Gold is widespread in low concentrations in all igneous rocks and has been discovered on every continent on earth Estimates for Its abundance in the Earth's crust vary between .001 to 0.004 parts per million. In seawater there are about 0.00001 ppm. The average human body (70 kg) contains 0.2 mg of gold with the bone containing .016 ppm and the liver .0004 ppm.
It is 75th in order of abundance of the elements in the crust of the Earth.It occurs mostly in the native state, remaining chemically uncombined except with tellurium, selenium, and possibly bismuth. The element's only naturally occurring isotope is gold-197. Two types of deposits containing significant amounts of gold are known: hydrothermal veins, where it is associated with quartz and pyrite (fool's gold); and placer deposits, both consolidated and unconsolidated, that are derived from the weathering of gold-bearing rocks.
The origin of enriched veins is not fully known, but it is believed that the gold was carried up from great depths with other minerals, at least in partial solid solution, and later precipitated. The gold in rocks usually occurs as invisible disseminated grains, more rarely as flakes large enough to be seen, and even more rarely as masses or veinlets. Crystals about 2.5 cm (1 inch) or more across have been found in California. Masses, some on the order of 90 kg (200 pounds), have been reported from Australia.
Gold occurs mostly in the native state, remaining chemically uncombined except with tellurium, selenium, and possibly bismuth. Gold is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold often occurs in association with copper and lead deposits, and, though the quantity present is often extremely small, it is readily recovered as a by-product in the refining of those base metals. Large masses of gold-bearing rock rich enough to be called ores are unusual. Gold also occurs in seawater to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in seawater is more than 9 billion metric tons, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.
Gold is found in nature in quartz veins and secondary alluvial deposits as a free metal or in a combined state. It 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 sulfur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulfide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in seawater to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in seawater is more than 9 billion metric tons, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.
The wear resistance is related to its surface hardness and age as well as to the actual metal. The annealed hardness of 9 & 18 kt golds are similar. 18, 14, 9 kt golds are considerably harder than 22 or 24 kt golds. Annealing softens them, and working (bending, twisting) tends to work harden metals (this explains why forged steel is much stronger than cast steel for the same alloy content).
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