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Electrum Mineral Facts:

Chemical Formula: Au(Ag) Gold and Silver alloy, more than 20% silver by weight.

Colors: Pale metallic gold, streak is the same.

Hardness: 2.5 to 3

Density: 12.5 to 15.5
The density is variable depending on the silver content. 

Cleavage: Electrum is ductile and mallable. Also sectile, and can be cut with a knife like lead.

Crystallography: Isometric, commonly octahedral.
Usually in irregular plates, scales or masses, and seldom definitely crystallized.

Luster:. Metallic luster.

Optics: (Refractive Index):  Opaque

Nevada Electrum, Tuscarora District

Composition, Structure and Associated Minerals:
Electrum is composed of an exceedingly intimate mixture (an isomorphous solid solution) of gold and silver. It is a natural alloy of the two metals.

The electrum known to Pliny the ancient Roman historian was probably named on account of its yellow color, from the Greek word for amber, of Strabo. It was defined as a natural alloy containing one-fifth of silver. The term is occasionally used by modern writers to cover natural gold-silver alloys, but has not met with general acceptance, its place being supplied by an extension of the term "gold." Electrum is derived almost entirely from the Tertiary andesitic goldfields of North, South, and Central America, New Zealand, and Hungary. Its color varies, with the percentage of silver present, from yellowish white to pale yellow.

Electrum is more commonly found in gold deposits with a high silver content, and is typically found in epithermal type deposits.  Here the gold is still associated with the metallic impurities of the sulfur bearing waters in which it has made its ascent towards the surface. Because of this, it is associated with silver minerals such as the ruby silvers, stephanite and other silver minerals as well as other sulfides such as galena.

Identification and Diagnostics
Electrum meets all the standard tests for metallic, native gold, but contains a much higher than normal silver content. It is usually distinguished by its pale golden color which is much lighter than typically seen in native gold. Factual confirmation requires a chemical analysis of the sample.

Occurrence, Localities and Origins:
Epithermal ore deposits form at shallow depth and are often associated with comparatively young volcanic rocks . Electrum has been found in epithermal gold-silver deposits of both high- and low-sulfidation styles. Electrum is found in silver-gold epithermal vein systems associated with silver and other sulfides  in many bonanza type silver mining districts. In Nevada it is an important ore at  the Comstock lode, Tonopah and at Round Mountain (where it is still actively mined). It is found with the rich silver ores of  Mexico, Chile, Bolivia and Peru. 

In Europe it is known from several  localities in Hungary where the deposits are associated with Eocene volcanic rocks. It is also found in epithermal deposits in New Zealand. Probably an unrecognized but significant amount of the free gold recovered from many epithermal deposits is actually electrum. In either case, it is an important ore of gold.

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