Thallium is a transitional metal element that was discovered twice in the same year, 1861. These simultaneous discoveries are not uncommon in science as one advancement builds upon another. In this case, the technique of flame spectroscopy had recently been established. See caesium.
So, in 1861, English chemist Willaim Crookes and French chemist Claude-Auguste Lamy separately detected evidence of a new element in residue from sulfuric acid production. Sulfuric acid was then derived from lead sulfide by use of selenium. Crookes, who made the first discovery, had received some selenium compounds. After removing the selenium, he examined the remainder with a spectroscope, expecting to see tellurium. Instead, he saw a brilliant and unfamiliar green line (right), the signature of a new element. He named it thallium, meaning "green twig", and eventually produced samples of the pure metal.
The second and independent discovery was by Lamy, from selenium compounds derived from pyrite after production of sulfuric acid. He observed the new green line in a spectroscope, then reduced what turned out to be thallium sulfide into an ingot of the actual metal. Lamy is often credited with the first isolation, while Crookes had first discovery.
Mining and Production
Thallium is not uncommon in the Earth's crust, but commercial extraction is usually a by-product of large-scale mining of sulfide ores. These include copper, lead, and zinc. The metal is also still produced from iron pyrite in the production of sulfuric acid. Production is usually not reported, but in 2018 was estimated to be less than 8 tonnes globally, primarily from China, Kazakhstan, and Russia.
Thallium compounds from any source are usually leached out and converted to thallium sulfate. This is subjected to electrolysis to plate the pure metal onto an inert conductor such as platinum.
Properties and Uses
Thallium as a metal behaves much like the adjacent lead. It is soft, conductive, and melts at a relatively low temperature. Since it reacts with oxygen, it will tarnish in air. Unlike lead, it is quite toxic as a metal or as a compound and should not be inhaled or touched. (See Agatha Christie's novel, The Pale Horse, in which the murderer's choice of weapon is thallium.)
Thallium has several uses in optics. The bromide and iodide are used in infrared lenses (right) and prisms. Thallium oxide increases the index of refraction of glass. Sodium iodide crystals in gamma ray detectors are doped with thallium. The element is also found in other special-purpose electronics.
Because of its toxicity, thallium sulfate was historically used as a rat poison, but this has been discontinued in many countries (right).