Background

A small miniature of deep red cubes of galkhaite (one of the very few thallium-containing minerals and a very complex sulfosalt) on matrix from the famous Getchell Mine, Humboldt County, Nevada.. (Source: Carnegie Mellon University)

Thallium is a soft, bluish-white metallic element. Its atomic number is 81 and its symbol is Tl. It looks much like lead, but chemically is very similar to aluminum. It is so soft that it can be cut with a knife. It reacts easily with air, water, and most acids. It does not react violently like the alkali metals. Thallium was discovered in 1861 by the English chemist William Crookes.

Thallium and thallium compounds are very toxic, so some of their earlier uses (such as a rodent poison and an insecticide) have been discontinued. They can enter a body through the skin, by inhaling dust or fumes, and by direct ingestion. As a result, strict rules about the use of thallium and thallium compounds have been created by the U.S. Environmental Protection Agency (EPA).

Adding thallium to mercury lowers mercury’s freezing temperature, permitting its application in low-temperature thermometers.

Name

When an element is burned, it creates a very specific spectrum of light. Thallium’s spectrum includes a distinctive bright green line. The name thallium comes from the Greek word thallios which means a green twig, which is a reference to this green line.

Sources

The thallium concentration in the Earth’s crust is 0.7 parts per million (ppm). It forms a small number of rare minerals, including crookesite and lorandite. These minerals form with the zinc mineral sphalerite. As a result, thallium is recovered as a by-product of processing zinc ores. It is also recovered from lead and copper ores, and from the dust that accumulates in the flues of the copper, zinc and lead smelters.

It is estimated that the thallium resources worldwide total approximately 17 million kilograms. These resources are found in Canada, Europe and the United States. As late as 1999, thallium was not recovered from ores in the U.S. Thallium is imported by the U.S. from Belgium, Mexico, Germany, and the United Kingdom.

In addition to these resources, approximately 630 million kilograms of thallium is contained in coal. As with other commodities, a way of recovering thallium from coal at a reasonable cost has not yet been developed.

Manganese nodules that form on the ocean floor contain thallium. However, it is still too expensive to gather these nodules, so they are presently not a source for thallium.

Uses

Thallium is used in a number of electronic devices. It is used in selenium rectifiers, gamma radiation detection equipment, and infrared radiation detection and transmission equipment.

It also has non-electrical uses. For example, thallium is added to glass to increase its density and refractive index (that is, its ability to break light into its component colors). It is also used as a catalyst to create certain organic compounds. Radioactive thallium compounds are used in medical applications.

As mentioned above, thallium is no longer used to make insecticides or for rodent control.

Substitutes and Alternative Sources

The supplies of thallium are more than enough to meet the demand for this element. As a result, there is presently no need to search for or to develop substitutes or alternative sources for thallium. Should these resources be used up, retrieving thallium from coal or from the deep ocean manganese nodules may one day become possible or even necessary.

Further Reading

• Common Minerals and Their Uses, Mineral Information Institute.
• More than 170 Mineral Photographs, Mineral Information Institute.