Gold (#46)

Crystalline gold on massive white quartz.
North America 16:1 Mine, Alleghany, Sierra County, California, USA 3 ¾" x 3" x 2 ½"

 

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Gold is both a "noble metal" and a "precious metal." Noble metals are those that exhibit high resistance to oxidation and corrosion - they don't react appreciably with other elements. They include gold, silver, platinum, and five others with names that are probably less familiar to you. Precious metals are rare and have high economic value, and there is high overlap between the two groups. Pure gold is quite soft and not suitable for jewelry that will experience abrasion, such as a wedding ring. So almost all gold jewelry is made of gold alloyed with some other metal to make it harder. The purity of gold in jewelry is measured in karats (not the same as the carat, used to measure the mass of a precious stone). Pure gold is 24 karats, 18 karats is 75°/o pure gold, and so forth.

All of the minerals of lower number in this display have one thing in common: they are nonmetallic. To be sure, many of them contain metals like Fe or Cu in their chemical formulas, but they have nonmetals in their formulas as well, and they do not have the properties of metals: they do not have a metallic luster, they are nonconductors of electricity, they conduct heat poorly, and they do not form alloys.

Many of the physical properties of minerals arise from the way in which their atoms are held together - chemical bonding. For the previous minerals, the bonding has been on the continuum between ionic and covalent. For metals, it is different. The outer electrons, those that participate in bonding, are "delocalized" - free to roam about within the bulk metal. This is sometimes described as "cores of atoms surrounded by a sea of electrons." This results in metallic luster and high conductivity for heat and electricity.