by U.S. Geological Survey Wednesday, January 28, 2015
David E. Guberman, a mineral commodity specialist for the U.S. Geological Survey, compiled the following information about thallium, an extremely toxic metal used in a limited number of commercial applications.
Thallium, a grayish-white metal similar to tin in appearance, was discovered spectroscopically in 1861. Like lead, it is heavy yet soft, and can be cut easily with a knife. When exposed to air, thallium’s luster quickly tarnishes to a blue-gray color owing to the formation of a film of thallium oxide. Its concentration in Earth’s crust is estimated at 0.7 parts per million; mostly it is found in association with potassium minerals in clays, soils and granites, but in general it is not commercially recoverable from those sources. Manganese nodules, found on the ocean floor, also contain thallium.
Commercially, thallium is recovered as a byproduct in flue dusts, along with other metals such as cadmium and germanium, during smelting of copper, lead, zinc and other sulfide ores, in which these metals occur in trace amounts. Thallium metal has not been produced in the United States since 1981, when a sulfide smelter in Colorado stopped recovering it from flue dusts. According to industry sources, China and Kazakhstan are leading producers of thallium metal. In 2011, a Brazilian minerals exploration company discovered a substantial thallium-rich deposit in northwest Bahia, Brazil. According to the company, the deposit is unique because it is the only known occurrence of thallium with cobalt and manganese. The feasibility of economic production of thallium from the site has not yet been confirmed.
Thallium metal is sold in various shapes and with purities ranging from 99.9 to 99.999 percent. Prices have recently increased substantially — at least three-fold from 2005 to 2011 — owing to relatively limited world supply. It alloys well with other metals to form materials that feature desirable properties, such as low coefficients of friction and acid resistance. Thallium alloys are used in bearings, contact points and solders, and an alloy of thallium and mercury is used in switches and seals designed for low-temperature applications in the polar regions and in space. Thallium compounds are used in glass lenses for digital cameras, prisms, windows for infrared detection equipment, and in repeaters used in fiber-optic networks. In recent years, thallium compounds have also been used in the development of high-temperature superconductive films, tapes and wires for magnetic energy storage, imaging and propulsion applications. Thallium is used as a dopant to increase the performance of thermoelectric materials that convert heat to electricity, and thallium isotopes are used in cardiovascular imaging.
Exposure to minute amounts of thallium can be extremely harmful for living organisms. Ingestion of as little as 800 milligrams of thallium is enough to kill a human being. Consequently, disposal of thallium-bearing wastes is controlled by federal, state and local regulations, and storage and transportation of thallium compounds are federally regulated. The leading sources of thallium released into the environment are coal-burning power plants and smelters of copper, lead and zinc ores. The major sources of thallium in drinking water are ore-processing sites and discharges from electronics, drug and glass factories.
For more information on thallium and other mineral resources, visit http://minerals.usgs.gov/minerals.
Prior to 1972, when its use as a pesticide was banned in the United States, thallium was used extensively as a rodenticide and ant killer.
Before its human toxicity was known, thallium was used medically for the treatment of ailments such as malaria, ringworm in the scalp, tuberculosis and typhus.
Thallium has in recent years become an agent of murder in crime dramas, where it is used to poison victims. The only known antidote is Prussian blue (iron hexacyanoferrate), a dark blue pigment, which, when ingested, absorbs certain heavy metals.
In 2011, U.S. apparent consumption of thallium compounds was estimated to be slightly less than 1 metric ton.
China, Japan and the Republic of Korea were the leading consumers of thallium in 2011.
World thallium metal production was estimated to be 10 metric tons in 2011. China and Kazakhstan were the leading producers.
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