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a group of minerals forming natural solid solutions of osmium and iridium. Two iridium and two osmium minerals are known: osmiridium (68–80 percent Ir) and nevyanskite (55–80 percent Ir), siserskite (55–80 percent Os), and native osmium (more than 80 percent Os). The principal admixtures, by weight, include Ru (up to 4–18.3 percent), Rh (up to 4.5–11.3 percent), Pt (up to 7.4–13.8 percent), and Au (up to 19.3 percent in osmiridium). Secondary admixtures include Fe (up to 1.6–2.6 percent), Cu (up to 1 percent), Ni (up to 0.4 percent), and Pd (up to 0.8 percent).
Osmiridium crystallizes in the isometric system and exhibits an iridium crystal lattice structure. Nevyanskite, siserskite, and native osmium crystallize in the hexagonal system and have an osmium crystal lattice structure. The minerals occur as irregular grains and crystals ranging in size from microns to mm, sometimes as concretions of grains and crystals. Nevyanskite forms the largest crystals (maximum weight 2–7 g).
Iridosmine minerals are opaque and nonmagnetic and have a metallic luster. Iridium minerals are white, and osmium varieties are dark gray. On Mohs’ scale, nevyanskite has a hardness of 5.7–7, siserskite 5.4–6.5, native osmium 5.1–5.2, and osmiridium 4.7–5.8. The minerals range in density from 17,600 to 22,400 kg/m3.
Iridosmine minerals are of endogenic origin (pneumatolytic-hydrothermal and magmatic processes). Native deposits occur only in peridotite serpentinites of foldbelts (for example, in the Central Urals and Tasmania). These minerals are also found in placers, where the iridosmine content reaches several hundred mg/m3. The minerals are extracted primarily from eluvialalluvial placers, where they often occur in association with native platinum and native gold (in the Urals, British Columbia, California, and on the islands of Borneo [Kalimantan] and New Guinea). They are also extracted from conglomerate deposits of metamorphic origin in the Witwatersrand of the Republic of South Africa.
The most widely distributed iridosmine mineral is nevyanskite, followed by siserskite; osmiridium and native osmium are very rare.
REFERENCESVernadskii, V. I. “Opyt opisatel’noi mineralogii.” Izbrannye sochineniia. vol. 2. Moscow, 1955.
Zviagintsev, O. E. Geokhimiia platiny. Leningrad, 1936.
L. V. RAZIN