capacitance of electrolytic capacitors. This thin aluminium oxide protective film on the metal may be deliberately thickened by an electrochemical process called anodising. Powdered aluminium burns in a shower of brilliant white sparks (used in fireworks) producing a white powder, aluminium oxide, Al2O3, or alumina, and a great deal of heat. Powdered aluminium mixed with a metallic oxide, when ignited (usually with magnesium) reduces the metallic oxide to the pure metal by exchanging the oxygen, a highly exothermic reaction producing temperatures up to 3500 Celsius. This works even for metallic oxides which are reduced with difficulty, such as those of titanium or molybdenum. This is called the thermite reaction and, using iron oxide, is used to weld railway track. Alumina is a refractory material.
Crystalline aluminium oxide, or corundum (not to be confused with carborundum, silicon carbide), is used as an abrasive because of its great hardness, 9 on Mohs scale, second only to diamond, 10. Corundum is often fluorescent, glowing red or orange under UV light, and also triboluminescent, emitting orange flashes when impacted. Common corundum is bluish grey to brown, but highly prized gem varieties exist: sapphire is bluish due to iron or titanium, and was used as sapphire gramophone needles and bearings in watches because of its great hardness; ruby is red due to traces of chromium and is used in high-powered pulsed red lasers; leucosapphire which is used as imitation diamond; yellow corundum or yellow sapphire; and the very rare mauve corundum. Corundum with trivalent iron impurities produces pale yellow sapphires, divalent iron paired with tetravalent titanium gives brilliant blue variety, divalent magnesium pairs with a hole in the lattice producing yellow, whilst chromium produces red sapphires. Red sapphire is generally called a ruby, but authorities differ in the amount of redness required. Corundum when disseminated with fine-grained magnetite is known as emery, and used to coat heavy paper when it is known as emery paper which is as an abrasive.
A milky white or brownish variety of corundum called geuda can be converted into a blue gem-varity sapphire by heating it to 1500 Celsius, or into other coloured varities by baking it for days in furnaces with other impurities present, where the impurities diffuse through the crystal. Fake pink varieties can be made by heating it with the beryllium-containing beryl.
Spinel is a magnesium aluminium silicate, MgAl2O4, which is used as a precious stone. Pure spinel is colourless, but the magnesium can be replaced by zinc to produce a blue variety, by iron to produce a green variety, or by chromium producing the usual red and pink varieties.
Aluminium has many uses: soft drinks cans, kitchen & 'tin' foils; omnibus bodies; and because of its lightness, aeroplanes; and because of its good thermal conductivity, heatsinks and frying pans; and, when suitably protected from the air, as the reflector in telescopes. Aluminium has little structural strength and is usually alloyed with a trace of silicon or iron to harden it. Strengthened by a steel core, it is used in overhead electrical power transmission cables because it is much lighter and cheaper than copper and is a good conductor of electricity. X-rays can pass through aluminium windows in X-ray tubes. It is used as an electrode in gas discharge tubes because it does not sputter like other metals. Elemental aluminium may be non-toxic but its compounds are implicated in Alzheimers disease. Symptoms of aluminium poisoning also include bone disease and anaemia. Alumina poisoning was the subject of an old black & white film, "Dead on Arrival". Acid rains cause the mobilisation of Al3+ ions in soil, which can combine with soluble phosphorus compounds in soils forming the insoluble aluminium phosphate, AlPO4, which limits the availability of essential phosphorus for plants and trees and has caused the death of some Scandinavian forests. Aluminium alloyed with copper and magnesium produces strong lightweight alloys and diecast alloys. AlNiCo, an alloy of aluminium, nickel and cobalt, makes permanent magnets. An exotic alloy of lithium and aluminium is used in jet planes and spacecraft because it is 38% lighter than aluminium. The alloy Al65Co20Cu15 has the forbidden pentagonal symmetry.
Bauxite, is an earthy mixture of gibbsite (or hydrargillite), Al(OH)3, diaspore and boehmite, AlOOH, and alumogel, AlOOH+nH2O, smells of clay when wet, and is an important ore of aluminium from which it is extracted by electrolysis, guzzling electricity from hydroelectric power stations. Aluminium is also present in many other rocks including the aluminosilicate feldspars (the most abundant minerals of the Earths crust, being major constituents of granite) and micas (also present in granites). Cryolite, Na3AlF6, is uncommon and was once used as a flux for the electrolytic recover of aluminium from bauxite ore. Cryolite is unusual in that it has a low refractive index which is close to that of water, in which it becomes almost invisible. Ultramarine, a deep blue pigment used in paints and 'dolly blue', is obtained by crushing the blue stone, lapis lazuli, or can be made artificially by calcining silica, sulphur, soda ash and china clay. Lapis lazuli, or lazurite, not to be confused with the similar looking and sounding lazulite, is a mottled blue and white stone highly prized for decorative carvings. It is a sodium aluminosilicate with sulphur which is the cause of its vivid blue colour, Na8S(AlSiO4)6, often containing white limestone or dolomite, and associated with a similarly speckled blue and white stone, sodalite, Na8Cl2(AlSiO4)6, a complex sodium aluminosilicate with chlorine. Lazulite, a basic aluminium-magnesium phosphate with iron, (Mg,Fe)Al2(OHPO4)2, was often passed as the more valuable lazurite (lapis lazuli) by Arabs. China clay, kaolin, which is white and has long been used by the chinese to make porcelain, contains kaolinite as its main constituent, a complex basic aluminosilicate, Al4(OH)6Si4O10, used also for filling paper and paints. Kaolin is also present in Cornwall.
Topaz is an aluminium fluorosilicate, Al2F2SiO4, an extremely hard and valued gemstone measuring 8 on Mohs scale of hardness. They are variously coloured, being either clear, or commonly yellowish, but can be blue, pinkish, or pink to violet, the richly coloured varieties can fade in sunlight, but due to the heat rather than the light from the sun. Jewellers now artificially alter the colour of rich yellow topaz gems to rose red by heating them, which gives it a high refractive index and strong pleochroism (displaying different colours from differing directions).
Both aluminium sulphate and sodium aluminate, Na2Al2O4·H2O, are used as coagulants in water purification. The compounds precipitate out as aluminium hydroxide, taking suspended matter with them. Aluminium sulphate was the subject of an accidental poisoning of the water supply to Camelford, where a whole tanker load of it was emptied into the water distribution tank by mistake, which reacted with copper in domestic piping turning consumers hair green. Aluminium dissolves in a solution of sodium hydroxide producing hydrogen gas and precipitating aluminium hydroxide, Al(OH)3, which is useful as a mordant in dyeing. Alums are used to tan leather. Aluminium carbide, Al3C4, evolves methane when exposed to water. Aluminium antimonide is a semiconductor used at temperatures up to 500ºC.
Aluminium consists of just one stable isotope, Al-27. Al-28 and Al-29 are strong gamma ray emitters with very short halflives. The positron emitter, Al-26, created in trace amounts in the atmosphere by cosmic ray bombardment, has a half life of 0.7 Myr. Altogether, 12 radioactive isotopes are known, from Al-22 to Al-34.
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