40 Fun Facts About Iridium

A thin layer of iridium-rich clay found in rocks around the world helped explain the disappearance of which animals?

Dinosaurs died out 66 million years ago, just when that clay was laid down. Asteroids carry far more iridium than Earth's crust, so the layer pointed to a giant impact.

Iridium is named after Iris, a winged messenger of the Olympian gods and the Greek goddess of what?

The rainbow suited an element whose compounds turned a striking variety of colours as its discoverer dissolved them. The pure metal itself is a plain silvery-white.

A golf-ball-sized cylinder of platinum and iridium, locked in a vault near Paris, defined which unit until 2019?

The kilogram was simply whatever that one cylinder weighed, for 130 years. Three separate keys were needed to open its vault, and a definition built on constants of physics replaced it.

Many long-life versions of which small car engine part are sold with a fine tip of iridium?

Spark plugs lose a little metal with every spark, and iridium stands up to that erosion far better than most metals. The same toughness makes iridium plugs a favourite in aircraft engines.

A hard iridium tip, first fitted to gold in 1834, became the traditional wearing point of which writing instrument?

Fountain pen makers still call the tipping material 'iridium', though a modern nib seldom contains any. Cheaper hard metals such as tungsten have taken its place.

The Iridium satellite phone network was named because its planned 77 satellites matched which property of the element?

Atomic number 77 means 77 electrons circling a nucleus, and the planners pictured their satellites circling Earth the same way. Only 66 turned out to be needed, but the name stayed.

The asteroid that spread iridium worldwide left a buried crater under the Yucatán Peninsula of which country?

Mexico's crater takes its name from the nearby town of Chicxulub. Geophysicists hunting for oil first spotted it in the late 1970s, before anyone knew an impact site was missing.

Doctors place tiny sealed sources of radioactive iridium-192 inside the body to treat which disease?

Cancer cells are killed by the source's gamma rays, and putting it inside or right beside a tumour cuts the dose that reaches healthy tissue. The technique is called brachytherapy.

Which American company, maker of the first handheld mobile phone, bankrolled the Iridium satellite network?

Motorola turned out a new satellite every 4.3 days at the project's peak. The business still filed for bankruptcy in 1999, nine months after it opened, for want of subscribers.

Iridium is about twice as dense as which heavy metal, long used for fishing weights and roofing?

Lead feels heavy, yet a block of iridium the same size weighs about twice as much. Only one natural element is denser still, and by barely a tenth of one percent.

Iridium is the second-densest natural element, narrowly behind which metal found in the very same laboratory residue?

Osmium edges ahead by about a tenth of one percent, a gap so slim that chemists long argued over which of the pair was denser. X-ray measurements of their crystals settled it.

Which English chemist discovered iridium in 1803, after helping to prove diamond and charcoal are one substance?

Smithson Tennant had far more of the mysterious black residue to work on than the French chemists who had also noticed it. That let him pull two new elements out of it.

Iridium was found in the black sludge left when crude platinum dissolved in which gold-dissolving acid mixture?

Aqua regia, a blend of nitric and hydrochloric acids, eats gold and platinum yet cannot touch iridium. Earlier chemists had written off the leftover powder as graphite.

Which two-letter symbol marks iridium on the periodic table, where it sits just before platinum and gold?

Ir occupies the 77th place, in the sixth row, among the dense, unreactive metals that chemists group with platinum.

Iridium outranks every other metal for which property, the reason it is used in deep-water pipes?

Corrosion resistance is iridium's great strength: acids do not attack it, not even the mixture that dissolves gold. It holds out at temperatures as high as 2,000 °C.

Which Nobel Prize-winning physicist led the team that in 1980 traced Earth's iridium-rich clay to an asteroid strike?

Luis Alvarez took up the puzzle with his geologist son Walter and two nuclear chemists. His Nobel had nothing to do with rocks: it was for work on subatomic particles.

The iridium-rich clay behind the asteroid theory was discovered in a gorge outside Gubbio, a town in which country?

Italy's Bottaccione Gorge exposes an unbroken stack of ancient limestone, laid down layer by layer. An American geologist working there in the 1970s found one band unusually rich in iridium.

Iridium is 'siderophile', meaning it bonds readily with which metal and sank with it deep into the molten young Earth?

Iron dragged iridium down towards the planet's centre while Earth was still molten, which is why so little is left in the crust. Many space rocks still hold their full share.

Like gold, iridium is thought to be forged mainly in collisions between which ultra-dense stellar remnants?

Neutron stars that merge fling out a storm of loose neutrons, which nuclei soak up faster than they can decay. That rapid capture builds elements far heavier than an ordinary sun can fuse.

Iridium catalysts drive the Cativa process, which makes the acid that gives which kitchen staple its sour tang?

Vinegar is dilute acetic acid, a chemical that industry makes by the million tonnes for glues, films and fabrics. The iridium route largely took over from an older one built on rhodium.

Iridium compounds give off the light in which display technology, found in premium smartphones and televisions?

OLED pixels shine when electricity excites their organic molecules. Molecules built around a heavy metal such as iridium can reach an internal efficiency of nearly 100 percent.

Which country owned the dead satellite that destroyed an Iridium craft in 2009, the first crash between two satellites?

Russia's Kosmos 2251, a military satellite out of service since 1995, hit Iridium 33 above Siberia at about 26,000 mph. The crash left more than 2,000 large fragments in orbit.

Which rocket company launched 75 second-generation Iridium NEXT satellites between 2017 and 2019?

SpaceX won the job in 2010 with what was then the largest commercial launch deal ever signed. Every one of the new satellites rode a Falcon 9 rocket from the California coast.

The largest known reserves of iridium lie in the Bushveld Igneous Complex, a vast rock formation in which country?

South Africa's Bushveld holds the world's largest reserves of all six platinum-group metals. Iridium is only ever a minor by-product, with just a few tonnes produced worldwide each year.

Which radioactive element, the fuel that powers space probes such as Cassini, is sealed inside iridium capsules?

Plutonium pellets give off steady heat, which a probe far from the Sun turns into electricity. The iridium-clad fuel modules are built to survive a launch-pad explosion or a high-speed re-entry.

Which NASA space telescope, launched from shuttle Columbia in 1999, focuses X-rays with mirrors coated in iridium?

Chandra's mirrors are nested like barrels, because X-rays would simply be absorbed by a normal mirror. The rays reflect only when they graze a smooth surface at a very low angle.

Iridium crucibles survive the 2,000 °C melt used to grow artificial crystals of which gemstone for lasers?

Sapphire is grown by dipping a seed crystal into the molten oxide and drawing it slowly upward as it turns. Iridium's very high melting point lets the crucible hold that melt.

Iridium-coated electrodes endure the electrolysis of brine, the main industrial source of which yellow-green gas?

Chlorine is freed at an anode that has to survive both the brine and the gas itself. A thin coating of iridium oxide on titanium carries the current and speeds up the reaction.

Pipeline inspectors use gamma rays from iridium-192 like an X-ray machine to check which metal joints for hidden flaws?

Welds are checked by placing the source on one side of the joint and film on the other, so that hidden cracks and gaps show up in the image. Most industrial radiography is of this kind.

The platinum-iridium bar that defined the metre from 1889 gave way in 1960 to the light of which gas?

Krypton gave science a metre that any laboratory could reproduce, by counting wavelengths of the gas's orange light. No one needed to compare rulers with a metal bar in France any more.

Critics of the asteroid theory argued that Earth's iridium layer came from which source, such as India's Deccan Traps?

Volcanic eruptions do bring iridium up from deep inside the planet, and a volcano on the island of Réunion still releases it today. A 2010 review by 41 scientists still favoured the impact.

Earth's iridium-rich clay marks the K–Pg boundary; K stands for which geological period, the last of the Mesozoic Era?

Cretaceous takes its K from Kreide, the German word for chalk, after the chalk beds laid down late in the period. Pg stands for the Paleogene that followed.

Iridium shares group 9 of the periodic table with rhodium and which lighter metal, known for a deep blue pigment?

Cobalt heads the column, with rhodium beneath it and iridium third. A fourth member, meitnerium, lies below them, but only minute quantities of it have ever been made in laboratories.

Iridium holds the record for the highest value of which chemical measure ever found in any element, at +9?

Oxidation state counts the electrons an atom has, in effect, handed over to its neighbours. Iridium's +9 has been seen only in a gaseous ion, while its common compounds sit between +1 and +4.

Iridium's discoverer was killed in France in 1815 when what gave way beneath him as he rode across it?

A bridge collapsed under the chemist near Boulogne, two years after he was made a professor at Cambridge. He had lived to give only one course of lectures there.

Wires of iridium alloyed with ruthenium have been used since 1933 in which devices for measuring very high temperatures?

Thermocouples of this alloy can read temperatures up to 2,000 °C in open air. They work by producing a tiny voltage at the junction of two unlike wires.

John George Children first melted iridium in 1813 with what was then the world's largest example of which device?

A battery described as the greatest ever constructed gave Children enough current to melt a small sample. Nearly fifty years passed before anyone melted iridium in quantity.

Finely divided iridium makes 'iridium black', an intense pigment traditionally used for painting which material?

Porcelain painted with it turned so dark that, in one old account, all other black colours looked grey beside it. The pigment is nothing more than the metal as a very fine powder.

Which German physicist won the 1961 Nobel Prize for a gamma-ray effect he discovered in a sample of iridium?

Rudolf Mössbauer found the effect during his doctoral research and was just 32 when the prize came. It is now a routine tool for studying minerals and metals.

Iridium-clad pellets of radioactive fuel power which NASA Mars rover, which landed in Gale Crater in 2012?

Curiosity takes both its heat and its electricity from a single generator rather than from solar panels. The iridium cladding blocks the fuel's alpha particles, so no further shielding is needed.