Tellurium (Te)

Tellurium is one of the rarest stable elements in the Earth's crust — around one part per billion, comparable to platinum and scarcer than gold. What makes that striking is that it is not rare in the universe. Tellurium is produced in quantity by neutron capture in dying stars and is reasonably abundant in the solar system as a whole; the shortage is local.

The explanation lies in what tellurium did while the planets were forming. In the hot solar nebula it formed volatile hydrogen telluride, which was blown away with the gas rather than condensing into the dust that became rock. Selenium, immediately above it in the group, suffered the same fate less severely. The Earth is depleted in tellurium not because the universe is short of it but because the young Sun took most of ours away.

The gold ore nobody could read

In the 1780s the mines around Zlatna in Transylvania were producing an ore that behaved wrongly. It looked metallic, it assayed as containing gold, and yet it would not yield gold by any of the standard treatments. Miners called it aurum problematicum, the problematic gold.

Franz-Joseph Müller von Reichenstein, chief inspector of mines for the region, worked on it from 1782 and concluded it contained a new metal — but he could not persuade anyone. He sent a sample to Torbern Bergman in Sweden, who could not resolve it either, partly because the quantity was too small. The Hungarian botanist and chemist Paul Kitaibel independently reached the same conclusion from a different ore in 1789 without knowing of Müller's work.

Martin Heinrich Klaproth settled it in 1798, confirmed the element, named it tellurium after Latin tellus, the earth, and did something unusual for the period: he publicly credited Müller with the discovery rather than claiming it himself, and later acknowledged Kitaibel too. The naming and the credit therefore belong to different people, which is why the record is unusually clean for an eighteenth-century element.

Kalgoorlie built its footpaths out of it

Gold is famously unreactive, and in nature it is overwhelmingly found as the metal. The great exception is tellurium: calaverite, sylvanite, krennerite and petzite are gold tellurides, and they are the reason a rock can be extremely rich in gold and look like nothing at all.

This caused an expensive misunderstanding at Kalgoorlie in Western Australia in the 1890s. Prospectors working the Golden Mile recognised free gold and discarded the telluride-bearing material as waste. It was used to fill potholes, level yards and build up the tracks around the town. When the assays were finally understood in 1896, the streets were dug up and the rubble carted to the mill. Cripple Creek in Colorado has a nearly identical story, and both districts went on to become among the largest gold producers in the world.

Heavier than iodine, and standing in front of it

Tellurium presented Mendeleev with one of the most serious objections to his table. By atomic weight, tellurium is heavier than iodine. By chemistry, tellurium plainly belongs with selenium and sulfur, and iodine plainly belongs with bromine and chlorine, which requires tellurium to come first.

Mendeleev backed the chemistry and asserted that tellurium's accepted atomic weight must simply be wrong. It was not. Careful redeterminations confirmed the inversion, and it stood as an unresolved embarrassment for forty years.

The resolution came in two parts. Henry Moseley showed in 1913 that the ordering principle is nuclear charge, not weight, which made the question of relative weights irrelevant to position. Isotopes explained the weight itself: tellurium's mixture is dominated by its two heaviest isotopes, tellurium-128 and tellurium-130, which between them make up around two-thirds of the element, while iodine is entirely iodine-127. Three other pairs in the table are inverted the same way — argon and potassium, cobalt and nickel, thorium and protactinium — and the tellurium case is the one that mattered historically because it was the one Mendeleev had to defend.

Tellurium-128 carries a second distinction. It decays by double beta emission with a half-life of order 2 × 10^24 years, the longest ever determined for any nuclide — and nobody has ever watched it happen. No detector has registered a single tellurium-128 decay. A kilogram of the pure isotope would produce on the order of one event a year, at an energy where ordinary laboratory background produces vastly more, so counting is hopeless.

The number comes from geology instead. Tellurium minerals hundreds of millions of years old — melonite and altaite are the ones that have been used — have been quietly accumulating the xenon-128 the decay produces, and the half-life is read out of the ratio of trapped xenon to tellurium in the ore. Different ores and different laboratories give results spread across a factor of three, which is the honest cost of the method. The record for a half-life established by actually counting decays is about a hundred times shorter and belongs, fittingly, to a xenon isotope.

Solar panels, coolers and rewritable discs

For an element this scarce, tellurium has an unusually modern set of applications, and all of them exploit the semiconducting behaviour of its compounds.

• Cadmium telluride photovoltaics. This is now the largest single use. CdTe has close to the ideal band gap for converting sunlight, absorbs almost all incoming light within a couple of micrometres, and can be deposited quickly over large areas. It is the dominant thin-film solar technology and a substantial fraction of utility-scale generation in the United States. Its scaling limit is not physics but supply: there is only so much tellurium.
• Thermoelectrics. Bismuth telluride is the best thermoelectric material anywhere near room temperature and has been for decades. Every Peltier cooler — portable drinks fridges, CPU coolers, laser diode temperature stabilisers, seat coolers — is a stack of bismuth telluride couples. Lead telluride works at higher temperatures and powered radioisotope generators on earlier deep-space missions.
• Phase-change memory. Germanium-antimony-tellurium alloys switch between amorphous and crystalline states with different optical reflectivity and electrical resistance, and switch back. That is the recording mechanism in rewritable optical discs, and the storage mechanism in phase-change electronic memory.

Beyond electronics, small additions of tellurium make steel and copper cut more cleanly, and tellurium compounds vulcanise rubber and pigment glass and ceramics.

The garlic that lasts a month

Tellurium has one property that anyone who has worked with it remembers. The body methylates absorbed tellurium into dimethyl telluride, which is volatile and is excreted through the lungs and skin, producing a powerful garlic odour on the breath.

The threshold is extremely low — quantities far too small to cause any other effect are enough — and it persists for weeks after a single exposure, because the body clears tellurium slowly. It has been reported from amounts a laboratory would consider negligible, and it is the reason tellurium chemistry has a reputation among chemists out of all proportion to its actual hazard.

Recovered from copper, and not much else

Almost all tellurium comes from the anode slimes produced when copper is refined electrolytically, where it accumulates alongside selenium and the precious metals. A smaller quantity is recovered from lead refining and from bismuth production in China.

The consequence is that tellurium output is set by how much copper the world refines and by whether individual refineries have bothered to install recovery circuits — many have not, because the quantities are small and the processing is fiddly. Estimates of how much tellurium is simply discarded in slag run high. For a material underpinning a growing share of solar manufacturing, that is an uncomfortable position: the supply is not limited by geology so much as by who troubles to collect it.