Native metal

Kondyor Mine Region Chabarowsk: native platinum nugget

A native metal is any metal that is found in its metallic form, either pure or as an alloy, in nature. Metals that can be found as native deposits singly and/or in alloys include aluminium, antimony, arsenic, bismuth, cadmium, chromium, cobalt, indium, iron, nickel, selenium, tantalum, tellurium, tin, titanium, and zinc, as well as two groups of metals: the gold group, and the platinum group. The gold group consists of gold, copper, lead, aluminium, mercury, and silver. The platinum group consists of platinum, iridium, osmium, palladium, rhodium, and ruthenium.

Only gold, silver, copper and the platinum metals occur in nature in larger amounts. Over geological time scales, very few metals can resist natural weathering processes like oxidation, which is why generally only the less reactive metals such as gold and platinum are found as native metals. The others usually occur as isolated pockets where a natural chemical process reduces a common compound or ore of the metal, leaving the pure metal behind as small flakes or inclusions.

Non-metallic elements occurring in the native state include carbon and sulfur. Silicon, a semi-metal, has been found in the native state on rare occasions as small inclusions in gold.^[1]

Native metals were prehistoric man's only access to metal, since the process of extracting metals from their ores, smelting, is thought to have been discovered around 6500 BC. However, they could be found only in relatively small amounts, so they could not be used extensively. So while copper and iron were known well before the copper age and Iron Age, they would not have a large impact on humankind until the technology to smelt them from their ores, and thus mass produce them appeared.

Native gold partially embedded in quartz gangue

Occurrence

Gold

Gold is the most well known of the native metals. Mined gold is almost invariably portrayed as solid nuggets or veins. Indeed, most gold is mined as native metal and can be found as nuggets, veins or wires of gold in a rock matrix, or fine grains of gold, mixed in with sediments or bound within rock. The iconic image of gold mining for many is gold panning, which is a method of separating flakes of pure gold from river sediments due to their great density.

Silver

Silver with quartz matrix (5 x 3 cm)

Native silver occurs as rare cubic, octahedral, or dodecahedral crystals. It more commonly as elongated dendritic silver wires or as coatings or massive. It may occur alloyed with gold as electrum. Typically occurs with silver sulfides and sulfosalts.^[2][3] Various amalgams of silver and mercury or other metals and mercury do occur rarely as minerals in nature. An example is the mineral eugenite (Ag₁₁Hg₂) and related forms.^[4]

Platinum group

Natural alloys of the platinum group metals include: native osmium (Os,Ir,Ru), rutheniridosmine (Ir,Os,Ru), ruthenium (Ru,Ir), palladium (Pd,Pt), platinum Pt, and rhodium (Rh,Pt). In addition gold, copper and iron occur in these alloys.^[5]

Copper

Native copper

Main article: Native copper

Main article: Copper mining in Michigan

Native copper has been historically mined as an early source of the metal. The term Old Copper Complex is used to describe an ancient North American civilization that utilized native copper deposits for weapons, tools, and decorative objects. This society existed around Lake Superior, where they found sources of native copper and mined them between 6000 and 3000 BC.^[6] Copper would have been especially useful to ancient man as it was much stronger than gold, hard enough to be made into useful items such as fishhooks and woodworking tools, but still soft enough to be easily shaped, unlike meteoric iron.

The same deposits of native copper on the Keweenaw Peninsula and Isle Royale were later mined commercially. From 1845 until 1887, the Michigan Copper Country was the leading producer of copper in the United States. Masses of native copper weighing hundreds of tons were sometimes found in the mines.

Iron, nickel and cobalt

Nickel iron meteorite

Most of the native iron on earth is actually not in fact "native", in the traditional sense, to earth. It mainly comes from iron-nickel meteorites that formed millions of years ago but were preserved from chemical attack by the vacuum of space, and fell to the earth a relatively short time ago. Metallic meteorites are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content. However, there are a very few areas on earth where truly native iron can be found.^[7][8]

Native nickel has been described in serpentinite due to hydrothermal alteration of ultramafic rocks in New Caledonia and elsewhere.^[9][10]

Metallic cobalt has been reported in the Canadian Lorraine Mine, Cobalt-Gowganda region, the Timiskaming District, Ontario, Canada, and in the Aidyrlya gold deposit in Orenburgskaya Oblast of the Southern Urals.^[11]

Others

All other native metals occur only in small quantities or are found in geologically special regions. For example metallic cadmium was only found at two locations. One place where metallic cadmium can be found is the Vilyuy River basin in Siberia.^[12] Native molybdenum has been found in lunar regolith and in the Koryakskii volcano in Kamchatka Oblast of Russia.^[13]

References

1. ^ Silicon, Mindat.org
2. ^ Handbook of Mineralogy
3. ^ Webmineral data
4. ^ Eugenite on Mindat (see related minerals and localities)
5. ^ Iridium on Mindat (see: Relationship of Iridium to other Species - Nichel-Strunz
6. ^ Thomas C. Pleger (2000). "The Old Copper Complex of the Western Great Lakes". UW–Fox Valley Anthropology. http://www.uwfox.uwc.edu/academics/depts/tpleger/oldcopper.html. Retrieved 2007-08-15. 
7. ^ Mindat with location data
8. ^ Native iron in the Handbook of Mineralogy
9. ^ Nickel on Mindat with location data
10. ^ Native nickel in the Handbook of Mineralogy
11. ^ Cobalt on Mindat
12. ^ Fleischer, Michael; Cabri, LouisJ.; Chao, Georg Y.; Papst, Adolf (1980). "New Mineral Names". American Mineralogist 65: 1065–1070. http://www.minsocam.org/ammin/AM65/AM65_1065.pdf. 
13. ^ Native Molybdenum - Mindat.org

• The Native Elements Class