Scandium

• 1. Introduction

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  Scandium sits at atomic number 21 on the periodic table, carrying the symbol Sc and a reputation that defies its abundance. It is not actually rare in the Earth's crust. Estimates place it at 18 to 25 parts per million, putting it in roughly the same league as cobalt. Yet the global trade in scandium oxide amounts to only 15 to 20 tonnes per year. That gap between what exists and what reaches the world raises a question worth sitting with: what is it about this element that has kept it so stubbornly out of reach?

  The story starts in Scandinavia in 1879, with a scientist named Lars Fredrik Nilson and two minerals pulled from the ground. It runs through a Cold War arms race, a US patent filed in 1971, a set of Soviet fighter jets, and a gun company in Massachusetts. It touches dental drills, oil refineries, and the Strategic Defense Initiative. Scandium is a silvery-white metal that burns with a brilliant yellow flame. Its price per gram rivals that of precious metals. And almost everything made with it could have been made with something cheaper, if only someone could find enough of it.

• 2. Predicted and Found

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  Dmitri Mendeleev, who is referred to as the father of the periodic table, predicted in 1869 that an undiscovered element he called ekaboron was waiting to be found, with an atomic mass somewhere between 40 and 48. Lars Fredrik Nilson was not looking for Mendeleev's missing piece when he went to work on the minerals euxenite and gadolinite from Scandinavia. He was simply doing spectral analysis. In 1879, he detected the new element hiding in those minerals and managed to prepare 2 grams of scandium oxide of high purity.

  Nilson named the element scandium, drawing on the Latin word Scandia for Scandinavia. He had apparently no idea that Mendeleev had described it a decade earlier. It was Per Teodor Cleve who noticed the correspondence between Nilson's discovery and Mendeleev's prediction, and who sent word to Mendeleev directly. The fact that a metal predicted from the logic of a table could be found precisely where it was supposed to be was one of the early vindications of the periodic law itself.

  Producing the metal in pure form took much longer. Metallic scandium was first isolated in 1937, by electrolysis of a eutectic mixture of potassium, lithium, and scandium chlorides at temperatures between 700 and 800 degrees Celsius. It took until 1960 before anyone produced the first pound of scandium metal at 99% purity.

• 3. Scarce Despite Abundance

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  Scandium occupies a paradoxical position in the periodic table of the Earth. It ranks 50th among the most common elements on the planet but climbs to 23rd in the Sun and 26th in the stars. In our own crust, the problem is not total quantity but distribution. Scandium is spread thinly across many minerals rather than concentrated in a few rich deposits.

  The minerals thortveitite, euxenite, and gadolinite, found in Scandinavia and Madagascar, are the only known concentrated sources. Thortveitite can hold up to 45% scandium oxide, which sounds promising until one considers how rarely it appears and in what small deposits. Madagascar and the Iveland-Evje region of Norway hold the only known deposits with high scandium content, but as of recent reporting, neither is being exploited commercially.

  In 2003, only three mines in the world were actually producing scandium: uranium and iron mines in Zhovti Vody in Ukraine, rare-earth mines in Bayan Obo in China, and apatite mines in Russia's Kola Peninsula. In every case, scandium was not the target. It came out as a byproduct of extracting other elements and was then sold as scandium oxide. That byproduct status has never translated into reliable supply. The absence of secure, stable, long-term production has been the single largest brake on commercial development. NioCorp Development in the United States has proposed a niobium mine at an Elk Creek site in southeast Nebraska that could potentially yield as much as 95 tonnes of scandium oxide annually, backed by a plan to raise one billion dollars, but this remains a proposal.

• 4. What the Metal Actually Does

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  Scandium's one major industrial application, by weight, is in aluminium alloys. Those alloys contain between 0.1% and 0.5% scandium. The effect of even that small fraction is notable: it limits grain growth in the heat zone of welded aluminium components, produces smaller crystals than those found in other aluminium alloys, and reduces the volume of precipitate-free zones at the grain boundaries. The resulting alloy is described as strong as titanium, light as aluminium, and hard as some ceramics.

  Titanium alloys with similar properties exist, and they are cheaper and much more widely used. Still, scandium-aluminium alloys found a home in Russian military aircraft, specifically the Mikoyan-Gurevich MiG-21 and MiG-29. The aerospace connection eventually extended to sports equipment: baseball bats, tent poles, bicycle frames, and lacrosse sticks have all been made with scandium-aluminium alloys. Smith and Wesson, the American firearm manufacturing company, produces semi-automatic pistols and revolvers with frames of scandium alloy and cylinders of titanium or carbon steel.

  Since 2013, Apworks GmbH, a spin-off of Airbus, has marketed a scandium-containing aluminium alloy processed through metal 3D printing under the trademark Scalmalloy. The alloy was developed for use in laser powder bed fusion processes and is promoted for its strength and ductility. Production of aluminium-scandium alloys had begun back in 1971 following a US patent, and similar alloys were developed independently in the USSR.

• 5. Light, Medicine, and Fuel

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  Beyond alloys, scandium appears in a handful of specialized applications that are easy to overlook but quietly significant. The first scandium-based metal-halide lamps were patented by General Electric. One type is made from scandium triiodide and sodium iodide, producing a white-light source with a high color rendering index that closely resembles sunlight. Television cameras can reproduce color well under these lamps. About 80 kilograms of scandium is used globally per year in metal-halide lamps and light bulbs, while the United States alone accounts for roughly 20 kilograms annually in high-intensity discharge lamps.

  Dentistry uses erbium-chromium-doped yttrium-scandium-gallium garnet lasers for cavity preparation and for work in endodontics. Oil refineries use the radioactive isotope 46Sc as a tracing agent. Scandium triflate serves as a Lewis acid catalyst in organic chemistry. In the 1980s and 1990s, laser crystals of gadolinium-scandium-gallium garnet were used in strategic defense applications developed under the Strategic Defense Initiative.

  Scandium has also been proposed for use in solid oxide fuel cells as a dopant in zirconia electrolytes, where it can improve ionic conductivity, thermal stability, and efficiency. Scandium-stabilized zirconia already faces growing market demand in this area. The 12.4 keV nuclear transition of 45Sc has been studied as a reference for timekeeping, with a theoretical precision suggested to be as much as three orders of magnitude better than the current caesium reference clocks used as the global standard.

• 6. Chemistry and Character

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  Scandium's chemistry is almost completely controlled by one ion: Sc3+, the trivalent form. This makes it simpler than many of its neighbors on the periodic table, but it also places it in interesting company. The ionic radius of Sc3+ sits at 74.5 picometres, which is closer to the yttrium ion at 90.0 than to the aluminium ion at 53.5. That proximity to yttrium is one reason scandium is often grouped with the lanthanides rather than treated as a purely transitional metal.

  A diagonal relationship exists between scandium and magnesium in terms of chemical behavior, mirroring the better-known relationship between beryllium and aluminium. Scandium oxide and scandium hydroxide are both amphoteric, meaning they can act as either acids or bases depending on conditions. The halides of scandium are very soluble in water, with one exception: scandium fluoride is insoluble. In contact with air, the metal develops a slightly yellowish or pinkish cast from oxidation. Scandium turnings ignite in air with a brilliant yellow flame to form scandium oxide.

  Scandium does not dissolve in a 1:1 mixture of nitric acid and 48% hydrofluoric acid, which is thought to result from the formation of an impermeable passive layer. Compounds in which scandium takes oxidation states other than +3 are rare but documented, including a blue-black compound that adopts a sheet-like structure with extensive bonding between scandium(II) centers. Scandium borides and carbides are non-stoichiometric, a trait shared with neighboring elements on the table.

• 7. Price, Toxicity, and Prospects

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  Between 2015 and 2019, the US Geological Survey recorded the price of small quantities of scandium ingot at between $107 and $134 per gram. Scandium oxide, the form in which it is most commonly traded, ran $4 to $5 per gram over the same period. Both figures reflect a market constrained more by supply than by demand: world production stays in the range of 15 to 20 tonnes of scandium oxide per year, while demand runs slightly higher and both have been growing.

  Elemental scandium is considered non-toxic. The median lethal dose of scandium chloride in rats was determined at 755 milligrams per kilogram for intraperitoneal administration and 4 grams per kilogram orally. Extensive animal testing of scandium compounds has not been carried out, so the general guidance is to handle them as compounds of moderate toxicity. The body appears to process scandium similarly to gallium, with similar concerns around its poorly soluble hydroxide.

  Scandium was born in a supernova. The stable isotope 45Sc is created through the r-process inside those stellar explosions and also through cosmic ray spallation of iron-peak nuclei. An element forged in the death of stars, predicted by a Russian chemist in 1869, named for Scandinavia by a Swedish scientist in 1879, and still waiting for the production infrastructure that would let it be used at scale, scandium sits at a peculiar frontier where the physics is well understood and the economics remain unresolved.