"Rock of the Month #177, posted for March 2016" ---

Native Osmium

from the Urals of Russia. Precious metals often occur as small grains and flakes in river gravels, so-called placer deposits. The placers are concentrates of heavy mineral grains, and osmium is the densest of all the elements and a platinum group mineral (PGM). The location is given as "Neiva River, Newjansk, Ekaterinburg" (Nizhnii Tagil, Sverdlovskaya oblast, Middle Urals). The Ural Mountains, parts of Siberia and Kamchatka are all noted placer PGE (platinum group element) districts.

The Mineral

The silvery flakes do not show a clear crystal habit, though the thin "tile" appears to preserve parts of four crystal faces. This is a good clue: osmium is hexagonal, and some crystals exhibit perfect hexagonal outlines. Osmium forms alloys with other PGE, especially iridium and ruthenium, as in the species iridosmine. The "asteroid" grain responds strongly to a powerful magnet, whereas the "tile" flake does not.

An electron microprobe or microscope examination is in order, to prove the identity as osmium, and to see what other elements or mineral species may be present. Iron may alloy with PGE, most particularly in the case of platinum itself: could this explain the difference in magnetic behaviour (?). The "tile" may be a corroded osmium crystal, whereas the "asteroid" may be a combination of associated metallic mineral species.

Native osmium (Cabri, 2002, pp.76-77) commonly forms thin hexagonal platelets, often exsolved within grains of Pt-Fe alloys, such as isoferroplatinum, Pt₃Fe. There is complete solid solution with ruthenium, and partial solid solution towards iridium. Some grains now defined as osmium are referred to as iridosmine in older publications. Weiser (2002) provides SEM images of fine examples of osmium crystals and platelets from Colombia and Burma.

A note on the Ural Placers

Placers of the Nizhnii Tagil dunite massif, located circa 115 km N.N.W. of Sverdlovsk, display varied mineralogy (Cabri and Genkin, 1991). The placer grains include isoferroplatinum with laths of osmium and grains of iridium, chromite, as well as laurite and other PGM species. The Ural Mountains are the surface expression of an eroded orogenic belt, with at least 15 major ultramafic complexes extending for about 900 km northwards from Ekaterinburg, at 60°E, 56-64°N (see map in Garuti et al., 2002a,b). These include tulameenite, a secondary phase formed during serpentinization of ultramafic massifs. With an ideal formula of Pt₂FeCu, it is usually found as a rim on primary Pt alloys. Associated minerals include laurite, tetraferroplatinum, isoferroplatinum and native osmium. Some of the PGM, including Pt-Fe alloys, found in the Urals placers may be derived from chromitites in the ultramafic complexes (Garuti et al., 2002a,b: see also Okrugin, 2011).

Worldwide Occurrence and Distribution

Native osmium and associated PGM are commonly derived from zoned, "Alaskan-type" mafic-ultramafic intrusions, or with sub-seafloor ophiolite complexes. Weiser (2002) reviews the kinds of PGM species found as heavy grains in placer deposits worldwide. Localities are documented worldwide, in such places as: the Choco district of Colombia; British Columbia, Yukon and Alberta, Canada; Alaska and California, U.S.A.; Ethiopia; Sierra Leone; Burma; South Kalimantan, Borneo, Indonesia; Papua New Guinea; Tasmania, Australia; and a number of localities across the vastness of Russia.

According to the informative summary by O'Neill and Gunning (1934, pp.115-122) placer platinum was discovered in the Urals in 1823, and production began the following year. These authors reported that nine ultramafic centres, pyroxenite and especially dunite, were of economic importance, giving rise to platiniferous placers. The olivine of the host rock weathers readily, and is seldom found in the placer grains, though chromite may be attached to the PGM. Native platinum associated with pyroxenite may be especially irregular in habit, and may have attached magnetite. The first Urals discovery may actually date to 1819 (Okrugin, 2011, p.1409), and, as mentioned above, there are at least 15 significant ultramafic complexes in the Middle and Northern Urals. In the early days of the Russian placer mining operations, a number of large nuggets (>1 kg) were recovered (e.g., Okrugin, 2011).

One of the other Russian occurrences may be noted here. The Guli placer deposit contains native osmium and iridosmine. The Guli intrusion, in the northern Siberian craton, may be the largest clinopyroxenite-dunite complex on Earth. It has an exposed area of about 600 km², including 380 km² of dunites. This may be the world's largest exposure of dunites: the total of all dunites in the Ural PGE belt is estimated at 100 km² (Lazarenkov et al., 1994).

Native osmium is comparatively rare: as of 01 November 2015, the MINLIB bibliographic database held 5,964 records on all PGE, of which 1,343 referred to osmium, including 510 to osmium isotopes (used in the rhenium-osmium geochronometer) and just 44 for native osmium, though a few more would doubtless be found under the older names of iridosmine / osmiridium. Osmium is composed of seven stable (or exceedingly long-lived) isotopes, ^{184, 186, 187, 188, 189, 190, 192}Os.

Osmium the Element: the Metal and its Uses

Osmium was discovered by Smithson Tennant in 1803 (Newton Friend, 1961). The silvery metal may be volatilised as the tetroxide, OsO₄, a fact which must be considered by analysts to this day. Explorers for PGE and related base metals commonly assay for platinum and palladium alone, as a cost-saving matter, reasoning that the other four, less-common PGE are unlikely to be present in significant quantities if Pt and Pd are not abundant. Commercial assay labs often add gold to offer a convenient 3-element, precious-metals "package".

Osmium is element 76, atomic mass 190.2, Pure osmium has a specific gravity of 22.59. This makes it the densest element known, though iridium is not far behind (Street and Alexander, 1998, p.208). Recalculated specific gravities of Os and Ir are 22.590 and 22.560 respectively, which confirmed Os as the densest metal (Arblaster, 1989). Osmium also is one of the most refractory elements, with a melting point of 2700°C. Despite this, it oxidizes readily, and thus may weather more readily than other PGE.

Osmium has been recovered in placer deposits in the Urals and elsewhere, as a heavy mineral in the Witwatersrand gold fields of South Africa, and in concentrates from a number of nickel-copper-PGE mines. The annual world production of osmium is so small, perhaps on the order of one tonne (about 30,000 ounces), that uses are necessarily limited (Pt and Pd, the most abundant PGE, are used extensively as catalysts in automotive exhaust systems and the chemical industry, as well as in jewellery and dental alloys). The metals business reports in detail on Pt and Pd, and sometimes Rh, rather less on Ir and especially Ru and Os.

Osmium has been used in speciality alloys of extreme hardness, in "iridium" tips for fine fountain pens, as an electroplated reflective coating for searchlights, and in light filaments (Newton Friend, 1961). Os-It-Pt alloys find use in some electrical contacts. Hard alloys may also be used in gramophone needles and in fine instrument pivots. Osmic acid is used as a tissue stain for electron microscopy (Simons, 1967, pp.128-130; Gray, 2009, pp.174-175).

References

Arblaster,JW (1989) Densities of osmium and iridium. Platinum Metals Rev. 33 no. 1, Johnson Matthey, 14-16.

Cabri,LJ (2002) The platinum-group minerals. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 13-129.

Cabri,LJ and Genkin,AD (1991) Re-examination of Pt alloys from lode and placer deposits, Urals. Can.Mineral. 29, 419-425.

Garuti,G, Pushkarev,EV and Zaccarini,F (2002a) Composition and paragenesis of Pt alloys from chromitites of the Uralian-Alaskan-type Kytlym and Uktus complexes, northern and central Urals, Russia. Can.Mineral. 40, 357-376.

Garuti,G, Pushkarev,EV and Zaccarini,F (2002b) Composition and paragenesis of Pt alloys from chromitites of the Uralian-Alaskan-type Kytlym and Uktus complexes, northern and central Urals, Russia. Can.Mineral. 40, 1127-1146.

Genth,FA (1853) On a probably new element with iridosmine and platinum, from California. Amer.J.Sci. ser.2, 15, 246-248.

Gray,T (2009) The Elements: A Visual Exploration of Every Known Atom in the Universe. Black Dog & Leventhal Publishers, Inc., New York, 240pp.

Lazarenkov,VG, Malich,KN and Lopatin,GG (1994) Geochemistry of the ultramafites of the Guli platiniferous intrusion, Maymecha-Kotuy province. Geochemistry International 31 no.6, 1-9.

Newton Friend,J (1961) Man and the Chemical Elements. Charles Scribner's Sons, New York, 2nd edition, 356pp.

O'Neill,JJ and Gunning,HC (1934) Platinum and Allied Metal Deposits of Canada. GSC Econ.Geol.Ser. 13, 165pp.

Okrugin,AV (2011) Origin of platinum-group minerals in mafic and ultramafic rocks: from dispersed elements to nuggets. Can.Mineral. 49, 1397-1412.

Simons,EN (1967) Guide to Uncommon Metals. Frederick Muller Limited, London, 244pp.

Street,A and Alexander,W (1998) Metals in the Service of Man. Penguin Books, 11th edition, 300pp.

Weiser,TW (2002) Platinum-group minerals (PGM) in placer deposits. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 721-756.