I.

Introduction

Gold, symbol Au (from Latin aurum, “gold”), soft, dense, bright yellow metallic element. Gold is one of the transition elements of the periodic table; its atomic number is 79.

II.

Properties

Pure gold is the most malleable and ductile of all the metals. It can easily be beaten or hammered to a thickness of 0.000013 cm (0.000005 in), and 29 g (1.02 oz) could be drawn into a wire 100 km (62 mi) long. It is one of the softest metals (hardness, 2.5 to 3) and is a good conductor of heat and electricity. Gold is bright yellow and has a high lustre. Finely divided gold, like other metallic powders, is black; colloidally suspended gold ranges in colour from ruby red to purple (see Colloid).

Gold is extremely inactive. It is unaffected by air, heat, moisture, and most solvents. It will, however, dissolve in aqueous mixtures containing various halogens such as chlorides, bromides, or some iodides. It will also dissolve in some oxidizing mixtures, such as cyanide ion with oxygen, and in aqua regia, a mixture of hydrochloric and nitric acids. The chlorides and cyanides are important compounds of gold. Gold melts at about 1,064° C (about 1,947° F), boils at about 2,808° C (about 5,086° F), and has a relative density of 19.3; its atomic weight is 196.967.

III.

Occurrence

Gold is found in nature in quartz veins or seams, nuggets, flakes, and secondary alluvial deposits as a free metal or in a combined state. There are several chemical and physical processes that may cause these formations, and it is also likely that colonies of soil bacteria and fungi play a part in gold agglomerations. Gold is widely distributed although it is rare, being 75th in order of abundance of the elements in the crust of the Earth. It is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold occurs, in chemical combination with tellurium, in the minerals calaverite and sylvanite along with silver, and in the mineral nagyagite along with lead, antimony, and sulphur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulphide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in sea water to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in sea water is more than 9 billion tonnes, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.

IV.

Uses

The metal has been known and highly valued from earliest times, not only because of its beauty and resistance to corrosion, but also because gold is easier to work than all other metals. In addition, gold was easier to obtain in pure form than the other metals. Because of its relative rarity, gold became used as currency and as a basis for international monetary transactions (Gold Standard). The unit used in weighing gold is the troy ounce; 1 troy ounce is equivalent to 31.1 grams.

The major portion of the gold produced is used in coinage and jewellery (see Metalwork). For these purposes it is alloyed with other metals to give it the necessary hardness. The gold content in alloys is expressed in carats. Coinage gold is composed of 90 parts gold to 10 parts silver. Green gold used in jewellery contains copper and silver; white gold contains zinc and nickel, or platinum metals.

Gold is also used in the form of gold leaf in the arts of gilding and lettering. Purple of Cassius, a precipitate of finely divided gold and stannic hydroxide formed by the interaction of auric chloride and stannous chloride, is used in colouring ruby glass. Chlorauric acid is used in photography for toning silver images. Potassium gold cyanide is used in electrogilding. Gold is also used in dentistry. Radioisotopes of gold are used in biological research and in the treatment of cancer (see Isotopic Tracer).

V.

Gold Mining

The simplest process used for mining gold is panning, using a circular dish often with a small pocket at the bottom. The prospector fills the dish with gold-bearing sand or gravel, holds it under a gentle stream of water, and swirls it. The lighter parts of the gravel are gradually washed off and the gold particles are left near the centre of the pan or in the pocket.

As gold mining developed, more elaborate methods were introduced and hydraulic mining was invented. The hydraulic method consists of directing a powerful stream of water against the gold-bearing gravel or sand. This operation breaks down the material and washes it away through specially constructed sluices in which the gold settles, while the lighter gravel is floated off. For mining on rivers, elevator dredges are generally used. The elevator dredge is a flat-bottomed boat that uses an endless chain of small buckets to scoop up the material from the river bottom and empty it on the dredge into a trommel (a container built of screening). The material is rotated in the trommel as water is played on it. The gold-bearing sand sinks through perforations in the trommel and drops on to shaking tables, on which it is further concentrated. Dredging can also be used in dry beds of ancient rivers if ample water is within a reasonable distance. A pit is dug, and the dredge is moved in and floated on water pumped from the adjacent source.

Extensive underground deposits of gold-bearing rocks are often discovered by a small outcrop on the surface. Shafts are sunk, as in coal mining, and the ore is brought to the surface. It is then crushed in special machines.

Gold is extracted from gravel or from crushed rock by dissolving it either in mercury (the amalgam process) or in cyanide solutions (the cyanide process). Some ores, especially those in which the gold is chemically combined with tellurium, must be roasted before extraction. The gold is recovered from the solution and melted into ingots. Gold-bearing rock with as little as 1 part of gold to 300,000 parts of worthless material can be worked at a profit.

The rarest form of gold is a nugget. The largest known nugget, the Welcome Stranger, weighing about 70.8 kg (about 156 lb), was turned up accidentally, just below the surface of the ground, by a wagon wheel in Victoria, Australia, in 1869.

VI.

Gold Production

Gold production dates from the Etruscan, Minoan, Assyrian, and Egyptian civilizations, when placer gold was derived from alluvial sands and gravels by simple processes of washing or panning. Gold was produced in this manner at an early period in India, central Asia, the southern Ural Mountains and in the regions bordering the eastern Mediterranean. With progress in mining technique, primary auriferous (gold-bearing) veins were exploited; this type of gold mining attained some importance before the Christian era. During the Middle Ages little progress was made in gold production and mining.

At the time of the discovery of the Americas the value of the total gold stock of Europe was probably less than $225 million. During the succeeding 350 years, from the end of the 15th century to about 1850, the world gold output totalled about 4,665,000 kg (about 150 million troy oz). South America and Mexico became large producers of gold during this period. Spain's domination in South America resulted, in the 16th century, in a large increase in gold produced in the New World; some resulted from simple seizure of gold from the Native Americans, who had long mined the metal. In the same century Mexico contributed about 9 per cent of the total world production. Gold was discovered in Australia in February 1851, and rich fields were found there.

South Africa is the world's leading supplier of gold, producing about 600 tonnes annually; its most important gold mines are in the Witwatersrand region. Some 70 other countries produce gold in commercial quantities, but about 80 per cent of the total worldwide production now comes from South Africa, the United States, the former Soviet republics, Australia, Canada, China, and Brazil.