IRIDIUM

Name: Iridium (from the Greek goddess of the rainbow, "Iris," due to the strong hue of its salts)

Symbol: Ir

Group: 9

Period: 6

Block: d

Category: Transition metals

Atomic number: 77

Atomic mass: 192.217 u

Electrons per shell: 2, 8, 18, 32, 15, 2

Electronegativity: 2.2

Density: 22.56 g/cc

Melting point: 2466ºC

Boiling point: 4428ºC

Thermal conductivity: 150 W (m·K)

Electrical conductivity: 2.1 × 10^7 S/m

Magnetic order: Paramagnetic

Ordinary state: Solid

States of Oxidation: +3, +4

Mohs Hardness: 6.5

Vickers Hardness: 1760 MPa

Brinell Hardness: 1670 MPa

Most stable isotopes: Ir-191 (37.3%), Ir-193 (62.7%)

Discoverer: Smithson Tennant, English (1803)

The story of iridium is intertwined with that of other equally rare and noble metals, such as osmium and, more tangentially, rhodium. At the beginning of the 19th century, Smithson Tennant, working closely with William Hyde Wollaston, undertook the study of native platinum-rich alloys, probably from South America. His initial objective was to separate and isolate the individual components in these samples, something that had already allowed Wollaston to discover palladium and rhodium. In the case of iridium and osmium, the work was joint: Tennant focused on the insoluble residues left after subjecting the original alloy to reagents capable of dissolving platinum, palladium, and other less resistant metals.

What remained was a solid, hard, and stubborn portion that refused to yield to any known acid or corrosive mixture, including the dreaded aqua regia. This resistance, which even today remains one of iridium's most distinctive features, made it evident that it was a distinct metal, extremely noble, and virtually impenetrable to the chemistry of the time. Tennant proceeded to refine it, thus obtaining the metal in its elemental state. During this process, he also identified iridium compounds (salts) that displayed a surprising range of vivid and intense colors, from deep yellows to brilliant greens and blues. It was precisely this chromatic diversity that inspired its name: “iridium,” derived from Iris, the Greek goddess messenger of the gods and personification of the rainbow.

The discovery not only enriched the platinum group metals with a member of unparalleled chemical resistance but also opened the door to research into alloys with extraordinary properties. However, like other metals in its family, iridium would remain for decades more of a scientific curiosity than an industrial resource of wide use, reserved for applications as rare as itself.

Iridium is, without exaggeration, one of the most unique and demanding metals on the periodic table. Silvery white with a slight golden hue, it is distinguished by a deep, clean luster that remains unchanged over time, immune to the corrosive agents that ruin the luster of other metals. It is located in the platinum group, with atomic number 77, and belongs to the third series of transition metals. It is extraordinarily rare in the Earth's crust, although it is relatively more abundant in metallic meteorites composed of iron and nickel. This fact, far from being a minor curiosity, has had notable scientific implications: iridium, present in high concentrations in these meteorites, has become a key tool for estimating the age of the planet and for identifying geological events of extraterrestrial origin, such as the famous impact that marked the end of the age of the dinosaurs.

Its hardness is remarkable, reaching 7.0 on the Mohs scale, and it combines with a corrosion resistance that has historically been considered the highest among all metals. However, this same hardness coexists with a brittle nature: iridium is not malleable in the traditional sense, and under excessive blows or pressures, it can fracture rather than deform. This places it in a mechanical range similar to other extremely robust metals such as tungsten, rhenium, and osmium. Its density, only slightly surpassed by that of osmium, gives it a surprising weight for its size, and its face-centered cubic crystal structure remains stable at any temperature. Curiously, despite being a noble metal, it is a poor conductor of both heat and electricity, which differentiates it from other precious metals like silver or copper.

Economically, iridium has had moments when its value exceeded that of gold and platinum, although its price fluctuates irregularly due to its limited demand and the difficulty of obtaining it. It is not a frequent protagonist in jewelry, and when it appears, it usually does so as an alloying agent in high-end platinum pieces or as a coating to impart a noble and resistant finish to other metals. Working it in its pure state is practically impossible due to its hardness and brittleness, and the manufacture of solid iridium pieces is reserved for very specific and expensive applications.

In the technical field, its ability to maintain its mechanical properties even at high temperatures makes it ideal for elite components, from long-life electrical contacts to critical parts of engines and scientific devices. However, it does not alloy with metals such as copper, silver, or gold, and its chemical affinity leads it to combine better with high-melting-point metals, particularly refractories and other members of the platinum group. As a siderophile metal, it exhibits great solubility in liquid iron, which explains why most of its natural reserve is found in the Earth's core, probably in greater quantities than gold or silver, although far beyond our practical reach.

Iridium is distinguished in the field of chemistry by its extraordinary inertness, making it one of the most corrosion-resistant metals. This noble metal remains virtually unaffected by the most powerful acids, capable of dissolving metals such as gold and platinum or penetrating the protective oxide layer on self-passivating metals such as titanium or tantalum. Unlike these, iridium's resistance does not decrease with increasing temperature, a trait that distinguishes it even from champions of chemical resistance such as tantalum, whose capacity is lost above 150°C, a temperature barely above the boiling point of water (100°C). This thermal stability makes iridium an ideal material for industrial applications beyond jewelry, where its high cost as a noble metal is justified by its exceptional durability.

At room temperature, iridium is virtually invulnerable, although no substance is completely immune to all chemical agents. In its case, certain alkaline solutions, especially in molten state or when applied as salts under heat conditions, can attack it. This phenomenon, common in metallurgy, occurs because increased temperature excites atoms, facilitating chemical reactions with external elements. However, iridium's ability to resist corrosion in a wide range of extreme environments positions it as a material of choice in industrial and scientific sectors, where its chemical nobility guarantees unparalleled performance. 

Iridium has gone through moments of discreet glory and long periods of technical exclusivity. While today its name may be associated with fountain pen nibs, those ancient allies of elegant writing, its use goes far beyond luxury stationery. This metal, more prestigious than osmium or rhenium but not reaching the symbolic reign of platinum, has a limited field of action by its very nature: its hardness and fragility make it unsuitable for many everyday applications, but perfect for those where extreme durability is an absolute requirement.

In jewelry, its role is usually that of a discreet but effective partner. As an alloying agent in platinum pieces, it increases hardness and toughness without sacrificing luster or corrosion resistance. White gold, however, is hostile to it: both metals are chemically incompatible, and any attempt to combine them yields mediocre results. Where it can appear is as a coating, especially on white gold or sterling silver, although in this field rhodium is often preferred for its colder and more reflective luster, despite the latter sometimes being even more expensive.

In industry, its use is restricted to contexts where cost is not an obstacle and where its unique qualities justify the investment. In the chemical sector, for example, it is used in crucibles and utensils that must withstand extreme corrosive conditions. It also acts as a dopant in superalloys of nickel, cobalt, or combinations of both, improving mechanical and thermal properties without the risk of carbide formation, a common problem in other hard alloys that lose strength with heat. The military has used it in critical components such as missile warheads, and NASA uses it in parts designed to withstand extreme stresses, both thermal and mechanical, where failure is not an option.

Its fame in fine writing originated in the mid-20th century when ink pen nibs were made with iridium due to its superior hardness to steel and its wear resistance. The idea was simple: a good pen should last years, even decades, and iridium ensured that the nib would withstand thousands of strokes without deforming. Today, ballpoint pens dominate the market, but mechanical pens survive as symbols of status and tradition, and it is still possible to find high-end pieces with iridium nibs.

Outside these contexts, iridium is an exotic metal that lives more in the realm of curiosity than in daily life. It occasionally appears natively on the Earth's surface, almost always accompanied by osmium, ruthenium, or platinum. It competes with osmium for the title of the densest element — a technical tie in practice — and is considered “extraterrestrial” due to its relative abundance in metallic meteorites. Its resistance is legendary, but its brittleness, paradoxically, prevents it from being malleable or ductile like other metals with a face-centered cubic structure. Its industrial processing requires extreme techniques, such as sintering or the use of electron beam furnaces in an inert atmosphere, which further increases its already high price.

And if gold seems scarce, iridium redefines the word. Obtaining a single gram of this metal involves processing quantities of material that would make gold mining seem like a light pastime. Therefore, when it is found, whether in geological depths or in a meteorite, iridium does not go unnoticed: it is, without exaggeration, a rare and valuable guest in any scenario.