Osmium

Osmium has a blue-gray tint and is the densest stable element; it is approximately twice as dense as lead and slightly denser than iridium. Calculations of density from the X-ray diffraction data may produce the most reliable data for these elements, giving a value of 22.587±0.009 g/cm for osmium, slightly denser than the 22.562±0.009 g/cm of iridium; both metals are nearly 23 times as dense as water, and ​1 ⁄6 times as dense as gold. Osmium is a hard but brittle metal that remains lustrous even at high temperatures. It has a very low compressibility. Correspondingly, its bulk modulus is extremely high, reported between 395 and 462 GPa, which rivals that of diamond (443 GPa). The hardness of osmium is moderately high at 4 GPa. Because of its hardness, brittleness, low vapor pressure (the lowest of the platinum-group metals), and very high melting point (the third highest of all elements, after only tungsten, and rhenium), solid osmium is difficult to machine, form, or work.

Osmium is the least abundant stable element in Earth's crust, with an average mass fraction of 50 parts per trillion in the continental crust. Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridosmium (iridium rich). In nickel and copper deposits, the platinum group metals occur as sulfides (i.e., (Pt,Pd)S)), tellurides (e.g., PtBiTe), antimonides (e.g., PdSb), and arsenides (e.g., PtAs2); in all these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, osmium can be found naturally in alloys with nickel or copper. Within Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld Igneous Complex in South Africa, though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of osmium. Smaller reserves can be found in the United States. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum group metals. The second large alluvial deposit was found in the Ural Mountains, Russia, which is still mined.