Other Forms of Gold
© AMNH / Craig Chesek
Breckenridge gold: Breckenridge, Colorado. The Arkenstone.
The Campion Collection: Colorado's Bounty
Mine owner John F. Campion was a founder of the Colorado Museum of Natural History, now the Denver Museum of Nature and Science. His personal collection of minerals, donated to the museum in the early 1900s, includes examples of spectacular crystalline wire and leaf gold from the Farncomb Hill area of the Breckenridge district. The examples have been cleaned, probably with hydrochloric acid, to remove the surrounding minerals and reveal the natural structure of the gold crystals. These examples weigh as much as 628 grams (20 troy ounces).
Some wire and leaf gold formations come from pockets--openings as much as two or three feet in diameter--in the gold-bearing veins of the host rock. Pressure from hydrothermal--hot water--fluids forces open the crack and fracture in the rock. Then fluids deposit minerals where the gold crystals have room to grow.
Almost all gold mined on Earth is native gold; that is, the gold is in a pure or nearly pure state. Unlike copper, silver, iron and other metals, gold rarely combines with other nonmetallic elements to form complex minerals. It is this quality that also makes it resist corrosion.
However, a few minerals occur in nature that are chemical combinations of gold and nonmetallic elements such as sulfur, selenium and tellurium. These nonnative minerals are rare. Many, though beautiful in their own right, have lost the color and luster of native gold.
In the periodic table of the elements, gold (AU) is centrally located.
Gold With Few Elements
Some samples contain gold combined with tellurium or silver, or both. Many gold deposits are associated with silver. More silver occurs in Earth's crust than gold; for every seven parts of gold there are 100 parts of silver.
The mineral electrum contains gold and silver (at least 20 percent by weight) in combination. If the silver content is less, then the mineral is considered native gold.
Gold With Many Elements
Some samples combine gold with even less common elements such as antimony, tellurium, palladium, platinum and others. These nonnative gold minerals are less common than the first group.
Montbrayite is very rare. It has only been found in five mines worldwide.