by John Betterton

Iridium, Ir, osmium, Os and ruthenium, Ru are among the rarest of the elements and thus they produce few minerals. Hence we treat them together.

Iridium is one of least common metals with a crustal abundance of about 3 parts per trillion. Most of this metal is extracted from the by-product of nickel refining. Canada is the main producer along with South Africa and Russia. About 3 tonnes are produced each year.

The main uses are in the chemical, electrochemical industries, in crucibles, spark plugs, various alloys, in fountain pen nibs, compass bearings and in certain porcelains etc.

Osmium possesses a crustal abundance of around 1 part per billion. Again most of this element is derived from the by-product of nickel refining. About 100kg are used annually.

Canada is the leading producer along with Russia and South Africa. The metal has few uses but includes the following: various catalysts, staining organic tissues in electron microscopy preparations, some alloys and geological dating.

Ruthenium is very rare with an average crustal abundance of about 1 part per billion. Again most ruthenium is derived from by-product of nickel refining. Canada and South Africa are the main producers with an annual production of around 12 tonnes. Ruthenium uses are on the increase with such uses as in the manufacture of solar cells, in various chip resistors, chemical industry for anodes, catalysts, jewellery, corrosion resistant pipes and in eye tumour treatments. The majority of the minerals containing the above elements are described below.

Iridium, Ir, can occur as cubic crystals and commonly as rounded or angular grains. Twinning has been observed in polysynthetic groups. It is opaque and silver-white colour with a yellow tinge. The mineral has a metallic lustre with a hackly fracture. It is somewhat malleable and
possesses a high hardness of between 6 and 7. The specific gravity is extremely high at 22.6-22.8.

Chemistry, grand physical properties and environment aids in its identification. Its common occurrence is in placers derived from zoned ultramafic intrusions and from ophiolites. Main associates are platinum and various Pt-Fe alloys as inclusions. It is widespread and is reported from the following countries: Columbia, Canada, USA, Pap New Guinea, Indonesia, Australia, Russia, China, South Africa and in other minor occurrences.

Iridarsenite, (Ir,Ru)As2, is a monoclinic mineral that forms irregular inclusions. Its colour in polished section is medium grey with a brownish tint. It has a metallic lustre with hardness of 5-5.5. The mineral is opaque with a high specific gravity of 10.9.
Chemistry and x-rays are necessary for conformation. It is found in nuggets and in fragments of natural Os-Ir-Ru alloys.
Iridarsenite is found with irarsite, ruthenarsenite and rutheniridosmine.

Various nations have identified this species and include the USA, China, South Africa, Russia, Papua New Guinea, Greece and Cuba.

Shuangfengite, IrTe2, is a very rare mineral that forms small massive aggregates and veinlets. This species crystallises in the trigonal system. It is black in colour with a black streak. The specific gravity is high at 10.1 with a perfect cleavage. This mineral has a hardness of 3 and is quite brittle. Chemistry, physical properties and x-rays help here. Shuangfengite occurs in placer deposits accompanied by iridium, osmium, platinum, erlichmanite, chromite, magnesite, ilmenite, gold, irarsite and gaotaiite. It is only found currently at the Shuangfeng village location in Hebei Province, China.

Osmium, Os, is found as euhedral prismatic inclusions that belong to the hexagonal system. Is white in colour and has a metallic lustre. The streak is grey and the hardness is high at between 6 and 7. Its specific gravity is notable as being the highest of any mineral at 22.48. Chemistry, physical properties and x-ray methods are used for characterisation. Osmium is found in ultramafic rocks and placers. It is
associated with many species like rutheniridosmine, bowieite, platinum, laurite, isoferroplatinum, cuprorhodsite, malanite, cuproiridsite, erlichmanite, cooperite, sperrylite, chalcopyrite and bornite.
This mineral is found worldwide at numerous locations. The main sites occur in Australia, Canada, China, Japan, Papua New Guinea, Norway, Russia and the USA Erlichmanite, OsS2, is a cubic mineral that develops pyritohedral and octahedral crystals up to about 1 mm in length with faint striations. It also occurs as tiny grains.
The mineral is grey in colour with a metallic lustre. The hardness is variable from 4.5 to 5.5 and the specific gravity is 8.2. The above
properties, chemistry, x-rays and mode of occurrence help in identification. It is a typical placer mineral with platinum, laurite, hollingworthite, irarsite and chromite. About 22 countries in total have reported erlichmanite to date. Example occurrences are to be found in Brazil, Canada, China, Colombia, Greece, Madagascar, New Caledonia, Russia, South Africa and the USA.

Omeilite is a mineral of Cu-Ni sulphide deposits associated with ultramafic rocks. It is accompanied by pyrrhotite, pentlandite, chalcopyrite, violarite, cubanite, bornite, sphalerite, galena, testibiopalladite, sudburyite, sperrylite, kotulskite and other mineral species. Five single occurrences are currently known for this uncommon mineral. These are located in Brazil, China, Greece, Ukraine and the USA.

Ruthenium, Ru, is found in placer deposits associated with rutheniridosmine, platinum and other Os-Ir-Ru alloys and Pt-Fe alloys. It forms minute tabular grains that crystallises in the hexagonal system.
The colour is white with a light creamy tint. The hardness is 6.5 and the calculated specific gravity is very high at 12.4. Chemistry and x-rays are best for certain identification. Specimens have been obtained from the following countries: Australia, Bulgaria, Canada, China (several sites known), Columbia, Dominican Republic, Japan, Russia (several sites known), South Africa and the USA (several sites known).

Ruthenarsenite, (Ru,Ni)As, occurs as irregular grains embedded in the matrix of ruthenium alloys. In polished section it is pale-orange-brown to brownish grey in colour with a metallic lustre. The hardness is 6 to 6.5 and the calculate specific gravity is very high at 10.4. This species crystallises in the orthorhombic system. Chemistry and x-rays are required for characterisation.
Ruthenarsenite occurs as inclusions with various Os-Ir-Ru alloys and in Alpine-type ultramafics accompanied by rutheniridosmine, irarsite, iridarsenite and sperrylite. This uncommon mineral has been identified from a few occurrences in Papua New Guinea, China, Russia, South
Africa and the USA.

Ruarsite, RuAsS, is a monoclinic mineral that is found as irregular grains or as aggregates with roughly textured surfaces.
It is a lead-grey to dark grey in colour with a greyish black streak. The mineral is metallic in lustre and brittle. Its hardness varies from 6-7. The calculated specific gravity is high at 7.7 and is brittle and nonmagnetic.
X-ray and chemical methods are best for identification. It is a mineral of chromium ores of Alpine-type ultramafics and in related placer deposits. The following species have been reported with ruarsite: rutheniridosmine, osmium, sperrylite, laurite, iridarsenite, anduoite,
spinel, pyrite, lollingite, magnetite, chalcopyrite, molyddenite, galena and millerite. Eleven separate locations have supplied specimens of this rare mineral.
They are to be found in China, Austria, Finland, Spain, Greece, Brazil, Canada, Ukraine and in the USA.
We will next discuss the element chlorine and its varied minerals in this series.

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