Precious Metals: Rhodium Jewelry
Rhodium Jewellery
Source: Canada, Russia (Ural Mountains), South Africa, USA
Rhodium is a rare, and highly reflective transition metal that is a member of the platinum group of metals (PGMs). The name "rhodium" was derived from the Greek work rhodon, meaning "rose" which refers to the red color of its salts. Up to 80 percent of the world's supply of rhodium comes from mines in South Africa.
Rhodium is a very hard metal, but it is not used as a stand-alone (solid) metal in jewelry, due to its high melting point and poor malleability. Rhodium is used as a hardening agent in the alloying of platinum and palladium, or as a highly-reflective plating (known as "rhodium flashing") for sterling silver or white gold.
Rhodium is electroplated onto the surface of other metals to increase their reflectiveness, and due to it high reflectance rhodium is used as an element for the preparation of "silvered" surfaces such as mirrors/reflectors used in motion-picture projectors and searchlights. Rhodium plating is also used to prevent black tarnish on .925 sterling silver. As a noble metal, rhodium is totally inert making it a perfect hypoallergenic plating for white gold.
Northam platinum mine (photo: www.northam.co.za) |
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Rhodium Ring Plated over .925 Sterling Silver |
Rhodium has a lower density and weight than platinum, but it is heavier than gold. Due to its low electrical resistance, rhodium is also used as an electroplate finish on electrical contacts. Rhodium is almost as resistant as iridium to chemical attack by acids, and is not dissolved by aqua regia which does dissolve both gold and platinum. In 2007, eighty-one percent of global rhodium production was consumed to produce three-way catalytic converters for automobiles and trucks.
In 1803, English chemist William Hyde Wollaston (1766-1828) first identified the element rhodium and palladium, which he found within platinum ore from South America. Wollaston's extraction technique involved dissolving the platinum ore in a corrosive solution, to create palladium cyanide from which the pure palladium was extracted.
Physical Properties of Rhodium
Rhodium Mining
Rhodium is a difficult material to extract as the metal occurs in small quantities within ores that are mixed with other metals such as gold, silver, platinum and palladium. It is found in platinum ores and obtained "free" as a white inert metal. The main producers of rhodium are South Africa and Russia (Ural Mountains), and Canada (Sudbury, Ontario).
One of the largest mining operations in South Africa is the Northam Platinum Limited mine (listed on the JSE Limited) in Limpopo Province. The mine specializes in platinum group metals (PGMs Ð platinum, palladium and rhodium), and comprises underground hard-rock mines alonk the "Merensky reef," at an average depth of 1 750 meters [1]. The harsh underground environmental conditions are mitigated by the use of giant surface refrigeration systems. Raw metals are refined at the W.C. Heraeus precious metals refinery in Port Elizabeth, South Africa, or the thier refinery in Hanau, Germany.
Bibliography on Rhodium
1. Northam, Platinum & Rhodium Mining in South Africa  . www.northam.co.za
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