Mineral Resource of the Month: Indium

Amy C. Tolcin, a mineral commodity specialist for the U.S. Geological Survey, compiled the following information on indium, a minor metal that is important in flat-panel displays.

Indium-tin oxide thin films are used in flat-panel displays, such as in touch-screen mobile phones. Credit: ©iStockphoto.com/tiridifilm

Geologically, the occurrence of indium minerals is rare. The element most often occurs as a sulfide inclusion or substitutes in other base-metal minerals, including cassiterite, chalcopyrite, sphalerite and stannite. Indium’s abundance in the crust is estimated to be 0.05 parts per million, which makes it more abundant than silver, but it is so widely disseminated that it does not occur in high enough concentrations to form mineable deposits. Therefore, indium is most often recovered from byproduct residues produced during the refining of lead and zinc. But only about one-quarter of the indium mined worldwide is refined into metal, as many indium-bearing concentrates are sent to refineries that do not have the capability of recovering the metal.

Indium was discovered in 1863 after the invention of the spectroscope allowed scientists to distinguish it from other metals by analyzing the light the element emits when heated. In fact, the word “indium” originates from the distinctive indigo blue lines the element emits in the spectroscope. Indium was initially used in small quantities as a coating material. In the 1930s, indium began being used on a commercial scale for dental alloys.

Currently, indium is mostly consumed to produce indium-tin oxide (ITO) thin films, which are desired for their transparency and electrical conductivity and used in flat-panel displays for TVs and computer screens. ITO thin films have limited or no commercial substitutes for many of the products they are used in, including touch screens and liquid crystal displays in consumer electronics. ITO thin films are also used in architectural and automotive glass and solar panels.

Other indium compounds are used extensively in semiconductors and batteries. Most indium semiconductors are used for light-emitting diodes (LEDs) and lasers. In batteries, the addition of indium prevents corrosion and improves the battery’s energy density. Other significant end uses for indium include alloys and solders.

To meet the growing demand for indium, the metal has been increasingly sourced from ITO scrap. China, Japan and the Republic of Korea lead the world in ITO recycling.

For more information on indium and other mineral resources, visit minerals.usgs.gov/minerals.

Indium production and consumption:

• China is the leading producer of indium from ores and concentrates, and Japan is the leading producer of indium from recycled materials.
• Japan is the leading consumer of indium.
• The U.S. imports 100 percent of its indium, primarily from China, Canada and Japan.

Fun facts:

• Indium metal produces a high-pitched “tin cry” while being bent. The sound is produced as the metal crystals shear against one another. Tin, gallium and niobium also have this unusual characteristic.
• The first ingot of indium was displayed at the World Exposition in Paris in 1867.
• Alkaline batteries contain indium to control certain chemical reactions within the battery, which improves the electrical discharge rate and prevents rupturing.
• Indium is used in fusible alloys found in automatic sprinkler heads. A valve in a sprinkler head is closed by a plug of the fusible alloy. When the temperature increases above a certain point, the alloy plug melts within the valve, allowing water to be released.
• Indium is used in CIGS (copper-indium-gallium-selenide) thin-film solar cells, which are cheaper to manufacture than the traditional silicon solar cells.