Gold is a mineral that is a natural solid solution of silver (traces, up to 43%) in gold; common impurities (traces, up to 0.9%) of copper, iron, lead, less often - bismuth, mercury, platinum, manganese, etc. There are varieties with an increased copper content - up to 20% (cuprous gold, cuproaurite), bismuth - up to 4 % (bismuth gold, bismuthaurite), platinoids (platinum and iridous gold; porpecite - Au, Pd, give birth - Au, Rh), natural amalgams (Au, Hg).
It crystallizes in a cubic system, in the form of octahedra, rhombododecahedrons, cubes and more complex crystals; often they are distorted, strongly elongated, forming "wires", "hairs", or flattened parallel to the face of the octahedron. Native gold is especially low-grade, characterized by a variety of growth forms, it is usually in the form of skeletal crystals, dendrites, whiskers and twisted-whiskers. Vein-like and irregular lumpy, "hooked" discharge is widespread; on their surface, imprints of crystals of other minerals are often preserved, the aggregates of which included accumulations of native gold. Etching reveals the crystalline-granular structure of the gold particles.
Gold is a very heavy metal: the density of pure gold is 19.32 g / cm³ (a ball of pure gold with a diameter of 46.237 mm has a mass of 1 kg). Diamagnetic, that is, the magnetic field in gold is weakened. Among metals, it occupies seventh place in density after osmium, iridium, rhenium, platinum, neptunium and plutonium. Tungsten has a density comparable to that of gold (19.25). The high density of gold makes it easier to mine, which is why even simple technological processes - for example, washing at sluices - can provide a high degree of gold recovery from the washed rock.
Gold is a very soft metal: hardness according to the Mohs scale is ~ 2.5, according to Brinell 220-250 MPa (comparable to the hardness of a nail).
Gold is also highly plastic: it can be forged into sheets up to ~ 0.1 microns (100 nm) thick (gold leaf); with such a thickness, gold is translucent and in reflected light has a yellow color, in transmitted light it is colored bluish-greenish, complementary to yellow. Gold can be drawn into wire with a linear density of up to 2 mg / m.
The melting point of gold is 1064.18 ° C (1337.33 K), boils at 2856 ° C (3129 K). The density of liquid gold is less than that of solid gold, and amounts to 17 g / cm3 at the melting point. Liquid gold is quite volatile, and actively evaporates long before its boiling point.
The gold content in the earth's crust is very low - 4.3 • 10-10% by weight (0.5-5 mg / t), but deposits and areas sharply enriched in metal are very numerous. Gold is also found in water. One liter of both sea and river water contains less than 5 • 10-9 grams of Au, which roughly corresponds to 5 kilograms of gold in 1 cubic kilometer of water.
Gold deposits occur mainly in areas of development of granitoids, a small number of them are associated with basic and ultrabasic rocks.
To obtain gold, its basic physical and chemical properties are used: presence in nature in a native state, the ability to react with only a few substances (mercury, cyanides). With the development of modern technologies, chemical methods are becoming more popular.
In terms of its chemical resistance and mechanical strength, gold is inferior to most platinoids, but it is irreplaceable as a material for electrical contacts. Therefore, in microelectronics, gold conductors and gold electroplating of contact surfaces, connectors, printed circuit boards are used very widely.
Gold is used as a target in nuclear research, as a coating for mirrors operating in the far infrared range, as a special shell in a neutron bomb. A thin layer of gold (20 nm) on the inner surface of window and stained glass significantly reduces unwanted heat losses in winter and protects the interior of buildings and vehicles from infrared heating in summer.
Gold solders are very good at wetting various metal surfaces and are used for soldering metals. Thin gaskets made of soft gold alloys are used in the ultra-high vacuum technique.
The traditional and largest consumer of gold is the jewelry industry. Jewelry is made not of pure gold, but of its alloys with other metals, significantly superior to gold in terms of mechanical strength and durability. Currently, Au-Ag-Cu alloys are used for this, which may contain additives of zinc, nickel, cobalt, palladium. The corrosion resistance of such alloys is determined mainly by the gold content in them, and the color shades and mechanical properties are determined by the ratio of silver and copper.
Dentistry consumes significant amounts of gold: crowns and dentures are made from alloys of gold with silver, copper, nickel, platinum, zinc. These alloys combine corrosion resistance with high mechanical properties.
Gold compounds are part of some medicines used to treat a number of diseases (tuberculosis, rheumatoid arthritis, etc.). The radioactive isotope 198Au (half-life 2.967 days) is used in the treatment of malignant tumors in radiotherapy.