The Gold Group
Gold was undoubtedly the first metal to be used by primitive man; for, occurring as it did in the stream beds, its bright color quickly attracted the eye, and it was so soft, that it was easily worked into various shapes, which, because they did not tarnish, became permanent ornaments. The metal is associated with the very earliest civilizations, being found in such ancient tombs as those at Kertsch in Crimea and in northern Africa and Asia Minor. It was used in the cloisonné work of Egypt 3000 years B.C. In America the Indians, especially to the south, were using it long before the continent was discovered.
Of all the metals gold is the most malleable, and its ductility is remarkable, for a piece of a grain’s weight (less than the size of a pin head) can be drawn out into a wire 500 feet long; and it can be beaten into a thin leaf as thin as ¹/₂₅₀₀₀₀ of an inch in thickness, and thus a bit, weighing only a grain, can thus be spread over 56 square inches.
It forms very few compounds, but has a considerable tendency to make alloys (i.e., mixtures with other metals without the resulting compound losing its metallic character). In Nature gold is never entirely pure, but is an alloy, usually with silver, there being from a fraction of 1% up to 30% of the silver with the gold, the more silver in the alloy, the paler the color of the gold. Australian gold is the purest, having but about .3% of silver in it, while Californian gold has around 10% and Hungarian gold runs as high as 30% of silver. Another alloy fairly abundant in Nature is that with tellurium, such as calaverite (AuTe₂) which is a pale brassy yellow, similar to pyrite, but with the hardness of but 2.5. Another combination includes gold, silver and tellurium, sylvanite, (AuAgTe₄) a silvery white mineral with a hardness of but 2. Such combinations are known as tellurides and the calaverite is mined as a source of gold at Cripple Creek, Colo., while the sylvanite is one of the important ores of gold in South Africa. Occasionally gold is also found alloyed with platinum, copper, iron, etc. Jewelers make several alloys, “red gold” being 3 parts gold and 1 of copper, “green gold” being the same proportions of gold and silver, and “blue gold” being the combination of gold and iron. Our gold coins are alloys, nine parts gold and one of copper, to give them greater durability. Most of the gold recovered from nature is found native, i.e., the pure metal, or with some alloy.
[Gold]
Au
[Pl. 5]
Usually non-crystalline, but occasionally showing cube or octahedral faces of the isometric system; hardness 2.5; specific gravity 19.3; color golden yellow; luster metallic; opaque.
Gold is mostly found as the metal and is readily recognized by its color, considerable weight, hardness, malleability, and the fact that it does not tarnish. It usually occurs in quartz veins in fine to thick threads, scales or grains, and occasionally in larger masses termed “nuggets.” It is insoluble in most liquids so that when weathered from its original sites, it was often washed down into stream beds, to be found later in the sands or gravels, or even in the sea beaches. When thus found it is termed “placer gold,” and its recovery is placer mining. Most of the original discoveries of gold have been in these placer deposits; and from them it has been traced back to the ledges from which it originally weathered. In the placer deposits the size of the particles varies from fine “dust” up to large nuggets, the largest found in California weighing 161 pounds; but the largest one found in the world was the “Welcome Nugget,” found in Australia, and weighing 248 pounds. When gold was discovered in California in 1848, this became the chief source for the world, but later this distinction went to Australia, and now belongs to South Africa, which today yields over half the annual supply.
The ultimate source of gold is from the lighter colored igneous rocks, like granites, syenites, and diorites, throughout which it is diffused in quantities too small to be either visible or worth while to extract. It becomes available only when it has been dissolved out by percolating waters and segregated in fissures or veins, either in or leading from these igneous rocks. Generally this transfer of gold has taken place when the rocks were at high temperatures, and by the aid of water (and perhaps other solvents) which was also at high temperatures. The presence of gold in sandstones, limestones, etc., is secondary, as is also its presence in sea water, in which there is reported to be nearly a grain (about five cents worth) in every ton of water. Beside the direct recovery of gold from gold mining, a great deal is obtained from its association with iron, copper, silver, lead and zinc sulphides, in which it is included in particles too fine to be visible, but in quantities large enough to be separated from the other metals after they are smelted.
In the United States gold is found in the Cordilleran region from California to Alaska, in Colorado, Nevada, Arizona, Utah, the Black Hills of South Dakota, and in small quantities in the metamorphosed slates of North and South Carolina, Georgia, and in Nova Scotia.