Compounds of gold

Appearance

Due to its relative inertness, gold is often found in nature in an unbound form. It usually occurs as a trace element in minerals, but sometimes also in the form of nuggets (nuggets), and sometimes in the form of a large nugget. The largest nugget ever found is the Welcome Stranger Nugget with a weight of over 71 kilos. It was found in 1869 in Moliagul, Victoria, Australia at a depth of almost a metre and has been melted down. The Serra Pelada (Canaã) Nugget was found in 1983 near Serra Pelada, Pará, Brazil. With a weight of over 61 kilos, this is the largest nugget still existing in its original form.

In almost the entire earth’s crust, gold is found in very low concentrations in the minerals Petzite, Calaverite and Sylvanite. However, the quantities are completely unprofitable for commercial extraction.

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The highest concentrations have been found in South Africa since 1880. Other large gold mines are located in Nevada and South Dakota in the United States. Australia, Indonesia and the People’s Republic of China are among the most important gold producing countries (in 2001). Approximately two thirds of the extraction takes place in open mines (open-cast mining) and produces large quantities of waste. Annual production from the mines in the years 2010 to 2017 was between 2745 and 3270 tonnes or at least two thirds of the total supply. The extraction of the gold from a 10-gram ring with an 18-carat content produces 18,000 kg of waste. Recycling old gold accounts for a quarter to a third of the total supply.

Two well-known gold mining methods use the toxic sodium cyanide and also toxic mercury by first dissolving the gold according to:

4 Au + 8 NaCN + O2 + 2 H2O → 4 NaAu(CN)2 + 4 NaOH.
The toxic sodium cyanide has caused a major environmental disaster at the Summitville mine in Rio Grande County, Colorado in the United States.

Especially in the past, mercury was used because gold dissolves in it (gold-mercury amalgam).

Usually gold occurs as fine grains, scattered in a rock. To be visible to the naked eye, the ratio of gold in gold ore must be more than 30 mg/kg (30 ppm). For most gold mines this means that the gold in the ore is not visible.

Origin
Gold has been present on Earth since the creation of the planet itself, but it has not been formed on Earth because its production requires fusion reactions. There are nuclear fusion reactions in the core of stars, but these nuclear fusion reactions do not provide enough energy for the production of gold.

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Stars that are heavy enough (from about 8 times the mass of the Sun) end their life in a supernova explosion. During this explosion, heavy metals, including gold, are formed through nuclear reactions. After the explosion, dust particles containing tiny bits of gold are thrown into space. When the dust particles enter the gravitational field of a star like the Sun during the formation of the planets, the dust particles become part of the aggregating matter around the star that eventually forms the planets.

Isotopes
Most stable isotopes
Iso RA (%) Half-life VV VE (MeV) VP
195Au syn 168,10 d EV 1,220 195Pt
197Au 100 stable with 118 neutrons
One stable isotope of gold is known and about 18 unstable, of which 195Au with a half-life of more than 168 days is the most stable.

Toxicology and safety
Pure gold is harmless to the human body and most gold compounds are not particularly toxic. However, liver and kidney damage, dermatitis and colitis can occur at relatively high doses of gold compounds (in the order of several tenths of a gram per week) as required for the treatment of rheumatoid arthritis, sometimes with fatal consequences. Colloidal gold can be absorbed by so-called macrophages, cells that play an important role in keeping inflammatory processes going (such as RA), after which they are killed (and the inflammation is inhibited). For this purpose, this gold is often injected into patients (because high doses are required for effect).

Furthermore, gold (as a so-called trace element) can play an important positive role in all kinds of enzymatic processes; and this can probably occur at low levels in the body. The human body (at a weight of 70 kg) contains approximately 0.2 mg of gold by nature.