Osmiridium and iridosmine are natural alloys of the elements

osmium Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...

and

iridium Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of ...

, with traces of other platinum-group metals. Osmiridium has been defined as containing a higher proportion of iridium, with iridosmine containing more osmium. However, as the content of the natural Os-Ir alloys varies considerably, the constituent percentages of specimens often reflects the reverse situation of osmiridium describing specimens containing a higher proportion of osmium and iridosmine specimens containing more iridium.

Nomenclature

In 1963, M. H. Hey proposed using iridosmine for hexagonal specimens with 32% < Os < 80%, osmiridium for cubic specimens with Os < 32% and native osmium for specimens Os > 80% (the would-be mineral native iridium of >80% purity was not known at that time). In 1973, Harris and Cabri defined the following names for Os-Ir-Ru alloys: ruthenosmiridium was applied to cubic Os-Ir-Ru alloys, where Ir < 80% of (Os+Ir+Ru) and Ru > 10% of (Os+Ir+Ru) with no single other element >10% of the total; Rutheniridosmine was applied to cubic Os-Ir-Ru alloys, where Os < 80% of (Os+Ir+Ru) and Ru is 10–80% of (Os+Ir+Ru) with no single other element >10% of the total; the Ru-Os alloys be known as ruthenian osmium (>50% Os), osmian ruthenium (>50% Ru); the Ru-Ir alloys be known as iridian ruthenium and ruthenian iridium where the boundary between them is defined by the alloy's miscibility gap (a minimum 57% Ir for ruthenian iridium and a minimum of 55% Ru for iridian ruthenium). The nomenclature of Os-Ir-Ru alloys were revised again by Harris and Cabri in 1991. Afterwards, only four names were applied to minerals whose compositions lie within the ternary Os-Ir-Ru system: osmium (native osmium) for all hexagonal alloys with Os the major element; iridium (native iridium) for all cubic alloys with iridium the major element; rutheniridosmine for all hexagonal alloys with Ir the major element; and ruthenium (native ruthenium) for all hexagonal alloys with Ru the major element. The mineral names iridosmine, osmiridium, rutheniridosmium, ruthenian osmium, osmian ruthenium, ruthenium iridium and iridian ruthenium were proposed to be retired.

Related compounds

Natural alloys also occur in the systems Ir-Os-Rh, Os-Ir-Pt, Ru-Ir-Pt, Ir-Ru-Rh and Pd-Ir-Pt. Names used in these systems have included platiniridium (or platinian iridium) and iridrhodruthenium. However, there is no universally accepted method of plotting these compositions and their names, especially in the ternary systems.

Properties

The properties of all these alloys generally fall between those of the members, but hardness is greater than the individual constituents.

Occurrence

Os-Ir alloys are very rare, but can be found in

mine Mine, mines, miners or mining may refer to: Extraction or digging * Miner, a person engaged in mining or digging *Mining, extraction of mineral resources from the ground through a mine Grammar *Mine, a first-person English possessive pronoun ...

s of other platinum-group metals. One very productive mine was operated at Adamsfield near Tyenna in

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

during the

Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

with the ore shipped out by railway from

Maydena Maydena is a locality in Tasmania, Australia, alongside the River Tyenna. Maydena is on the Gordon River Road, south west of New Norfolk, through the Bushy Park Hop Fields, turn left at Westerway, past Mount Field National Park and Russell Fa ...

. The site of the mine is now totally reclaimed by dense natural bush. It was once one of the world's major producers of this rare metal, and the osmiridium was mostly found in shallow alluvial workings.Bottrill, R. S.; 1989, Economic Geology Appendix A; Tasmanian Geol. Surv. Explanatory Rept. 73 ( Huntley.) Cited a
Adamsfield, Tasmania, Australia
Mindat.org. Accessed May 5, 2010. The alloy is currently valued at about US$400 per

troy ounce Troy weight is a system of units of mass that originated in 15th-century England, and is primarily used in the precious metals industry. The troy weight units are the grain, the pennyweight (24 grains), the troy ounce (20 pennyweights), and the ...

Isolation

It can be isolated by adding a piece to aqua regia, which has the ability to dissolve

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

and

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...

but not osmiridium. It occurs naturally as small, extremely hard, flat metallic grains with hexagonal crystal structure.

References

Osmiridium
��Mindat.org {{Osmium compounds Iridium minerals Native element minerals Osmium compounds Precious metal alloys Superhard materials